c-/u Tf-XE ~~87 /SE"86 P~TIO CONTIt-m!~hL It~ OCEAf.\lIC AND BASALTS jU~D THE or.J:GI~~ 0'2 IGtmOUS ROCKS by GtJN1rER FAURE B.Se., Unive~sity of Westezn Ontario (1957) ) LI ..~~~ , .1NDGll£N SUBl'-D:TTED IN PA1:tTIAL fMJI2ILL!i1ENT OF THE REQUIlRE.I'f~~""TS FOR THE DEGREE OF OOCCrOR OF PHILOSOPHY at. tha MASSAChVUSEftS ImSTITUTE OP TECHNOLOGY • June, 1961 Certified by ~ , - Thesis Supervisor - t . Accepted by ----------------Chairmal1, Departmental Committee of G~aduata Students. THE Sx-81jsx-86 RATIO IN OCEAtilIC Al~D CO~~!~m~""X'AL !3ilS}~LTS 1\~'"D ~'HE ORIGlt~ 01'1 IGNEOUS by ROCI(S Guni:er Fat!X"e Submit~ed ~o the Depa~~~en~ of ~~ology snd Geophysics on May 15# 1961, in paztial fulfillment of the requi~e~ ments fo~ ~he aegree of Doctor of Philosophy. Abst~ The isotopic eqmpositions of strontium in 2S basal~s and related volcanic rocks f~~~ oceanic and continental localities have been detG!!~linedtl the abundance of srS7 in basal'ts. ratio, is constant Wi~li~ n~rzow of the sr87/sr86 ratios of eleven teen cQiil'tinental basalts 0.0003c respectively. 'Arne &"esults indicate '\.':ha't 67 e~tpg'es:;edas the SX ;S'IC86 11mi~s. The average values oceanic basalts and ~our- arc O.7072:t 0.0003 and O.70B2.:t The diffe~ence in ~he abundance of Sr87 is not consis~ent and is not believed ~o be signif1can~o The overall average sx8ils~86 ra~10 xor all 2S basalts is o. 707a~g:gg~. 1\ reproducibility of:t 0.001 (.14%) fox- t'he 87 sr /sr66 ratio was acbieved by making an 1nst~uw~ntal isotope fractionation correc~ion. The accuracy of the ir~asure~ manta W&S monitored by periodic analyses of a srco3 standard. The concentxa~!onB of rUbidiam and strontium in a rep~~~ sentative number of basalts were dete~~ned by isotope dilu~ tio~. Five olivine basalts avezaged: Rb =17 t. 6 ppm, Sr 390.t 55 ppm, RbISr :: Oe044:t 0..018 whereas six t.hcleeitic basalts were found ~o contain 30~ 7 ppm Rbi 504% 167 ppm S~~ Rb/sr = 0.060% 0.024. The ~epzoducibi11ty of the rubidium = and atgon~ium analyses is~ 2%e The small, but statistically significant, diffe~ences in the abundance of sr87 in basalts from different locali~ies are in~erpreted as evidence of small latexal and/or ve~tical variations in ~hs Rb/Sr ratios of the source material of basalt magma. Numazical values of the Rb/Sr ratio w~~e cal~ culated with the assumption that the initial Sr87/Sr86 ~atio was O.7004Z 0.002 and that the age of the source material i6 4.5 x 109 years. The average value and its limdts of varia- = O.039~8:8i~. tion was found to be Rb/Sr The relative abundance of rubidium in the souzce regions of basalt magma is therefore approximately iden~ical to ~hat of olivine basalt, eclogite or ul~rabas1c 19naous rocks. The low value of ~he Rb/Sr ratio as "Jell as other geophysical and geochemi.- cal evidence suggest that basalt magma is ganerated upper mantle. in the The abundance of sr87 in the sialic CZU6~ waa estimated by making reasonable assumptions about 1~B Rb/Sr ratio, its age and its initial sz87/sr86 ~atio.On the basis of the mos~ reliable rUbidium ana stz~ntium analyses available in ~he geochemical literatuze and using PoldervaaztOe crustal model, ~he Rb/sr ratio of ~he sialic crust was estimated to be Oe25. If its age is 2 billion years and its initial sr87;srB6 ratio was 0.704, the pzesent value would be 0.725. This es~1mate ~aB substantiated by ~e results obtained for two composites of Paleo2oic shale which averaged sr87/sr86:: 0 7215 Z 0.001. 'I'his value is measu&"ablyhighsr than that. in oceanic and continGn~al basalts. CI On the basis of ~h1s evidence the hypothesis is advanced 'that the value of tIle sr87/S'1:86 X'atio of .igneous rocks, at. the time of crystalli~ation, can be used as a crite~ion fo~ the origin of the ma~erial. The initial sr87/sr86 ra~io o~ an igneous rock formed by ~e-msl~ing or gzanitization of sialic mategial is expected to be measurably higher ~1an that: of an igneous rock dGlrived frem the souX'es regions of basalt magma. ~e initial sr87/sr86 ratio of.a complex of intrusive igneous ~oc:ks or of .a series of lava flows of different. compositions is best dete~ned from the coordina~es of the point of convergence of the wholG-gOCk st:ontium development lines. The convergence of the strontium developmen~ lines is a teet for possible co~agm&t!c relat.ionships of associated igneous rocks. This teat is particularly applicable ~o the study of alkaline rocks, pegmat1tes, lamprophyres and cl1roonatites. A pre11mdnary investigation of ~ha 1n~ruBiva igneous roclts of 'the Monteregian hills, Quebec, indicated ~hat the different: rock t.ypes were derived from f:he sa..~ source about 11Sr 2S million years ago which.at that time had a Sr87/sr86 ratio of 0.7047%0.001. This unusually low value indicates tha~ these rocks originated in ~he source regions of basalt magma, perhaps well belo"J the base of the crust. Thesis Supervisora Titles l?atrick M. Hurley Professor of Geology page 2 12 LIST OF TABLES 16 18 ACKNOWLEDGIl/lEil'STS Part I TPu~ BE'87/S;g:86 RATIO It" OCEIUnC ,At'ID COI~Ii:-mm'AL BASAI.,TS ~JD '.rHE ORIGIN OF IGNEOUS PDCKS. Abstract 20 Iratroductlon 21 E~~erimental Errors and Accuracy 2S Analytical Data 30 The Isotopic Composition of Strontium in the Source Regions of Basalt Magma 36 The Isotopic Composition of Strontium in the Con~inental C~ust 41 Geological Applications 49 Evidence of Sub-crustal Origin of the Intrusive Hills, Igneous Rocks ~.n the Mont.ereg1an Quebec Discussion of E)~er1mental Results 54 56 Conclusions 61 Acknowledgments 64 References 6S Page 1?a~t II Chaoter 1 V~RiAT!ONS OF THE sr87/sr86 RATIO IN ROCKS AND THEIR GEOLOGICAL SIG1:1!~IChl~CE. 70 Introduction Derivation Equation of the strontiwn Development 73 The Primordial sr87/SrB6 Ratio 78 C~ol09ic&1 83 Applications The Origin of Granitic. Igneous 84 Magma 90 of the Upper Man~le 93 The Differen~1ation The Composition Rocks of Basalt ChapteE' 2 THE COUCENTRATIONS OF RUBIDIUM AND STRONTIUM IN IGNEOUS AtID SEDIMENTARY ROCKS AND IN THE CRUST OF THE EARTH. Introduction 95 ~le Concentrations of Rubiaium in Igneous and Sedimentary and strontium Rocl~s 96 Summary 107 Summary and Conclusions 113 -. The Concen~rationB of RUbidium in the Crust of ~le Earth and Strontium 11($ Introduction POldervaart's Model of the Crust Introduci:ion 118 119 Continental ~~ield Region 121 Young Woldea Eelts 122 SUb-oce~nic 124 Regions Volcanic Islands 126 The Crust ol the Ea~th 127 Summary and Conclusions 130 Chapter 3 THE ISOTOPIC COz.mOSITI01,J OF STROmZU~1 IN TIE CRUST OF fl'EEEnRTH. 133 Int.zoduction The Isotopic Composi~ion of St~ontitffil in Chemical Reagents and in Ocean ~;ateZ' 134 Estimate of the sx87jsz86 Ratio in the Crust 139 Egpezimental 142 Evidence Summary and Conclusions 144 Chapter 4 RUBIDIUM AND STJaONTXUf.l ]\l~t,YSES. 145 Intzooduction Experimantal pzocedures Preparation of Br8S Radioactive Trace~ Calibration 147 of the "Spiltec,a Solutions Introduction RubidiQ~ 145 Bnd Strontium Shelf Solutions 149 151 152 P~eparation of Dilute Rubid! lLtU Spilt8 Calibration of sra6 Spik~ Solution Calibration of Precision S1:84 of Rubidium Sp1Jte. 154 155 158 and Strontium Analyses Introduction Rubidium Chapter 160 and Strontium Blanks 160 Results of Triplicate Rubidium and Strontium Analyses 162 TiltonOs 164 Rubidium Shelf Solution 5 MEASURENEN'T OF THE ISOTOPIC OF STRONTIUM. COMPOSITION Introduction 166 Chemical Procedures 166 Mass Spectrometric Techniques 170 Discussion of Er~ors 174 Introduction Reproducibility 175 1\ccuracy 180 Conclusions 183 Chap'te&' 6 TEfE ISO~'OP!C CC~~OSITIOt;j AtiJDCONT!NEN~AL OF STROt~~rXnz-! IN CCEIHG!C BASArmS~ 185 Analytical 187 Resul\.:s lee Oceanic Basal'ts loSS 193 t?1sCOrt'3sion Isl~nd, and iGhe l1z0~es !!'.doeuAtlantic Ridge ~ 1.94 195 197 Continen~al Basalts 197 Daccan Plateau~ India 201 Columbia Rive~ Plateau, Oregon 202 Triassic Diabases of New Jezsey and Connectic\}.t. 203 !~ecellanaoua Localities 204 205 206 Conclusions Chapter 7 THE sz67/sz86 IU\TIO It~ PRECA!~r~AN BASIC I~f1'RUSlVES. 210 Introduction Tho Bushveld Igneous CGmpleJ~1 Introduction s. Africa 211 page 211 E:tpmximeni:al Results 213 Discussion 21.4 Conclusions 217 erne Duluth Gabbzo Introduction E~erimen~al 211 Results 218 Discussion Slurnnaxy and Conclusions Cllap'ter 8 Part 1 THE AGE OF AN ECLOGITE BY THE Rb-Sr )1ETIiOD. Introduction ~23 App~ica~lon of ~undances of 'Strontium Isotope~'in 'Basalts to th~ Eclogite Problem 226 Geological Summary 227 Exper1~n~al Data 227 Diacussionof the Results 230 Conclusions' 232 part 2 . ' THE THE AGE OP FOmlATION ~Et..Y GREENSTOtiE ~"D .. T$ SOUDAN I!l MINNESOTA •. Introduc:1:ion 233 Geological Summary. 233 " . Experimental Results 235 235 Discussion 239 pag~ 3 ElJIDEt~CE OP SUB-CRUSTAL OR!GIF! OF ROCI{S rfROr.-1 THE I~OI~EREGIAtq LLS s QtJEB;E;C.. m 24-0 Geological Summary 240 242 242 Discussion 247 Conclusions Chapter 9 VARIATIONS OF THE sr66/s~B9 At~ s~s4/sza8 RA~IOSo 249 Discussion of Re8ul~B 252 251 Conclusions Chaptez: 1.0 DESCRIPTION OF S~!.iPLES. Oceanic Basalts Con~inental Volcanic Rocks precambrian Easic Int~sives Monteregian Hilla 262 264 264 Eclogit.e 265 tt~ewat1n'Volcanics 2r~~ Minneso~a Shale Composi~es 265 266 BIELIOGRAPliY 267 BIOGRAPHY 2i7 26 Isotopic Compositi@n of St~on~ium in St~ont1um Reagen~so A~~lyt1cal Resul~~ for Oceanic ~a · 'co~tinental ~aealtso 26 ' 31 Concentrations of fu~oid1lnn ~nd St~ontium in.Igneou$ an.d Sedimentary RockeD ~~ncentra~iona of Rubidium and Strontium in the Crus~ as Defined by polaezvaag~Os ~-5 45 tttCdel. Estimate of the sr87/sr86 Ratio in a aypotbe~ical Geos¥nclinal ~ssemblage. 46 Iao~op1c ComposltifJO of Strontium in Composites. of paleozo~c Shale'. 47 Isotopic C~positicn Qf St~ontium in Igneous Rocks frQftl the Monteregian Hills. 57 1.1 The PZ"imoZ'dl~ls~87/sr~6 'RatiO. 80 1.2 Initial sr87/sg86 Ratio in Chondritic Meteorites. 82 2.1 Superior'Analyses of Rubid1um and S~rontlum in Igneous" Rcc'ks and Meteorii;ss. ' 97 P~t II Table 2.2 Summary of Rubidium and Stron~itim Concentrations in Igneous Rocks •. 106 2~3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2~11 2.12 2.13 1 Supe~ior nnalyseBof Rubidium and Strontium' ~n Sedimen~ary R~eka9 110 Surr~ar~of RUbidium and 8tron~iw~ Concen~r&tions in Sedimenta~ ~ockse 114 and S'tron'cium in Igneous a.nd Sedimentcu:y Rocks. 115 Concentrations of alm1dium and' Strontium in '~he Oceanic CZUBt. 120 Concentrations of Rubidi~~ and Strontium in the ~ontinental Shield. 122 Concentrations of Rubidi~~ aridSt~ontilli~ in the Young Folded Belts. 124 Concenerations of RUbid1wn,~,d Strontium in ~he,Sub-oceanic Regions. 126 Concant;gat1.ons of ~ubiaiu.li'Q Concentrations of Rubidium and Stronti~~ in OCeanic Xslands. 127 Concentrations of RubiCliumand Strontium in the Total Crust. 128 Concentrations of Rubidiumahd St~on~lum in Various l?ari:s of' 'the Crust. 129 Concentrations of Rubidium and St~ont1um in 19n~ous and sed!ment'ary ttocks. 2.14 Concent~at1ons of Rubidium and strontium in the Crust. 3.1 131 132 Isotop1eComposj.t1on of Strontium 1" Ch~m1cal R&agen:~ts. 135 302 Isotopic Composition of Sea Water Strcnt1um~ 3.3 The sr87/sr86 Ratio in a:ayPotbetlcal Geosynclinal Assemblage. 136 141 3 ...4 :!8~~i't:~pd.cCert~p"osi'~~.<n~ of: S'ti.:Z'Qn:citlE'..l i7tj C~28tal RG~ks B • 152 ~ Gp;lke(l lmalytical Da:ta ffnr: Calib~:a:;;i@!j o~: S~.86 ( t.L,4: 156 j~ilalyt:tcal Da'ta £04' ~..... 84 Calibza~~ ..cl1 of ~.'.'.. monaal and S'tt:on1::il:L>Q Elmn"tt D'8t(tn~111iwa"i::tcw~z Rub:tdifl:sG SpjJ{f9 .. 159 Q 161 lIr].agn1-cudeof RlWidiu.,~ and Stzon'tium 131a~1!~ 161 Co::goc'i:ions. 4.6 165 4.7 Inte~la1)CEa,toz" ~u~st.11.tsfor T!l'ton 0 s l~ubidilliu Sh~lf soluti\c)1!lt. 5.. 1 Isotopic 165 Composition of S1:E'ont:j.' ..un in Basalt from RilauasD Hawaii (R1292). St~ont1Um Carbonate Reagent~ Eime~ and 181 P~end, lot 492327. 5.3 !sotepic Composition cfSt~onti~n ~eagants. ' O~ean1c ~asalts. in G1s~cal ' 182 139 6.2 6.3 7.2 Continentat Ozigin., Volcanic RO.9kf3 of Sub~czustal 198 Summary of Data fo~ Continental Saaalts at ~he Time of ?~eir Ex~ru5!on. 205 The Bushveld 'Comple~tl!; 215 DUluth Gabbi:c.. 220 Page 228 The Ely Ggoeenstong and t.he Scud~n ~omation. 8.3 E:~rimantal 236 Results fo~ the Mcntegog1an Hij~ls, Quebec. 243 250 9.2 Chi Square 'lest. 256 -16<::> LIST OF FIG~ES. Figure A-l Page The sr87/sr86 Ratio in the Source Regions of Basalt. A-2 41 The sr87/sr86 Ratio in the Source Regions of Basalt Magma and in the Sialic Crust. A-3 The sr87Jsr86 Ratio in the Intrusives of the Monteregian Hills, Quebec. 1.1 58 Increase of the sr87/Sr86 Ratio with Time and the Rb/Sr Ratio. 1.2 Sl 77 sr87/sr86 Ratio in the Crust and the Upper Mantle. 85 Concentrations of Rb, Sr, K and Ca in Igneous ~ockso 108 Sr Development Lines for Red Granite, Bushveld Complex, South Africa, Schreiner, 1958,p. 114. 212 The Basic Igneous Rocks of the Bushveld Complex,S. Africa. 216 7.3 Duluth Gabbro 219 7.4 Initial 2.1 7.1 7.2 sr87/sr86 Ra~1os in Precambrian Basic Intrusives. 222 Eclogite Xenolith from Kimberlite Pipe, Robert Victor Mine, South ~frica. 229 8.2 Ely Greenstone and the S~udan Foxmation. 237 8.3 Monteregian Hills 244 9.1 Distribution of 8.1 sr8G/sraS Ratios. 251 -17Figure 902 9.3 Plot of Cumulative Distribution of the sr86/S~88 Ratios on Normal-Probability paper. Distribution of sr84/sr86 na~ios in Rocks, Tektites and srC0 3 8 9.4 Plot of Cumulative Ratios. _ Distribution 254 258 of s%84/sr88 259 It is a pleasure to acknOwl~~ge my~ebt.to my ~hesis ,suppliec1 most of 'the r:;>ck specimens and' made ava11abl~ a .' ,mass spsctrameter for 'the analyses. His comments and constructive suggestions did ~mch to clarify the problem and k~pt the investigation on a t~ue course. 'In addition, I wish to thank ~rof~ssor t~"H.. Pin~on, , the chemical procedures and inst~~ental ',' '\JJhotaught J-&., , ' .. me , , techniqu0s many Fellaw students have con- of which h~ developed himself. tributed to many aspec~,~~~ th~s ~h~sis throug~ 1nformal a1scussions , endabted comments. ,I am"pa.rticularly and 'crit~c~l to Dr. S.R~ ~art,. Mr. C.C. Schnei:zler and Mr. G.R. Beall ~ho in 'add~tionsuppl~ed several of tbe rock ,s~c1rnens from the Monteregian ~ills. ,I als~ wish to thank.Professol: J.D. Thompson, Jr. ot Harvard Univex:s1ty ... ' ... 'for ..the. rock 'specimens f:tom,the.. Bushv.eld Complex~ I . .' " "'11\18 'thesis w~~ the full' was in, fact of cooperation a my . • joint ven~ui:~u~dertak~n wife Barbara to .whom I 'OWe.a large debt of 9rat~tude; .' The financial,.support of Imperial Oil Limited . , throug~ an Imperial Oil Graduate 'Research 9ratef~11y aCkno~led9~d. ~ellol"ab1p is (Intendedfo~ Publication) ~pm S~~/SZ86 RATIO IN OCE~r~C CONTlNE1~AL B.r.~LT nt~ THEl AND 2RIG1N OF IGNEOUS ROC~S. By G. Faure and P. ~1.Hurley 0 Abstract. Tne isotopic compositions of ~trontium in 25 basalts and related volcanic rocks from oceanic and continental localities have been deterMined. The results indicate that the abundance of ar87 in basalts, expressed as the Sr87/sr86 ratio, is constant within narrow l!mi~s. The 87 86 average values of the Sr /sr ratios of eleven oceanic basalts and fourteen continental basalts are 007072+ 0.0003 and O.1082.:t0.0003, respectively •. The diff"erence in the abundance of sr87 is not consistent believed to be significant. ratio is 0.7078 + 0.002 The overall and is not average Sr87/sr86 -0.003. The concentrations representative of rubidium and strontium in a number of basalts. were dete~ned by iso- Five olivine basalts averageas Rb 17.t 6 ppm, Sr = 390 T. 5S ppm, Rbis!:' 13 O.044.:t. 0.018 whereas six tholeeltic basalts were found to contain tope dilution. = 301: 7 ppm xu" . 504:!:.167 ppm Sr, Pb/sr ciJ Oo060:t 0.024 • The small, but statistically significant, differences in the abundance of sx87 in basalts frcm d1fferen~ locali- ties are interpreted as evidence of smull lateral and/or vertical variations in the Rb/sr ratios of the source regions. Numerical values of the Rb/Sr ratio were calculated on the assumption tha~ the initial sr87/sr86 ratio ~)as O.7004:t 0.002 and that 4.5 x lO~ years. variation the age of the upper mantle is The average value and ii:s limits of was found to be Rb/Sr t1ve abundance of rubidiam magmais therefore s . 0.039+ 0.012. -00017 The rela- in the source regions of basalt approxima~ely identical to that of olivine basalts ;or ecloyite. On the basis of~he mos~ reliabl~ rubidium and stgontium analyses of.igneous and s~imentary'roeks available from the 11teratur~ .arid ,-,sing Po~dervaart 0'8. . crustal' model, the RbISr ratio of '~he .sialic cZust' was les.tin\~ted to be (L.2S. ,If the age of1:he sialic crust 1'6,approximately 2 b111i(~n yearsand1ts initial sr87/Sr86,' ratio was 0 •.704, the'preB~nt ave:age value woul~ be. 0.725. This estimate was substantiated by the results 'ob~a1ned for two composites of Paleozoic shale which averaged Sr87/srB6 0.7215 +. _.0.001. ' ' = . On 'the basis of this evidence-the hypothesis is advanced"that the value of tIle sr87/sg'86 ratio of igneous ro~ks',. at 'the t1~ of cry~t~11i2at!on, can be used. as a criterion for.the origin of the materialo Th~' initial S7 86 ,sr /sr . ratio "of an igneous rock formed by re-me 1ting or 9rani~za~1on of ~e sialic C~ust is expected to be mea~ur~ al3.lyhigher thantba,t. of atf .igneous ~ock °d,erived from the sub-crustal s~urqe',z.e910ns o~ basali: magma. sr87/sr86 ratio' of a 'complex of intrusive ~tjneo~s .rocks,,or of a 'series of lava flows of differ .. 'theinit.ial -ant' cotnposltions vergenr::e Of t.he The' cori"ergence 'also ~ .used to betwe«:tn is bestdeteJ.:lD1ned from the point of 90n- whole~3:oclt strontium development l.ines. lines can ' of i:ho strontium development test f~r pOssible co-magmat:1.c. rela-t1onships 8l3S0c1a'ted igneous, rocks. I~ is suggested that this f.t9thcd 1s particularly appl~CaPle to 'tbestudy of, .alkal~e r~ks, pe9ma~ites, iamprophyres' andcarbonatites. Introduci:1on:: ..'rile abundanc;e of sra7, expressed' as the 'ra't10 sr87/s~86! has increased by'the nat:u~ai'rad~oact1ve'decay of Rb87 ~o s~7 since the tlmeof formai:ion of the eleIr~nte. The J:;81:e'of 1ncreaB~ of 'the srS7/s.r86 ratio: of any system Whicb' is",close''' to rubidium andstront:1um duri~9 it:s,11fe- time is propo~ional to its RbISz ratio. pgocesaaa opezat1ng si~ce ~e have X'Gsulted in an enrichment Differentiation beginning of geologic time of rubidium .in the upper regions of the con~1nental crus~ relative to the base of the crust and the upper ma~'tle. As a result: tot is expected that in the course of geologic time the abundance of sr87 in the sialic crust has increased at a greater rate than in those parts of 'the upper mantle which are the source regions of basaltic magma. If the difference in tha sr87/sr86 ra~1os of the two environments is measurable, then the origin of intrusive ~9neouB rocks in the sialic crus~ can be de~erm1ned of their sr87/sr86.ratios In1:rus1ves wh1chwere from tbe values at ~he time of crystallization. formed by re-melt:ing or grani't1za- tiOD of sialic material are expected to have higher Sr87/sr86 ra1:ios than ~hose igneous rocks which originated in the upper mantle or the lower: s1mai:1cregions of the con1:1nen~al crust. A very similar BU9ge'stlon was made by Holmes who proposed t:hat: the abundance of the isotope could be used to differentiate bet"-rreen grani tee (1932) ca41 formed by recme1t!ng of granitic parent material from those which are derivatives of basalt: magma. This pE"oposal was based on the asaump~ion, now diapZ'ovedo 'that ~l The abundances of the 1S0~OpeC3 ~ela~1ve to SraG = 1, as decays to Ca41• Sg87 and Rb87, both a function of time in any system which rema1ned closed ~o rUbidium and s~rontium during ita lifatims can be expressed as foll~ ... er: ).-t" a ~ ao+ b(e (1) -1) whe:e a b = sr81/srS6 at time of analysis of ~e ma~er1al = Rb87/sr86 at time of analysis of the material sr87/S:86 at an 1n1~1al t1me'~c = ~1me measured backt:tJ8rds from A = 'the present decay constant: of llb87 1.47 x 10-11 ygS-l (Flynn and Glendan1n, 1959) - Before equation (1) may be used to calculate the abundance of ar87 in the sUb-crustal source regions of basal~ magma th:rOU9hou~ geologic time, appropr1ai:e values of musi: f1rs~ be determined by a judicious available data. :reg1ons wi~ 80 and to examinat:1on of Moreovez, the homogeneity of ~he source respect: to 80 to to should be demons$:rated. In this paper ~he xesul~s of 1sotop1e analyses of strontium in 2S basalts and related volcanic sub-crustal oriq1n are reported. rocks of The specimens were selected from oceanic and continen~al to test the lateral h~gene1~y localities in order of the sub-crustal source regions of basalt magma with respect ~o a, the present sr87/sz:86 ra:tio. The resul'ta will order to dete~ne also be exa.c...wrdnad in to wha~ extent they 1nd1ca~e homogeneity of the source regions with respect to So and to. Concentrations of rUbidium and strontium were meas- ured in a representative of isotope dilution. for their numbaX' of baaali:s by the method These analyses were made not only geochemical 1nteresi: but also in ordaZ' to compare the measured values of the Rb/Sr ratios in basal~s to ~hose of their sou~ce regions calculated by means. of equa'tion (1). Such comparisons can give evidence of chemical fractionation ing of the source material Experimental Errors during ~e partial melt- to produce magma. and Accuracv!! All 1'il88SUremen1:&were made on one 60° sector radius" solid source mass spectrometer sweep and a vibrating ,ree~.electrometer tion of ~e lon current. D 6 inch using a magnet for amplifica- The instrumen~ was built at M.I.T •. and has been described (1958)" and Herzog and Pinson previously (1956). by Herzog et al. Xt ~as dasi~able for the pu~osa of this investiga- tion to determine Sr87/Sr86 ratio3 with a reproducibility of O.l~ or better and ~o ma1~~a1n a high etsndard of accuracy of all measurements. The geproducibil!~y of the mass spectromatglc measurements was determined from the resul~s of "several analyses of a single sample repeated peIClodically throughout the course of this investig&tion. It was expressed as the standard deviation for a single analysis calcula~ed fram the results of the replicate analyses. Tha main fac'tors duc1b11ity are: during che~cal spectrometric t1h1ch c1etexmine ~he X'spro- variable amoun~9 of contamination processing of ~he samples and the mass analysis, effects of isotopic fractionation in the 1ns~rument., vaz:1ations of ~a ra~e of ion emission from the fl1amen~, 1nstabili~y of ~s magnetic field of the magnetic analyzer in the amplification and high-f:equency and recording electronic noiee systems. Several of the errors listed above may bs systematic for a single analysis but are random for a large number of repeated measurements. They thus affect single determination Z'~ndom errors. the xeproducibility and are "included with ~he truly of a An instrumental fractionation corgection to all sr87/sr86 ratios on the assumption ratio is a constant e~al was applied ~hat the Sr86/sz88 to O.1194(Nier, 1938). All szS6/sr88 ra1:ios were adjusted to this value bu~ measured only half of this correction was applied to the measured sr87/srS6 ratio bacauss the mass difference between SrB7 and sr86 is only one half ~a~ between sr8G and SraB• 1'" This correction resulted in a ma$ked ~provement reproducibility of the sr:e7/a~e6 ratio w1thou~ same time affecting ~e the value of the mean. of the at 'the Tha effect of fract1ona~ion correction and the resulting improvement of ~e reproducibility axe well illustrated by the analyses of an olivine basalt from Kilauea, Table A-l Hawaii (Table A-l). Olivine Basal~, Rllauea, Hawaii (R1292). No.of sr87/sr86 (srS'/sr86~ Date rr Bra6/SrBB sr84/sr88 scans 6/1/60 0.7062 0.7068 0.1196 0.0070 138 8/3/60 9/3/60 1/25/61 2/8/61 0.1102 0.7072 0.1184 0.0063 84 0.7090 0.7040 0.7091 0.7069 0.7058 0.7061 0.1187 0.1200 0.1184 0.0066 84 0.0065 Average 007077 0.7066 .0.1190 0.0066 c; 1:. 0.0011 i; 0.0003 1: 0.0003 -- 104 90 :to.OOOl :t 0.0006 :t 0'. 0007 %0.0003 '.Corrected by adjusting sr86/sr8S to 0.1194 and the sr87/sr86 ratio by half this' amount. (J 1: 0.0025 =21= In this ser1SB of mgasu~amants ~O.0006 a reproducibi11tyaf (0008%) haa bean achieved for the corxected sr87/sr86 ratios. This represents an improvement. by a factog' of about four over the reproducibility uncorrec~ed ratios. The importance ation in the mass spectrometer range of variation rected of the of isotopic fraction- is illus'trat:ed by the of the measured values of the uncor- sr87/S1:86 ratios as wall as the 8r86/8r88 sr84/sr88 ratios. A precision e~ror of O.l~ for the 8%,87/5X86 Z'atio can be achieved effects of isotopic and the only by removing fractionation the in the mass spectro- meter. The accuracy of ~e periodic analyses of a m9asurements was monitared by srco3 standard (Eimer and Amend, lot 492327) for which isotopic analyses ~ave been reported by Hegzog et ale (1953) as well as Aldrich The average of elgh1: analyse~ performed G~ during of this invest.igation is in excellent. agreemsnt earlier analyses 'the course with (Table A-2) as well as the original measuremen~8 repotted tium. ale (1953). by Nier (1938) for me~al11c st.ron- Table A~2 Isotopic Composi~!on of Strontium in Strontium Reagants AnalYBt: N1ero 1938, p. 277 0.712 ~O.007 0.1194 :to.OO12 Aldr1ch et al.~l) 0.711 1953, p.458 ~ 0.0004 Herzogat al., 0.1195 0.0068 .:t. 0.00014 0.0067 Z 0.0003 :t 0.0005 0.1196 0.0070 0.712 1953, p.462 This work(2) 0.7119 :t 0.7120 0.1195 0.0066 0.0006 :t 0.0003 :t 0.0002 Z 0.0001 0Corrected by adjusting sr86/Sg88 to 0.1194 and the sr87/8:-86 &'81:10 by half this smouni:. (1) Average of s1:st analyses, (2) Average of eight analyses. The c~nelstency of our results for t:he srC03 standard with those previously reported 1s an indication of the reliability of the analyses of the isotopic composition of strontium. in basalts. correc1:ed sr87/s1:86 ratios The reproducibility achieved in of the i:h1s case is 1:.0.0009 (0013%) • On the basis of 1:1118 evidence single tosasurementsof the sr87/sz86 ratio, corrected for isotopic fractionation, of good ~all~y :t 0.001 (0.14"). BEe assumed to bave'a reproducibility An analysis is judged 'to be of good of ~ality if it is basad on 80 O~ more consecutive scans recorded at. pressures of less tb&n 2 :t 10-6 romof Hg and if it is not otherwise affected by instrumen1:al diff1c\11ties. Concentrations of rUb1ditun and s~gontium we~e determined by means of isotope dilution using spikes enriched in 1£087and fh:.86, respectively. were calibrated by triplicate against shelf solutions The spike isotope dilution con~a1n1n9 analyses known amounts of rubidium and strontium of nozmal isotopic The isotopic solutions composition. cOO'lpoa1t:1onsof i:ha spikes ware confirmed by separate trip11cai:e analyses (Faure, 1961). Four blank. analyses for rubidium and stront:ium ware made in ~he c~'rse of this investigation ination corrections were applied to all (ab 0'. o 674/Agr..s. , 81: analyses. rUbid1um and strontium cate analyseso = and appropriate contam- = O.250~.9r.s:) The reproducibility analyses was dete:m1ned The results indicate by tripli- a reproducibility of :t ~ for both rubidium and stront1UiU. analyses m, ;1> lS ppm and Sr '7 150 ppm. (Faure. of 'the 1961). when Analvtical ~a~9~ The results of tl13 isotopic analyses of strontium in basalts together with 'their concentrations and strcnt:ium. are sho'\1nin Tsble ,ACl3. brackets of rubidium The figures in after the measured value of the sr87/srS6 ratio indicates the number of independent analyses perfc~ed Averages of 'the ratios of st:rontium for that speciman. isotopes were calculated for each local! ty from which The sr87/sr86 more tha\nona speciman was analY2ad. ratios ,,;ere weighted i.n the average by "the total number of specimens analyzad whereas the averages for the 6r86/sr88 and sr84/sr88 ratios \t;ere calculated total number of analyses performedo olivine Rb/Sr basalts = found to contain Rb/sr = of rubidium and strontium in five average Rb O.044t 0.018. Rb = O.060Z 0.024. All errors are the mean. standard deviations of.~a The concentrations = 17t.6 ppm, Sr = Rb c 6.9 ppm, Sr 390 -r. SS ppm, The six tholeei't1c basali:s ware 30'Z 7 ppm, Sr One eclogit.e = 504 :t.167 ppm and a kimberlite fZ'OID pipe in S. nfrica was analyzed in duplicate. are: from the I: 149 ppm, RbISr = The results 0.046. Rb/Sr ratio of the eclogite is identical ~o ~at olivine basal'ts. The in the -31- @r~ LO 0 ro '0 N ~ ~ N III N M i' • 0 • 0• 0 \t) (:') 0 ...... "'" " 0 :f-1j • l'f') q. .~La'" \D 0 ~ 0 0 0 \0 l!'t • M • a f-6 ~ \.00 • • ~ Ul ...r.s ~ .!J Ul ItJ CQ i2 ,..0 'C" ed. Ct S u 5w en' 1-4 ~ ....'" :$ fA .... 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N 0 At a ~ ~ 00 (j'a I' E' \0 \D 0 0 \J) \$) 0 \0 0 Q 0 t'- 6\ roo- ,..., P'4 mo 00 f'0 • • +1 rgra P4 ... co ..... ta s: c2'" t P) en ....0'\ ~ k Q) :> ~ • en .-1 • • +l r0- <.... 00 0 t"'" eo 1'0 N ,.. .... 0 I' CD +1 co ..., nJ ...to ¥t It3 ~ en en I1S ffi ~ N 0 0 N " • • +1 "• ('.) \ON mo 00 1'0 • • +1 q.,sO H ell \0 0'\ +1 !~ C m m I'll • .-4 tdto 1'0 00 1'0 • • "-" f' 'D ....- l' 0 • ('.) • • • I'Pl ..... .... en (j\ 0 i""- \t) ,.. .... ..... ....... 0\('1) LO .... 00 1'0 ""'0 ...40 E' • • 00 00 00 • ..... \00 r-I PI .... ~N • • i-t N coo coo • co 0 E' 0 C' G) g' <:) • • t\ ~ CDtn ........I'tJ- 18 (0 C1J \00 • • +1 .--.. -t-I • • m ;! ....0 ~ coo CI) N 0) N ..., • • to .. ...- i2'5J \00 ... ..., ..... ....., ..... ..... N S ~ 0 ~ Q) • ..... C") ::J C .... 0\ ..... CD fa .... m .....-4 ...c ~~ 'tJ en M l' 0 I' tv) ~ Ol""f 0\0 .-10 .-40 ..... • • 0'\<'1 ~ • I"'- 00 • c-t 0 Q 00 00 \0 .... pol tnf'"4 l" Q) "" ..... N \0 0 • • QJ .... ~, J!. ., . Q +1 ... t1It C.Q~ .. 0'\ ..... 0 0 I' .CD ~o .,....0 M (J , \0 0 .... ~n ~ 0 0 ~ ~ tn • • ... ...CD i&t "tn 00 ~ RI J: .-I N \00 00 ...ns U ('tl r-:. ex) ~ft° ! co ~ +\ ...... P) tIl OJ \Dc-4 woo ilt~ 0 \0 \!) U UJ i.104 u tJ ~ • mc .fJ t:: • C\ 0 rn ""0 M 0 0 tw ~ ....c • (j) tnN ~ ~ '8f\J • • A - tiS 0 - ~~ ..g u ~ tJ\ 0 IU 0 ...- wa to t2 tfl (l) t\) ~ ....~ .... 0 0:> roc t: .a tV :> ~ .... ~ &:: m o 0\ 0 ~ 0 ,m en N QJ ::J c c.w Q) ....CJ .... c: c ~ .... ...., It1 =' :> tJ cti ....... m (Xl 0 0'1 0'\ f'l'l " " k OJ &n ,.... ro 0 N ~ b" res k Q) ~ ~ r0- o • .. N o .-4 an ..0 o o o \0 o • \!) • '-O~ \0 0 o 0 (;)0 r~ P"4 0 .-of 0 .. N • i"- t\ 0 ....r- 0 o o ...... • o " Q "• to tn 0 COO CO s: .~ • +1• Ge .~ It) tn a.l 0 " 0 ...... C C1 00 • • +1 o 0\ C Q OeD • ..... 0 o P'f0 ''0 •• +1 , C'""! OJ o r-- • Q • PI o• n .... tI) "'~ \ON \00 00 00 ••+\ ..n \0 o o • O'\ttl tOO ...+1 0-40 ....to 1'0 l"- .. ___1-1 00 00 • 0 CD o • 00 00 • • +\ t\ 1'&-1 tOo P"40 ,...,0 0) .... ~+\ N " \00 •• 1' ..... \00 00 Ln.-i \DO &.n~ ..... Ntt\ 000 QO 1'0 • • •1-1 mo P-fO rot 0 • • +\ C%)N f'O 00 r-o •• +, M ~ 00 00.-3 00 i'Q ••• s: +1 \O~ COt') 00 ...,0 00 1"'-0 o • +1 0\0 r-..o • • tl -35- ,QI tat M\O \00 " 00 {r'l ~Q • • +1 ta M 8: m ~I CM . o to ~ ~ ~ "'r0- • lP'"t +1 ~ ~ N ~ om 0'0 • • G} \D ~ ~co ~ rgm Q',) l' .. 0) m ~ co c 0 ...c ~ ~ Ci' c-t C6 tlJ poof • \Dc-f 00 00 tf3(1) ~~~ Irt tDn 0....-. 0 U ~ ~ ~ UlCf.l or4 ....eo • • -+. frJ Q) co m coco . tQ en ~<4J t4 C!) ~~ \0 co .... men 0'\0 ~ ~ ~ :J Q'J 1'0 n1 ,C • • +• ~ ....c.c;~ ~ tC~tU 4JtOc s: ~o..s • ..., ~ Q) C 0 ~~ !'1 tt ... .... .. '6~ M I ~ Gl ..., ~ ~S (lJ c-I ~~ E1 fJ)tzf la, OJ U • co (t)...e>u .... ..... a..s~ ... .!J..t -afJ~.g g'CJ&.tflof .... fU .-J ...,:IsO 1a :rl0 ~~ '0\0 a~iui t't PI kCD o M C"'4 .... " C't) f.W m a 0 Q) S ~ 'W tJ 0 • c ~ CD 0 ~ m~ n1cSJ GUns U ...0 ;> .= t::: fnCU'O 'f'fOQJ oWrc •"" ~ ........ '" M n3 'tS ... res c Q) ctS .... ~ Q) ttJ tJ\& t: c &os u ..... 0 .e.I res ~¥tOJ (1) :>t f' s: ar3 ... .... .!J ~ \f) ~C &51 lU .. ~~ OIl ~ .... rn 00 fJ rar;: .eJ M C to :$ '" .... , to ~ 0 ~ wo 00 . f' 0 • • .&J ~ k W Pl ~~U)O'J • • Nf"l 0'\0 ...,0 1>4 .er .6J .... ~IOO m 0 • • CDk ~. film "'"' ,..." 0 \DO <X) W ,... +1 .... CD t1l4 m m c floG ItS fU ....'0 OJ ~~ Q) ~. 0 k ~ :3GJ M... N.aJ Q) &.If' CJA"'" r:~PI Hma; uco " .... QJ~ c ...... ~ (\,\ "- THE ISOTOPIC COr~OSITION OF 'S~ONTIm~ IN THE SOURCE REGIONS OF BASALT MAGMA. ~~e sz87isx86 ratios of basalts from different con- t1nantal and oceanic.localities vary between the limits 0.7047 (R2002, ~~ui Island) and 0.7101 (R4159, Iceland). The eleven oceanic basalts average s~87/sr86 = 0.7072 t 0.0003 whereas the four~:~n continental basal~s have a mean of O.7082 !:0.0003. '!'he difference between the sr87 of continental and oceanic abundances of baaal~s is no~ consistent and 1s probably not significant •. The combined average sr87/sr86 ratio for all oceanic and continental variai:ion basal ~s 1s o. 7078 t. 0.0002 o£'+ 0.002 'and -0.003. demonstrate bomogeneity wi ~h the 1im!ts of The results 'therefore of the source material ofbasal~ magma with respect ~o ~e sr87/sz86 ratio. '!'his homogeneity could be 'the reeul1: of fortuitous combinations of the variable!?! ao' b, and to in a multi- p11cl~y'o~ source materials dence that 80 1tcould be taken as evi- and 'to a.re constants of basal1: magma. The latter 11: implies or for all source regions interpretation 'that the small differences ratios of basalts from different is preferrable. in 'the sr87/sr86 localities are caused by corresponding regions. differences in ~he Rb/Sr ratios of the source The absolute values of the RbISr ratios calculated for the sou~ce material of basalt magma depend on the selec~ion of values for ao and ~o in e~at1on (1). The value of the init1~1 sr87/sr86 ratio (&0) and the time of the differentiation dete~ned of the earth (to) have been from analyses of meteorites. Direct measurements of this ratio cannot be made because crustal roclcs formed at that time have apparently not been pzeservad. of meteorites The use to detexmine "~alues of ao and to for ~he source regions of basalt magma can be justified by analogy with the results for lead isotopes. patte~son and his assoc~ ates showed that the abundances of lead isotopes in d1fferen~ stony meteori~es at t = 4.SS~O.07 preserved Hanbury and in Recen~ crustal rocks converge billion ye~rs t:o the primordial abundances in the tro11its phase of t.he iZ"on meteorite (Patterson~ 1956). The uranium"and thorium contents of this me~eor.tte are so low that the abundances of the lead isotopes have remained virt.~.ally unchanged since 'toSimilarly, ''the value of 'the in! t1al sr87/sr86 ratio can be determined directly from isotopic "analyses of strontium in !a)-poor stony me1:eorites such as 'the achondrite Pasamonte. ~2asurements of the sr87/sr86 ratio for this meteorite have been ~eported by P2KZog a~d Pinson (1956), Schumacher (1956), and Gast (1960a). GastOs value of 0.7004+ 0.002 is based on triplicate analyses on two mass spectrometers and is probably the most precise deta~ination. of 0.002 is he~e considered tone the uncertainty value as an estima~e of the true value of ~O. that The e~ror ao :: O.7004:tO.002 and to :: 4.55TO.07 of Gastes Assuming x 109 years, equa~ion (1) can be solved for (1Rb87/sr86) which is then transf01~d to ~e weight-ratio of Rb/sr. Using th~ae values for ao and to ~ha va~iation of the srS7/sr86 ratio in basalt from"different localities 1nd1cates a range of values of the RbISr ratio in the source material of basalt magma from 0.022 to 0.051. Rb/sr zat10 is 0.039. The average A comparison of the calculated values of. the Rb/Sr ratios of the source material Rb/Sr ratios in basalts indicates d~let1on material. of Sg) an enrichment with 'the of Rb (or of the basalt magma relative to the source The only exception to ~h1s are ~he basalts from Hawaii Island whose Rb/Sr ratio is 1den~ical to that calcula~ed for ~he1r source regions. 'lhe average of 'the sr87/srSG from HaltJa1i Island ratios of four basalt.s is O.7069;t 0.0003 whereas the basalt from the neighbo~1n9 island of Maui has a value of 0.7047 i 0.001. This di£ference~ if real, can be interp~e~ed as evidence that basalt magma was derived from two separate source regions having different Rblsr ratios. If the .con.... centration of alkali elements decreases with depth in the source regions, differences of srB7/sr86 ratios of basalts could also be 1nterp:certed in ~erms of depth of the" source material. A comparison of the calculated. values of ~he Rb/sr ra~ios of the source regions of basalt magma with the Rb/Sr ra1:1os of different types of igneous rocks may serve to identify tl1.enature of the source matexial. tiona~f Concentra- rubidium and strontium in different types of igneous and .eedimen~ary rocks wers compiled from the literature (Table A-4). The calculated averags Rb/Sr ra~io of the source regions of basalt magma is 0.039 and thUB lies in the range of basalt, eclogite, and pyroxenite. No reliable data exist for per1dot1~es •. ~ither nor tile achondrites materials which have been analyzed from which to derive basalt magma. evidence preeen~ed here the conclusion chondrites are suitable Based on the is justified that ~he source material of basalt magma has an average Rb/Sr -40- ratio which iUOSt closely rss2rnbles those observed in ult~a- basic igneou.s rocks and eclogite. The analytical results and tlleig interpre~ation sh~Jn in Figure A-i. the coordina~es ao years. Differences The initial = point are is assumed to have to == 4.5S:t 0.07 x 109 O.700~:t 0.002, in the 8:&-87/sr86 ratios of basalts f~om different localities are a~tributed to corresponding .differences in the .Rb/Sr ratios of ~a basalt magma. The.basalt from Iceland J:l1ghes'tvalue of the development source regions of sr87/sgS6 ratio. (R4159) has the '!'hes'tront1um line of its source material,'therefore give an upper limit of the sr87/Sr86 regions t;hroughout geologic for all source time •. '!1l16 limiting upper value will be used in comparisons ratios of 1n~rus1ve may wi~ initial srS7/sr86 xocks of unknown origin to determine their possible derivatton from the source reglons of basalt magma•. The slope of this strontium be dei:erm1ned independently development line be measuring the initial sr87/sr86 ratios of Precambrianbaaic intrusives. can -41- CD co'" U) ,,-' CO&.. U) 0 ,.... en 0 ,.... co ,.... U) 0 ,.... 0 0 ,.... 0 0 0 0 0 ,... ~ ,." Q CD ~ en Cl)- ,." V lO rt) C\I 0 ,.... 0 ,.... 0 ,.... 0 d 0 0 0 I': 0 I': 0 0 0 ,.... 0 V ~-=lt) N ~ <X f/) <X m LL 0 en z 0 ~ UJ 0:: UJ 0 0:: ::::> en 0 f/) 0:: C\I<X UJ UJ >- :I: lLL C\I (,!)O: WW 0::-;» o +1 en 0(1) .. 3t =~W <tZ LlJ =<t l=-Z <t <t lL ..J Ort) 25 -:ll: -=LlJ Z 0:: Z a. rt)_(,!) (I) 0 ~<t I- 0_ 0 <to. ~o::LlJ ~ ~ --(I) Z 0:: <t LlJ ~ 2 v-c cl o LlJ .. LlJ~>CD --Z ..J I_' (I) Z laJ - <t 0 ~ ~ ccj 1-==~owO::c LlJ ZZ(I) z_-:;I-o: z -rt)o....J <t<t<t <t ~(I)o Z 0 ~ ..J..J CD(I) 0 (I) ..J(I)o(l)3t Z (I)(I)Z --<t (I) 0 z<t Z tiwZLlJO~~ -0 laJ 10:~>..J<t~ ~ 0 <to 0.0 :::)~ 0 OOLlJ . ..J~..J 0: Z <tLlJ I-LlJOO ~ <t<t (I) -N I- <t -;»0 Z~(I) 4 ~<t2~>-(I)o t! -Nrt) V IOCDt-COC»O .. N en 0 z 0 I- V 0 0 t: 0 Z en o 0:: o >- ->ZW !!!~ o )( o ,.... lO o V .. o CD"'i:" COen n rt) - alCD OJ'" en ~ ,,-, CO'" 0 .... LaJ :e I- ,...' cooo )( <X 0:: -I -I en UJ :I: I- THE tSOTOPIC CO~~OSXTION OF STRO~U~ IN Tim CONTXNE~~~ CRUSTo The objective is to show that the average sr87/sr86 ratio of rocks in the upper regions of the continental crust is measurably higher than that of the source regions of basalt magma. To date only a small number o~ isotopic analyses of strontium in crustal rocks have been reported. Gast (1960a) analyzed ~en granitic roclts ranging in age from 200 to more ~han 2650 mdllion years and found ratios from 0.720 ~o 1.003, averaging 0.830. sr87/sr86 Schreiner (1958) reported va~ues ranging from 0.859 to 1.811 for . the red granite from the Busbveld Compls)t. On t:he other hand paleozoic limestones are known to have sr87/sr86 ratios close to 0.712 as does ocean water (Gast, 1960a). It: is clear- that existing the composition of crustal data are not adequate 'to define strontium but serve merely to demonstrate the extreme variations which exist. range .of values 1s large because the crust The is composed of _a great. variet.y of rock types of diffexosnt ages whose Rb/Sr ratios vary th%ough several orders of magnitude. In spit.e of this inhomogeneit.ymeaningful average values can probably be obtained by analyzing the strontium in those geological environments which a~e composed of or contain weathering products of a variety of crustal rocks. Such environments are geosynclinal sediments, metamorphic rocks, parsgneisses, glacial till, river water and ocean water. Isotopic analyses of s~ro~t1um in such rock types are not ~'et available. It is therefore necessary to estimate the sr87/sr86 r~tio in the crust on the basis of ~he average Rb/Sr and probable crust.. age of the continental Such an estimate will sho'~ that it is reasonable to expect that the average s~S7/srS6ratio of a mass of sialic rocks, which mdgh~ becama granitized during an orogeny, is significantly higher than that of ~he source regions of basalt magma. The actual value of the initial ratio of an igneous rock of crustal derivation of course, on the geological history of ~e ~e depends, material. average sze7/sr~6 ~atio of a,volume of geosynclinal sediments, for 1ns~ance,w111 depend on ~e proPortions of volcanic rocks and limestone on the one hand and de~rital sediments derived from wea~ber1n9 of ancient continental rocks on ~e other. An extensive survey of the geochamical was undertaken literature in order to obtain the best estimate of -44- -the RbISr ra'tios in different continental crust. of a precision types of rocks and in the Analyses were selected on the basis error of better than t:. 15%. The analyses were weighted in the average for each rock type by the total number of spec~menB rep~esented by each analysis (Table A-4). Table A-4 Concentrations of Rubidium and Strontium in Igneous and Sedimentary 2ockso Rock Type Granite Rb~~ 196 «290) 1,4,5,6,7,8 Granodiorite Syenite Diorite andesite Gabbro basalt Olivine basalt Eclogite Anorthosite pyroxenite Chondrites Shale Limestone Sandstone Deep sea clay 1,2,3,7,9 122 (9) 4,8" 440 (85) 136 (14) 4,5,10,11 88 (21) 4,5,8 32 (331) 1,4,5, 7,11,12,13 18 (11) 7,11 6.9 (1) 11 0.6 (1) 11 0.5 (1) 11 3~17 (12) 156 (2) 10,11 0.. 87 500 (13) 0.18 0.07 13,15,16"; 17,18 Achondri'tes Sr 'Ppm 197" '(245) 0.37 (6) 13,15,17,18 149 (29) 3;5,19 5 (7) 5 60 (4) 5 110 (8) 5 3 3 440 (612) 0.28 2,3,7,9,.11 440 (11) 7,11 149 (1) 11 280 (2) 11,13 49 (1) 11 10.6 (2) 16,17,18 92.'1 (1) 17 300 (69) 3 610 (160) 3,19 20 ( ) 3 720 . (98) 0.04 OoOS 0.002 0.01 0.30 0.004 0.50 0.008 3 0.15 3,20,21,22 ooze 10 (6) 5 Number of specimens .. Globdgerina ( I) 2 Index to authors. 800 (3) 22 0.012 -45= 120 Fairbairn at al., 1953 13G Cabell and Smales, 1957 14. Pinson et al., 1953 1. 2. 3. 4. Ahrens e~ al., 1952 Turekian, 1955 Turek1an and Kulp, 1956 Taylor et al., 1956 S. HOrstman, 1957 6. Sazhina, 1958 7. Gast, 1960a 8. 15. Gaat, 1960b and lQ1i te:nrov, 1958 Dem!n 9. Katchenkov & I'legontova, 1958 10. Wager and Mi~chell,1953 c. 16, Herzog and Pinson. 1956 17. Schumacher, 1956 18. Webs~er et al., 1957 19. Heide and Christ. 1953 20 ..Clarke, 1924 21. Hevesy et al., 1934 22. Goldberg et: al., 1958 11. This work Values from Table A-4 ware used to es~imate ~e con- centrat10ns of rubidium and strontium in the crust. as defined in PoldervaartOs model (Poldervaart:, results 1955). The are shownin Table A-5. Table A-S Concentrations of Rubidium and Strontium in the Crust as Defined by Poldervaart,°s Mod~l. Geoloqical Environment To~al crust above Moho 74 441 Rb/Sr 0.17 Continental crust only 87 442 0.20 Igneous rocks in continental shields 90 443 0.20 Sediments on continental shields 88 229 0.39 sediments in yeung folded belts Sediments in sUb-oceanic regions Deep seased1ments 100 352 0.28 114 388 0.29 5S 660 0.08 It]) J)"Pm Sr ppm The Rb/Sr ratio of the continental crus~ is estimated to be 0.20. This includes a laye7; of basalt: which is assumed to exist at the bottom of the continental Because we are primarily masses. concerned with ~be sialic portion of the continental is arbitrarily crust an average Rb/Sr ratio of 0.25 accepted as moat ~epresentative region .. This value is approximately for that 'that of gra.nodiorites shown in Table A-4• .The sr87/sr86 ratio of a hypothetical geosynclinal assemblage can be eat1ma~ed on ~e sented in Tables.A.;.4 and A-S. volcanic and sedimentary basis of the data pre- Such an estimate recks of PoldervaartDs for the young foided belt is shown in '1'ableA-6. Table ~-6 the Est1mat~ of sr87/sr86 Ratio in a Hypothetical Geosynclinal Assemblage. Sr x wt:." Roclt 'lyJ:)e Wt:." Sr D.Pm 100 ar87/sra6 S%% 44~3 .,. 0.733(1) 20 156 3 610 134 38.1 i 500 .25 Andesi'te 6. 500 Rhyolite. 2 197 Shale 52 13 .22 Graywacke Sandstone Limes~one 300 S~ 100 0.324 0.733 0.712 . 0.271 30 7.1 8.5 0.708 0.708 0.050 0.060 4 1.1 0.708 0.008 0.9 352ppm Total x 87/86 . 0.007 0.720 (1) Pure shale and sandstone derived from weathering 'of granodiorite 2 billion years in (sr87/sr86)oz 0.70S. The average ~r87/sr86 0.720 and 11:8 ftb/sr 8g8, Rb/Sr ratio for this assemblage ratio 1s 0.28. If ~a = 0.29, migh1: be average age of the continen~al crus~ is 2 billlo~ years and 1~s initial sr87/sr:86 rat:10at ~at time was 0.704, it would now have -47= an aver~ge sr87/sr86 ratio of 0.725 assumdng its Rb/sr ratio 0.25 as auggest;ed above. ~neee estimates ~~re confi~ad by tho isotopic analyses of strontium in two composites of Paleozoic shale. One of these ccmpos1ts8 is from 'the Appalachian l?rovince, the other one is The average from the west coa~t (Table A-7). srB7/sr86 zoatio 1s 0.7215 which is very close 1:0 the value predicted for ~ geosynclinal Table A-7 assemblage. These shale composites Isotopic Composition of Strontium in Ccmpos!~es of Paleozoic Shale. Sample Ro. of 87/86 Number Locality -R4184 East coast: 0.7204 0.7204 0.1194 0.0064 44 R4185 tlest Coast 0.7232 0.7226 0.1192 0.0066 90 0.7218 0.7215 0.1193 0.0065 Average shale (S7/86~orX' 86/88 84/88 contain calcite and probably same volcanic material. scans. They therefore resemble 'more closely the geosynclinal assemblage than a "pure" shale. Based ontbe evidence presented above ~be assumption is just1f1ed<tha~ the RbISr ratio of the upper regions of the co~1:1nen~al crue't 1s approximately sr8'/sr86 is of the ox:aer of 0.25 and that: the O.72S:tO.OOS. For age ae'tem1nation purposes by the Rb-Sr method it -49is customary to assume an initial sr87/sr86 ratio of 0.712 0 This value is basad on analyses of the isotopic composition of strontium in sea water and SrCO) reagents. meaningful It gives ages only when the ~aterial is strongly enriched in radiogenic sr87 so that the e~act value of tbe initial abundance of sr87 1s not importan~. An average crustal sr87/srB6 ratio of 00712 implies tl1at the continental cruet is composed predominantly young acid volcanic rocks. found 'that • 0 e. of basalt or of relatively Green and POldervaart (1958, p.l0S) 'the geochemistry of Na and K 1s incompat11"Jle with a theory of evolution of continents by weathering and sedimentation of basaltic source rocks. On the one hand there 1s too much sediment, on the o1:her there .is net enough Na and 1( in sea-water". Moreover, the age of the .con~1nen~al masses is known to be of the order of 2 billion years or more 0 An average sr87/sr86 ratio of O. 712 therefore does not seem compatible with 'the chemical compositon of the sialic crust and1ts abundance of sr87 in causes 1. probable 8geo The apparent low sea water maybe the resuli: of several & Recent and Tertiary volcanism in ~e circum-ocean1c belts and in the oceans makes -a major contribution to the total amount of strontium in solution 1n the oceans. This volcanic st.rontium was developed in 'the Rb-poor environment of the source regions of basalt magma. 2. Orogenic belts in which young volcanic rocks predom- inate fo~ the major topographic features actively undergoing arosion. An impor~ant groportion of sedlm9n~s introduced into the oceans therefore has low sr87/sr86 rat1oso . 3. 4. ~mestones go rapidly in~o solution thus recycling oceanic at%ontiwuo Rubidium is concantrated in clay minerals and micas of potassium. These minerals are by replacement deposited in sed1menta~y baains and may prevent accumulated radiogenic srB' from en~ering ~e oceans. All evidence ar~'\a11ableat the praaen't time favours the conclusion ~at rubidium is concentrated in the sialic upper regions of the continental crust and that the average value of i~e Rb/Sr ratio is sufficiently high a measurable enrichment of the sialic particillaJ: value of 'the.average , crust to in result in 5r87• 1'he sr87/sr86 ratio of a volume of crustal rocks depends on 1~s geologic histo~ and the Bb/sr ratios :1s reasonable of its component.rock 'types. t:o expect 1:ha~ t.he! init.ial 11: r:atio .of an 19neou~ rock of crustal deJ."iva~ion is higher than 'that of a rock which originated in the source regions of basalt magma. GeoloQ1cal Applications. 'the objectives geological materials of the st:udy of sr87/Sr86 ra.tios in can be sta~ed as follows. ~501. To (~~t1nguish'i9neous rocks formed by re-melt1ng croat from 'those originating from the source zeg10ns of basalt magma' in the upper mantle OIl:' 'the base of the crust. of the sialic 2. To relate diverse products of magmatic d1fferen~1a- 'tlon to a comm.on source. 3. To obtain information abou1:the chemcsl compos! t10n of ~e source regions of basalt magma. The use of the initial sr81/sr86 ratio of igneous rocks as a criterion to determine 'the origin ma~e&"1al is 111us~J:ated in Figure A-2. development line for ~e :1.8 drawn The strontium source regions of basalt magma from the initial point., based on met:eor1t1c .data, to the upper limit of, variation of the ra1:10 in basalts. fore represents there- the maximum value of the sr87/sr86 ratio It is implied that regions sr67/sr86 This line, 1f drawn correctly, in source reglons of basalt Ume. of the magma throughout the has remained constant tion may need modification RbIs!:' ratio with t~. geologic of ~e source This assump- but serves as a firs~ approximation. The most representat1 ve value of the szS7/8",86 ratio of the sialic crust at ~he pr,sent time is assumed , , 'to be O.72S:t 0.005 with an average' Rb/Sr ratio as indicat.ed by 'the slope of the dashed line in of 0.25 .. -5I- t-an eno C\ltt) 00 00 ::>0 0:. 00 U) COL- en ............. coL- en 0 ~ I'0 ~an do +- ....JC\I «I'- 0 C\I I'- 0 eno d 0 tt) to- ~ + I Oct CD -en 0 I':ct I'om d 0 0 t- d U) a:: ct I&J >- C\I lL o LLI t- U) o: o z z o o ....J ....J :I: o ct m tt) z FiguzosA-2. If tbe in! 1;1al 5r87/6r86 ratio of an intrusi va on or below the strontilhu or volcanic igneous rock falls development line of the source regions of basalt magma, a sub-crustal origin is indicated for 'that rock. On the other hand, if the initial ratio j~~s significantly above 'the s::87/sr86 ratio of 'the sub-c:ruetalsource cZ'Ust:alorigin is implied. regions a The actual value. of the lnl~1al 'sr87/sr86 ra~ioof such a rock will be a measure of ~e amoun~of young volcanic rocks of sUb-crustal ozigin.mixed with the sialic material. In order to detexmine the initial srB7/srB6 ra~io oian igneouB rock it 1s nacessa:y to know l~s present sr87/s~86 ra.tio, it:s Rb/sr ratio and its age. .value of 'the initial tiOD of the intrusive The ratio and the t.imeof crystallizaare also .glven by ...the ..coordina'tes of 'the point of convergence of the whole-rock developmentlines.of several c~gma~1c stront:1um rock ~ypes hav- _J Lng different Rb/sr ra~1os. If ~e rock was not re-heated after its initial t~e of crystallization, the componen~ minerals may also ar87/sr86 ra1:10. at10n of a vol~ be used to de1:ermine the 1n11:1al In general the products of d1fferent1- of magma 1n1~lally all have the same .. , ' . srS7/sr:86 ratio as t:heir l,areni; magma. Thereafter fraction develops its own value of ~e accordance with its genceof particular the whole-~ock number of associated each s~87/srS6 ratio in Rb/Sr rai:10. The conver- strontium develoPment lines of a igneous .:rocks is therefore evidence for a co-magmat1c relaf:lonship. strong The cQordiDates of the point of convergence are the time of d1ffexent1at1~n of the paren~. magmaand at; 'that. 'time. the sr87/sr86 rati.o of the magma The magnitude of this value can tilen be used 'to determ1ne the origin of the magma as indicated above. This convergence 1:he differeni:iat1on , t.ast can be applied of magma under d1fferen~ physical ..J conaitions as indicated - ~1ve1gneouB rocks. . by associated volcanic and 1n~ru- It can also be used 'to de'termine the r:e la't1onsb1ps of pegmat1tee, ~ etc. to the study of lamprophy~es, ccu:bon&tit:es , 'to the igneous intrus:J.ves with which 'they may be . associated. A co-magma'tic origin be shown tha1:f~e1r strontium 1s indicated if 11; can development; lines converge to the same point as do ~bose of the associated igneous rocks. In so far as basalt magmais faDled by partial meli:ing of source material in the upper mantle, 'the abundance of sra7 in basalts ana other basic and ultrabasic igneous rocks from d1ffexent geographic localities can be used to st.udy the homogenel ~y of :the uppex: mantle ...with :respect to its RbISr ratio. rai:1os of basic intrusives crust at: different The iniUal sr87/sr86 intruded into ~e sialic times recoxd the abundanceof " of the upper mantle throughout geolog1c t1meo plot of these points va geologic ~ Prom a independent dence of the bis1:ory of differentiation sra7 evi- of tile upper mantle" can be obtained. Evidence of Sub-crustal Origin of ~e Intrusive IqneouB Recks in 'the Monterec;s1an.HillB~ Quebec The Monteregian -,hills" are a series of eight small intrusives of alkali-rich igneous rocks e~endlng from Mont:eal{ Quebec, eastward across the St. Lawrence lowlands intrusives for a distance of about SO miles. and their associated ,These dikes and sills are cam- posed of a grea~ variety of rocks ranging in compos!'tion from basic essexite ~o 1ntexmedia~e They" constitute "province collectively which is believed nepheline syen1~e8. a d1stinc~ pe~rog%apblc to have formed as a result. -55- of crystal fractionation multiple injections of a single parent magma. and of the fractiona~icn products. A detail~ escript10n of ~e in~rusivesand a complete bibliography were given by Dresser and Denis, (1944, p. '455) •. These igneous rocks have intruded Precambrian crystalline rocks of ~e leas,t 1000 million foming Grenville Province which are at. years in age. Muchof the material the metamorphic rocks of 'this reglon probably came. from older Precambrian areas to 'the North, with ages of 1700 'to 2600 million years. ratio The s~87/sr86 in 'the host rocks is theref(ore undoubtedly well up in the range of values for ancient continental crus~al , reg10ns. The age of the intrusives is 122:t9m1l11ol1 years -according 'to It-Ar age de te m1nat:ion 'on biotl~e f:rom Brame Mount:a1n (Lowdon, 1960, ~.38)., This confizmed by the Geochronology Laboratory (Ann. Progress age has been a1: M.I.T. Repor1:, 1960, p. 283). Iso~oplc composi~1ons of stront1um in four specimens of igneous rocks from the Monteregian hills and in one sample of calc! 1:e from a ~arbona1:1 'te at Olea were . -56dete~ned. Concentrations of rUbidium and strontium were measured in dilution. two of the igneous rocks by isotope The experimental data are sbown in '1'able~-8. Discussion of EXPerimental RssultsA The average value of ~he corrected of the essexite, sr87/sr86 ratio the yamaskite and the 'tinguate is 0.7049. 'Ibis value is cons1dexablylower than tilat for 'the nordmark!1:0 which is o. 7156. However, when the st:ront1um developmen1:lines, with their error envelopes are plO~1:ed for 'the nordmarkite ana 'the yamas1d.1:a,an inursection ob'tained at t = 115:t 25 million years and O.7047:t 0.001 (Figure A-3). s'r87/sr86 is • Thie time is identical within. experlmen'tal error to the age of the biotite from Brame Moun~ain whlch was de1:exm1nedby ~e It-Ax" me'tbod. 'lhe 'time at which the s~ront1um development lines intersect is the z:efore,also the time a~ which 'the different rock types were separated from t:he parent magmaand :from each other ane) began to accumulate radiogenic w1ti\ their different Rb/sr ratios. s~ront1um in accordance 'fh1s is strong evidence that the nordmal'1citeis a magmatic differentiate of ~e same parent magma which gave rise to the yamaskite and ~e -57- &nro i!1~ I: m ::s " ~:.I: It) a.n to 0 c • auu Ul i .... H 0 l' ...en 0'\ I'Q ~ ...0'" \Otn N..t .-f0 ....0 CD","" 0 00 • • • ~ • +1 ........ 0\ • r- cno .... c-I C tW \0 \0 0 0 \00 00 Q~ +1 \Den 00 1'1"- • • \Dr- CD ~ ~ 'Cf\D 00 0 I' 0 I"'- • • ... r--I"'- • • +t • • 0 ....0 CD pf .. c-f • • • -4-1. 0 of'4 "l' • • • ., 0 \0 '0 \0 00 1"0 "'0 an .... \DO r-o • • • • +1 1""00'" I' ... r- PI 00 f'O .tI) ...... • • • CD ~ :t .... N 1.; Q ~ .8 ~ &.5 -~ o . C G) tIl ., .. .g"0 ...... :i~i CDsl fa U) CD • PI ~O'. .. ~co is nJ tel SO\r~~ ~ .... ~~ ~ ~ 0 \D ... ... ... ~ ~ ~ ... B (\e 8! .• \Opt ~ CJ 'tJ \0 ... PI P4 0 0 c-t u...., t~ o~ - c >e 'i\f~ ~O\ \S) • 0 .... Pl =" Q) . ..,a 'PI tJ tJ .8 ~ • (i)(J O. N -.n to N a an +1 co CD J.o (/) ~ ~ UJ z~ !~ CD • ~ co m ~:a ~~ ~ >e .... !~ .a(I), -i "' '2at GJ '94 u (I) iii +I,Q ... U tJ t:Jl ~~ ~~ ~ .2,.. 'NO') ~ cn~ GJ n3 co,c:: ~ G) ':3 k N&n 1-1 CD t~~ U3S" Q) ~ CD • • ~ 00 r--o 00 • • i-, UlCQ ~~ • cI 0 It) .... ~ 00 0 l"- ~ I' \.0 0 1'0'\ \.0\0 00 QO • • an ~ ~Q (0(1\ Ol .... ... pf ""r-o ~ N --= U) ~ 0 0\ • • &n~ men ..... " ... CD CO 18 O~ CD \0 0 ...." k 0 u ...tm ..0 pof fJ)~ ~.... +, -t I ~ r-- 'tS &n 0'\ .c r: Ulot ..., • QJ CIl~ en 0 0 ~ .oJ .... CD a) \0 fl}tQ ~~. ~ ~ ~B .. ~i in ... 18 ~. ~"u o \.0 CD ~ ~1j tJ\ ~ +1. \D \0. ~ CIlOl ~14 .... -}l c. ~il C 2.&JtrrJfa . M(f<.' \DR 0 g~ • +1• N 0 to Q.I NO • +\ J.o ....c:: m• qt\O \00 MO 00 w.-4 .cfl:$' 8~ • -58- U) CD" (/) 1'-' CD&. U) U) CD ,.... ;;: d 0 q- ,.... 0 (\J ,.... 0 0 C\I It) U) f'I) 0 0 "&. (J) 0 "..0 ~ « \ \ \ \ 0 (/) W > (/) ::) 0: m a \ .. \ \ (/) ..J - a: 0 LLJ t- « a:: IZ LIJ .... Z ...J cs: :E a:: 1'-' W co'" (/) :J: ::J CD(/) ,, ,, lIJ a.. a.. I I I 0 I- 0 (/) CD Z ..J w. >= (!)~ , elm \ exI " ~ I \ 1 \ : \ (!) ~ LaJ ~ 0 \ ...I Z --~----=-- - N t- 0 v W 0:: ::l 0 ~ +1 '\ « 0 0 :E I- W :J: I.L rt) I I \ 0 ::i U)~ 0 '\I, « l1J LL \ W :J: :J: t- Z - ex :E ex > ,, , >- CD (J) \ ::) 0 ~ \ l1J (!) , \ t- ..J Z z .... \ W W W \ (/) a:: 0::: LL rock types. other .. The only way in which the nordmarki te could develop a sr87/sr86 ~a~io of 0.7156 in 115 million years is to have had an initial ratio of 0.7047. the n~rdma:r:kite magmabeen contaminated Had at the. time of its intrusion with old crustal material having a sr87/sr86 ratio much greater.1:ban 0.7047. intersection with the yamasklte It could not give an at a time which is known t;o be the age of these rocks from other evidencee The sr87/sr86 ratio of tbe parent magma was 1:. 0.001. Th~.s 1s sugpgis1ngly low when compared to tbe average sx87/sr86 gat10 fo: oceanic basalts which is 0.7078 + O.0~2 abundance = 0.7047 and co~tlnental (Table A-3) •. The low ~O.003 of the srB7/@r86 ratio indicates that the source material from which the magma was formed had a very low RbISr J:at:io, even lower tilan the source material for average basalts. The Rb/sr ratio of the source mater- 1al 1s estimated by assumingan age of 'the earth of 4.5 billion-years and an initial Sr87/sr86 ra1:10 of 0.7004 ~ 0.002 (Gast, 1960a.). ~e this way is 0.022:t 0.011. only an ultrabas1c RbISr ratio calculated in This value 1s so low tha~ rock could ~a11fy as a source for the primary 'Monteregian hill magma. If the content of alkalis decreases downwar4 in the upper mantle, this value would sU9ges~ tbat the source region for the magma was at considerable depth in t.he upper mantle. The isotopic composition of strontium in calcite from Nb ore from a carbonat1~e concentrated at Oka, Quebec, was determined for comparison with the strontium1n the tgneous rocks of the Monteregian hills. The average of two independent 8r87/sr86 = 0-.7062'.:t 0,.0005. thecarbonatite dei:enrdna't:Lons gave The srB7/sr86 ratio is therefore notmeasu:cably from that: in the basic igneous rocks. not inconsistent with ~e conclusion different The results or1g1n rocks. This conclusion 1s si:rengthened by compar1son &\ Sr87/81:86 ratio of (1960a) for Grenville are that the calcite of the carbonatlt;e at Oka 1s of sub-crustal and is related 'to the igneous of O. 709:t:O. 002 reported limestone. with by Gast An impure Grenville 11~8~one containing a small amount of mica would be expected to have an even higher ratio. Gast also gave a value of O.713:tO.003 for an Ordov1c1an Pel:ry County, Texas. l1mes~one from -61Conclusions. The results of isotopic 2S basalts fram oceanic analyses of strontium in and cont1nen~al localities 1ndi- cate that the source regions of basalt magma in the upper mantle are homogeneous within narrow limits with respect to the sr87/sr86 ratio. ratios in basalts ~e. average value of this ratio is consisten~ with the conclusion the source regions were initially homogeneous that with .res- pect 'to the abundanceof Sr87•. The differences in the sr87/sr86 ratios observed in Recent and Tertiary ba~alts from different geographic have developed in response variations localities of the Rb/Sr ratio in the upper mantle sr87/sr86 ratio since Assumingthat the was O.7004:t 0.002 and t :: 4 5 0 x 10.'9 years, t:he Rb/Sr ratio of the source .regions of .basal't magmais calculated to be 0.039+ 0.012. . -0.017 value is in. the range of the RblSxrat10s observed eclogit.e, olivine basalts rocks. to ~o lateral and/or vertical the time the mantle solidified. in! t1al are believed This in and cexta1n ultrabas1c igneous . The Rb/Sr ratio of the sialic po~t1on of the cont1nental crust is estimated to be.O.2S ical evidence presently available. cantlyhigher from the best geoehemThis value is signifi- than that indicated. for the source .regions of basalt magma. Ii: is t:herefore expected that the average sr87/sr86 ratio of the sialic crust is measurably higher than that of the. sub-crustal Reasonable estima~es sources of basalt magma. of the age of the sialic crust and its initial sr87/sr86 ratio suggest a p%cbable present .average value of 0.725:t0.005 Based on this evidence for this ratio. 'the hypothesis 1s advanced that the sr87/sr86 ratios of intrusive and volcanic igneous rocks in the sialic crust at the ~1me of their crystallization, andgranitized can be used to dis~inguish sia11cmaterial from differentiation ducts of basal~ic magmaof sub-crustal applic~i11~y of the .initial re-melted origin. pro- 'I'he of this ~est depends on the actual value ratio of an igneous rock of possible sialic derivation and on the degree of certainty w1~h which it can be distinguished from the sr87/sr86 ratio of the sub-crustal source regions of basalt. The strontium'development lines of co-magmatie igneous rocks which are fractionation products of a -63- common parent magma converge back\'1ardsin time. The to a point when plotted coordinates. of 'this point: are ~he time of differentiation and the the parent magmaat: that time. relationships between lines. It: 1s suggested that the suites of diverse differentiation products can be de~e~ned vergence sr87/sr86 ratio of by testing for possible con- of their whole-rock strontium developmen~ Thls methodmay also be. used to determine the relationships of pegmat1tes, phyres with associated carbonatites intrusive The isotopic camposlt1ons intrusive igneous.rocks and lampro- igneous rocks. of strontium from the Monteregian in several hills, .. .~uebec, products indicate ti1at they.are of a commo)lpa~ent nated at considerable the dlfferentiat:1.on magma which prObably origi- depth below the base of the crust. The abundance of sr87 in a sample of calc1~e from ~e carbonatite at Oka, Quebec, is not inconsistent the conclusion that it is directly igneous rocks in 'the Mon~re91an w1th related to tbe hills. Acknowled~nts. It is a pleasure to ackn~~ledge our deb~ to Professors w. H. Pinson and H. W. Fairbairn who in many ways assisted in the chemical processing of the samples and the mass spec~romet:ry. G. Faure received financial assistance from Imperial 011 L1m1t:edthrough an Impe%lal 011 Graduate Research Fellowship. The support. of the Division of Research of the Atomic Ene~9Y C~ssion.1s acknottlledged. gratefully References. Ahrens, L.P., W.R. Pinson, and M.M. Rea~ns, Association of rubidium and potassium and ~he1r abundance in commonigneous rocks and meteorites, Geochim. e~ Cosmoch1m. Acta, 2, 229-242, 1952. Aldrich, L.T., L.P. Herzog, J.B. Dealt, and G.L. Davia, Variations in s~ron~1um isotope abundances in minerals, part I, mass spectrometric analysis of mineral sources of strontium, Trans. ~.m. Geophys. Union, 34, 457-460, 1953. Cabell, J.J. and A.A. Smales, The detexm1nat1on ° of rubi- dium and caesium in rocks, minerals and meteorites ~y neutron activation analysis, The Analyst, 82, 390-460, 1957., Clarke, P.W. and R.S. Wasbington, The composition of the ear~ho8 crust, U.S. Geol. Survey, Prof. Paper 127, 1924. Demin, A.M. and D.N. 1<h1'tarov, Geochemistry of X, Rb, and Tl applied to problems ofpe~rology, °Geokhimiya, 6, .570-581, 1958. Dresser, J.J\. and 'r.C. Denis, Geology of Quebec, Geologi- cal Report 20, vol. of Mines, Province PairbairD, 2, Descriptive Geology, Deparb!ent of Quebec:, 1944. H.W., L.H. Ahrens and L.G. Gorfinkle, element content of Ontario diabase, Cosmoc:b1m. Acta, 3, 34-46, 1953. MinoX' Geocblm. at Fauxe, ,Gunter, The sr87/sr86 ratio in coni:lnental and oceanic basalts and the origin of igneous rocks, Ph.D. thesis, Dept. of Geology and Geophysics, M.I.T., 1961. Plynn, K.F. and L.E. Glendenin, Half-life and beta spectrum of Rb87, Phys. Rev., 116, 744-748, 1959. Gast, P.W.,,, Limitat.ions mantle, J.Geophys. on the composl1:1onof the upper Research, 65, 1287-1297, 1960a. -66- Gast, P.w.o Alkali ma'tals in stone meteorites, et Cosmoch1m. Acta, 19, 1-5,. 19'60b. Geochim. Goldberg, BoD. and G.O.S. Arrhenius, Chemistry of Pac:1.fic pelagic sediments, Gsocbim. et Cosmochim. Acta, 13, 153, 1958.. Green, Jack and AJ:1e Poldervaar~, trends, ~och1m. Petrochemical fields and et'Cosmochim. Acta, 13, 87-122, 1958. Heide, F. and W. Christ, ZUr Geochemie des Stront1ums und Bariums, Chemie der Brde, 16, 327-330, 1953. Herzog, L.P., L.er. Aldrich, W.K. Holyk, P.S. Whiting and L.R. Ahrens, Variations. in strontium isoi:o~ abundances in minerals, paX'~II, J:.ad1ogen1~8r87 in biotite, feldspar and celestite, Trans. Am. Geopbys. Union, 34, 461-470, 1953. Herzog,L.f'. and W.H. Pinson, RbISr age, elemental and isotopic abundance studies of stony meteorites. Am. J. Sc~., 254, 555-566, 1956. Herzog, ~.P., W.R. Pinson and R.P. Coxmier, Sed1men~ age detem1na~1on by Bb/Sr analysis. of glauconite, Bull. Am. Assoc. Petroleum. Geolog1s~8, 42, 717-733, 1958. .. " Hevesy, G.V. and K. Wt1:r:st.lin, Uber die Stxon~lums, z. Anorg, Holmes, A., The origin 69, p.S50, 1932. Hauflgke1t des Allgem. ahem., 216, p.312,1934. of igneous rocks, Gaol. Magazine, Horstman, B.L., The distribution of lithium, rubidium, and caesium in igneous and sed1mentar:y rocks, Geochim.et Cosmochlm. Acta, 12, 1-28, 19S7~ Katchenkov, S.M. and' B.I. P1egontova, Minor elements in the basement rocks of eastern part of the Russian plat.form, Geochemistxy,2, p. 224, 1958. Lowdon, J.A .. (ed.), Age detexminaUons by the Geological survey of Canada, Report 1, Isotopic Ages, Paper 60-17, 1960. PAGES (S) MISSING FROM ORIGINAL webs'ter, KoR., J.W. Morgan and D.A. Smalea, Some recent Itazowellanalytical work on geochronology, Trans. Am. Geophys. Union, 38, 543-545, 1957. Wager, L.R. and R.L. Mitchell, Trace elements in a suite of Hawaiian lavas, C-eochim.et Cosmochim.Acta, 3, 217-233, 1953. -69- Part II -70- Chapter 1 RATIO IN ROCKS AND THEIR GEOLOGIC SIGNIFICANCE • ..... VARIATIONS OF THE SR87/SR86 Introduction. The element strontium has four stable isotopes, sraa, Sr97 , sraG, and sr84• namely: One of these, sr:87, is the daugh'ter product of the radioactive of Rb87 and its 'abundancehas 'therefore increased throughThis :Lncrease 1n the abundance of Sr87 out geologic: time. 1s conveniently decay expressed in terms of the As will be shown later, the ratio sr87/sr86 ratio. sr87/sr86 at any point in the earti1 increases in tima as a function of 'the RblSrrat10 a~ thai: point. mineral g:r:a1nsand I'OC~ This is true not only for specimens but also for large di v- 1s10n8 of the earth such as the continental mantle. > 'Both these large-scale crust and the geological environments have developed, throughout geologic 'time, characteristic sr87/sg86 ra~ios"in accordance with their Rb/sr ratios. A survey of 1:he geochemical 11i:erat:ure (Chap~er 2) suggests ~at the RbISr ratio of the continental approximately 0.25. crust is On 'the other hand, the isotopic -71- composition of strontium in basalts of sub-crustal demands a ~/sr (Chapter 6). ratios origin ratio of about 0.04 for ~he upper mantle Because of 'this difference !t is expected that developed significantly in the Rb/Sr the crust and the mantle have different ave~a9a Sr81/sr86 ratios. The act.~al value of ~h1s ratio in the crust is not , knownwith certainty age of ~e at the present time. If the average con~1nents is 2 billion years, their Rb/Sr ratio is 0.25, and the 1ni~ial sr87/sr86 ratio is 0.7004 1: .002, the present value of the Sr87/sr86 ratio should be 0.725. Chapter 3. This topic is discussed in more detail in On the other hand, .the average sr87/sr86 ratio of 25 oceanic and continen~al basaltic rocks of sUb-crustal origin is 0.7078 ,(Chapter 6). ment precis1on.of Mass spectromatric measure- the sr87/sr86 ratio can be as low as :to 0.001 for a single determination (Chapter 5) so t.ha~ the ,difference betwe;en crustal strontium and man~le- stro~Uum is clearly measurable. This difference between man~le-s~rontiumand crustal strontium has important geological implicationsl of magma formed by partial melting of ~e A body upper mantle and injected into the crus~ contains strontium which was developed in 'the low-~b mantl~ environment:. of magmatic d1ffexentiat1on of sub-crustal ~ll products magma will -72- likewise contain mantle~strontium. difference Because of 'the of the sr87/sr86 ratio in the crust, mantle- strontiwnc:en be recognized and iden'tifled as such. srS7/sr:86 ra.tio of igneous rocks at the t1meof The their c~'stal11zat1on is therefore a possible cr1terion for crustal or sub-crustal origin. Thepurpose of this study is to ascertain the exact value and the range of variation in ~he sUb-crustal magmatic done by analyzing origin continental localities. sr87/sr86 ratio source regions. the strontium sub-crustal of ~he in 2S basaltic rocks of from a la%ge number of oceanic 'rUbidium and ~trontlum In addition and concentra~1ons of were determined for a,representa- tlve number of basalts' in order to compare ra'tios of the rocks to those calculated regions. This 'was the Rb/Sr for their source Such comparisons are ~ne way in which 1nforma- tion about the. processes of magmaformation can be In add1~lon gated: several related topics were investl- The age of an eclogite, the origin and differentl" atlon of magma in the Monteregian study the variation mantle of the ' hills and an attempt to sr87/sr86 ratio of the upper 1n the course of geologic time by detexm1n1ng 'the -73- sr87/sr86 ratios in several Precambrian basic initial intrusives. The value of the half life of Rb87 used throughout is .T := 47I 1 x 109.years 'this thesis (Flynn and Glendenin, 1959) • Derivation of ~he Strontium ~velopment'Equation. Russel and Allan equations f01: isotopes. (1955) derived completely the variations ~ese general in the abundances of lead equations were adapted to the Rb87decay scheme by Gast (1960, p.1291). In 'the toll~Jin9 the a strontium developmen~ equation .1s simplified into more approximate but more easily useable fo~. The d1s1n~e9ratlon of Rb87 to Sr87 is subject ~o the law of nat.ural radioactive decays _,\t P == (1) P09 where P Po A t IQ =" -.. - number of parent a'tams at time t number of paren~ atoms at time t decay constant time. - 0 The number of radiogenic daughter atoms ""D is given by At SUbstituting appropriate chemical symbols for the numbers of daughter *sr87 (2) -i). :: P(e and parent. atoms: = Rb87(~t._l) (3) The total number of atoms of isotope sr97 in a given sample of matter is Sr87 where sr\' Therefore = sr~l + esrB7 (4) 1s the number of atoms of Sr87 at; t the total number of atoms of = O. sr87 in any 9~ven system at any .t1~1s Sr87 = sr8~ + Rb87 (jt (5) -1) .l\bile the numberof Sr87 atoms increases as a function of 'time and the nu."Bberof Rb87 atoms in the system, 'S 86 - f or-atoms remain unchanged. It 1s therefore pemiss1ble to divide both sides of equa't1on (5) sra, /sr86 . the number - (sr87/s1'86) + 0 by sreG• ~b87 /sr86(e At -1) (6) -75m Equation (6) is entirely . ) rigorous. .. imation is introduced by ~xpanding e A.+ The first approx- in a powsr series and neglecting all but the first two te~s. Ai Since e - 1+ - . 2 A..t + M..u 2~ JA t.l~ •• 3: 0 This approximation causes a maximum error of 3.4" when t 8; 4.5 billion years. the error lereduced If t:he 'third tem 1s included, to 0.07$. ttherefoX'e, ~r87 ~ _s_r8_7_ + srS6 (7) sr86g It is convenient ~o transfo~ the a~omic ratio Rb87 srst; into a Weight ratio: 87 • (Rb) Rb sr8a~.,._...... Sr '-~~ .. PhS7 "At. ~'e1gi~:t \\It. 51." x A At:ewt.RbxAS~ 6 (8) where RbS7 , A 1m isotopic abundance of R))87 at the p:esent time, expressed as a fraction of the total number of rubidium 86 sr A' = ~~cms. isotopic abundance of Sr86 in the sample of strontium for which the sr87/sr86 ratio was determined. -76- Substi'tut1ng (8) into (7) _8....,r8_7~ _ 8r86o (!1l) Ate~lt.Sr.A x Rb87 (9) At. ~~t Rb. ASr86 Srwt It If k := (10) The num{~;:ricalvalue of the factor k is calculated . from the atomic weights 'the abundances of rubidium and strontium and of Rb87 and sr86 a~ 'the present ~1meo Since the atomic wa1gb~ of strontium and the abundance of srB6 depend on the aw~unt of radiogenic sr87 present, ~e factor k is. not strictly a constant but should be evaluated for 'each sample. 1.35% when the isotopic The value of k increases composition ~~ween ~he limits sr87/sr86 For "normal" strontium by of strontium varies = 00712 to 0.850. It = 87. 710 :Ie 0.2785 - 2.8962. 85.557 x 0.0986 - The strontium encountered here does not differ greatly from "normal- strontium and 'thevalue for k = 2.8962 will be used in all calculations unless otherwise stated. The final form of the strontium developmen~ equation -77- CD .CD .. fn ' CD rt) CD~ ....... r--: (/) 0 CD I'- C\I en I'- 0 ci ~ 0 0 0 0 I'- 0 0 0 ~~. 0 ~ --~~ ":s' . 0 (/) en" :is a:: <t lLJ a: >- C\I LL 0 (/) Z 0 ...J ...J lIJ :I: .... LL 0 W U) ~ W lLI . ....- .... ~ :I: 0: ....- .. ~ , (/) 0:: 0 Z 0 ..0 0: -. CD :E 0 .... ~ LLI 0:: :I: WCD .... ~U) Q O::CD" C)I'-' Z -CD'" ~ LLCJ) ", Z lLJ :E - I- - sr67 sr86t Equation a:t ~1;'8.! 8&:86 -+ x(~) Srwt 0 A (11) t a family of straight lines which (11) describes have a common origin at t The slope of the lines 20896 = 0 when sz87/sr86 - is 91ven by 2.p\6(:~) tberefore a function of the RbISr ratio. straight lines is.shown in Figure 1.1. A (sr87/srcS6)o. A a.nd is system of such The value of tbe in1~ial sr87/sr86 ra~io is arbitrary on such a plot but has a definite fixed value for strontium in our Bolar system. The value of the initial sx87/sr86 of the differentiation ratio at the time of the earth has been detexmined from analyses of meteorites. Direct meaauremen'tB of this ratio cannot be made because crustal rocks formed at that t:1ma-have apparently not been preserved. Before the isotopic composition meteorite can be identified of terrestrial strontium as ~e of strontium primordial in any composi~ion 11: must be shown thatl 10 Different types ,of meteori~es ~are produced initially from homogeneous pa~ent material and that ~hey have existed as closed cheroical systems since that ~1me. 2. That terrestrial stzont1um originated from the same parent material and initially had the same abundance of srB7 as the'meteorites. Patterson and his associates demon~trated that the abundances of lead isotopes in different types of fGStecrites and in the crust of the earth satisfy both conditions (patterson, Goldberg and Inghram pa't'terson, Brown, Tilton (1953), Patterson and Inghram (1955), pat~erBon, Tilton and Inghram (1956) ). The above authors of lead isotopes in different young crustal rocks fom billion (1953), (1953), Patterson (1955), Patterson showed 'that 'the abundances stony meteorites an array converging and in at t = 4.5 years to 'the primordial abundancespreserved in the tro11ite phase of 'the iron meteorite Henbury. The uranium and thorium contents low that. the abundances of this meteorite are so of the lead isotopes have remained virtually ~nchanged. By analogy \oJ1 tb lead the assumption is terrss'tJ:lal strontium composition justified -initially had 'the same isot.opic as meteorite stront1umo The va.lue of the primordial sr87/~r86 ratio can be found directly by 'that -60analyzing strontium in P:b-poozostony meteori t.ea t'\1hich have prese~ved their initial sr87/sr86 ratio, from the point of convergence be dete1~nsd O~ it can of the development lines of strontium in several different at:ony meteorites. by The first approach has been prefeX"red several invest1ga~orB who have r0ported sr87/sr86 , . values foz achondritic meteorites wl10ae ~b/Sr ratios are less than 0.01 ~able 1.1). ~able 1.1 The Primordial sr87/Sr86 Ratio Herzog and Pinson, 1956, p. 560. Schumacher, p. 211. 1\chondr1te (Sr87/srS6)0 Invsat1qator 1956, Gast, 1960~ p.1290. Average pasamonta 0 ..703 0.691 psaamonte 0.687 Pasamonte 0.7011. 0.7005 0.6995 pasamonte 0.7004, It II V-:I: 0.0005 .1 O.7015 :t 0.002 Sioux Count~ Ie O.7027!:. 0.002 Nuevo Laredo Herzog and Pinson (1956) preferred a sr87/sr86 ratio 'of 0.703 for Pasamonte. This value is definitely than tha~ quoted by Schumacher(1956). higher Gast (1960) re- independent analyses perfo~ed on t~o mass spsctromaterso His avegoage value is 0.70041: 0.0005. This result a.grees with that of Herzog and Pinson (1956) within the limits of error stated by them. therefore Schumacheros analysis is now considered to be in error. GastOs values for tIle srS1/sr85 ~at1os in Siou~t County and Nuevo Laredo are 0.7015 and 0.7027, respectively. The error ofzO.002 assigned to these analyses was not fur~her explained by Gast and is ass\tm2d to be a limit. of uncertaini:y. It is unfortunate that no attempt has as yet been made to determine the initial sr87/sr86 point of convergence ratio from the of the strontium development of several stony meteorites. lines This would give an independent check of the value obtained by ~irect analysis of Rb-poor achondrites. Forest City and Homestead are the only chon- drites for which sr87/sr86 ratios as well as Rb/sr ratios have been determined. meteorites The analytical are shown in Table 1.2. data for these tt~O Initial sr87/sr86 ratios were calcula~ed assuming that ~e age of the earth 1s 4.5 x 109 years and using a decay constan~ for Rb87 of 47 t 1 x 109 years (Flynn and Glandenin, 1959). Table 1.2 Initial srB7/sr86 Ratio in Chondritic Rb/Sr Meteorite Forest. City Me~eori~es. o. 755tO ..003~ O.30tO.03S~~ O.69B!O.007 Homestead tJ Gust" 0* Table ~~* 1960. 2.1, Chaptsz 2 Herzog and Pinson, 1956. It 1s evident. from 'the data in Table 1.2 that publisl1~d values of the sr87/srg6 ~1d the RbISr ratios in stony meteo~~ ites do not have sufficient precision ~o calculate values of the initial sx87/sr86 ratio. zeliable The beat estimate of this ratio must therefore come from direct analyses of Rb-pooX"stony meteor! ~es such as Pasamonte, Nuevo Laredo and Sioux County. GaatOs value of O.7004~O.002 for pasamonte is here accepted as the best estimate of the initial terrestrial sr87/sr86 ratio not only because it is the most precise measurement but also because it has the lO"Jesi: value of the srB7/Sr86 ratio of the three achondrites. of PaBamon~e is ~O.OOS (Schumacher, The Rb/Sr ra~io 1956) and its Sr87/sr86 ratio has therefore remained virtually unchanged wi~h t1m2. On the other hand, ~he ?~/s~ ratios of Nuevo Lazedo and Siou)t Coun~y are not' ltnot~£'afaXt is .:;he2:efoE"s possible thC1'i: the slightly higl1er s~B7/Bx86 ra~!os in these ~wo ~~~eoritea are caused by smmll inc~eases of the Sr87/S~86 ratio with time. Nevertheless. a litnii: of uncertainty of t.. 0.002 will be accepted with the measured srS7/sr86 ratio of paeamonte in order to include, at least partly, the other .two achondriteso t An initial srS7/s&86 ratio of 007004 0.002 will be used in all calculations throughout ~his thesis. As early as 1932 Holmes proposed that the differences in the ab\.\ndanceof ca41 could be used to distinguish between granites formed by refusion of granitic pa~ent material and granites which are differentiation of basaltic magma. tion, now disproved, products This suggestion was based on th0 assump~ that: K41 decays by 13- emission to ca41• In the course of the thirty years sin~e this remarkable scheme was proposed radioactive widely used for age date~nation elements have become purposes. The abundances of daughter isotopes, however, have not been applied extensively to derive the geological info~ation reformulai:e Holmea ° s idea in tenas visua- of the abundances of the radiogen1~ isotope sr87 0 ~ne objactivea in ~he study of sr87/sr86 ~atios in geological materials are conveniently listed as follcws~ 1. To distinguish igneous rocks of crus~al o~i9in from tllose of sub-crustal origino 2. To relate diverse products of mmgmat1c d1fferentia~ tion to a common parent. 3. To obtain 1nfo~atj ..on about tIle chemical composition of the magmatic source ICGgions in the upper mantlell In the following pages these three topics will be discussea in soma detail. The Oriqin of Grani~1c Igneous Rocks. the present time is O.7078-+0.002 - 0.003 (Chapter 6). On tbe other hand, there is evidence this ratio in ~e O.72S:t .005. (Chapter 3) that continental crust 1s approximately Ac:hondritic lOOt.eozites indicate that the srS7/sr86 ra~104.5 billion years ago was Oo7004io.002. These facta are c~~ined Ord0% in Figure 1.2 in to illus- trate the changes in the sr87/sr86 ratio in the crust and -85- C\J", &0 00- 0 (0 cot.. U) co'"'"" U) 0 0 v r--ci ", r--0 0 ~~~ + 1 e:! +1 co en C\J 0 C\J ..... r--- ci 0 0 &0 r--- r--0 r--- 0 \ \ e:! m ci .. U) t- ::::> CI) ct 0: 0 (,!) W :J: 1&.1 to- \ Z 0 \ Z ~ en ~ (f) e:! """ , \. t- ": 0 \ t- 0 0 ,, 0:: « uJ C\J 1&.1 I- LL Q: 0 z 0 :I: 0 (f) 0 Z ex Q W ..J t- Z 0: ::e 0 t", « « (0 .. cou) ' ..... cot.. .(f) :E .... tZ LL v LLJ :J: o: :::> 0 (!) « m W a.. a.. N W ..J ..J Z 0: W ::::> . >- It) ~ in the upper mantls thzoughou~ geologic time o ~fuan a volume of 9~anitic magma is produced as a product of magmatic d1fforent1a~ion of basalt magma of sub-crustal origin, ita initial s~87/sr86 ratio at time t will be 0.709, or less. magma is produced by fusion On the other hand, if gran1~ic O~ grani~ization of average crustal rocks, ita sr87/sr86 ratio at time approximately 0.719. t Both magmas and their c~ystalliza- t10n products will then begin to accumulate arB7 will be radiogenic in accordance with ~heir Nb/Sr rat1os~ The ~Bub- crustal~ granite would now have a sr87/sr86 ratio of 0.724 whezeas the ~crustal" granite would have a ratio of 0.734. The initial sz87/srS6 ratio at any point in geolog1c tims t of any igneous rock now residing in the crust is there£o%e a potential criterion for a crustal or sub-crustal origin of this rock. The value of the initial sr87/Sr86 ratio of an igneous rock which is the produc~ of granit!zat1on sediments depends on the nature of ~ese not necessarily lie on the developmsnt of geosynclinal sediments and dces line of average crustal stron~1um as indicated 1n Figure 1.2. geosynclinal sediments rocks or the weathering consist predomdnantly If the of volcanic products of basic igneous rocks '., -87of sub~crue~al origin, the p~oducts of gran1tization will have initial sr8'/sr86 ratios lower than the crustal average at that 1:1meo On 'the ot;her hand, the 1n1i:ial ratio could be higher i:han the average, consist;ed largely of weathering products rocks if the sediment.s of old 9i:an1~ic 0 The actual value of the initical sr87/SrB6 ratio of such an igneous rock may be used to estimate portion of sub-crustal mi:er1al 1n 'the original assemblage, particularly if the the prosedimentary sr87/Sr86 of the continental rocks a~ that: t:lme can be determined. The developmen~ line for strontium in the mantle is ~rawn t~ ~he ypper limit of variation of ~e sr87/sr86 ratio in basalts as observedin this survey. Assuming that Figura 1.2 correc'tly indicates the time-variation of 'the sr:87/sr86 rat:10 in the upper mantle, a sub-crustal origin is i.ndicated for any igneous rock whose initial sr87/sr86 rati.o at any time in geologic history falls below this'line. In order ~o detexmine the initial ratio of an igneous rock it is necessary to know its age, its Rb/Sr rat10 and i.ts present sr87/sr96 ratio. If the age is not known, the init.ial ratio ca.n be determined by obtaining a point. of 1nters~ct1on of ~he develcpme.nt lines from two or preferrahly three ..whole rock specimens Rb/Sr ratios. . Fina.lly, / of differing' 1f the rock has not been meta- m<)rphozed after its czystal11za~1on, 'the initial ratio can also be found from the point of convergence of the developmf!n~ 11ntas for t:he minexalscompos1ng The ~i11 ty to distinguish the rock. igneous 2eocksof crustal and sub-crust;al origin w~ll cont:r1bute matezoially to the sQlution of ~any geologic problems. meth,ad offers a n~ approach of granite and will b::1ng Bowen's classic to the ~roblem of the C!r:1g1n us question; ~n~ how much metamorphic?" Specifically ~he closer to the. answer of tQlrDW much gran11:e is (1948, p. 80). magmatic 'I'he dlstlnc~ion between products of granl't1za'tion and igneous rocks added - ~o ~e crust from ~hemantle in,orogenic belts will also tilrow new light: on processes of the continen~s metl:1odcan .also 'the origin be of moun'ta1n fom8tion, and many other. geotect.onic 'applied to ~e of 'the porphyry problems. The study of ore deposits& copper. deposit.s, assoc:1a~1f?n of certain types gxcwth of ore deposi .'the pos~1ble 1:8 ~1th J.gneous ~nuusives of crus:tal or sub-crustal origin, and ~be possible volcanic, sub-crustal or1gin of confomable, sedimentary PAGES (8) MISSING FROM ORIGINAL Cu-Pb-Zndeposits as suggested by Russel and stanton~ ~9S9~ and Ofteda~l (1958). The Differen~la'tion of Basal~ Magma. When a body of basalt magma undergoes a~ion, 'the resulting crystal fractlon~ produc'ts w1ll dlffer among each otheX' with respect to their Rb/Sr .ratios. HOwever, all fractions will 1~it:1allY ha.ve the sama'sr:87/sr86 ratio as theiX'.m~9matte source mat:er1al in the uppex mantle. each fraction will develop accordance wlth its The increase Thereaft.er 1'tsown 8r87/8r86 ratio in par~1cular RbISr ratio. w1~ time of ~~e several igneous rocks which ~uld sr87/sr86 ratio in be' d.1fferentiation produc~s of basalt magma 1s illustrated in Pigurel.3. The slopes of ~he development lines for ~e different types of plu~n1c igneous rocks or t:he1r volcanic equ1valents .were calculated ,using estimates of tlleir Rb/Sr ratios pr~senteClin C?hapter 2, Table 2.2. '!'his metilod can be used to 'test. for commonparentage of a series of r$olcanic or plutonic rocks of different ccmposl~lons. Co-magmatic igneous rocks because .tbe strontlumdevelopment fOB an "array' lines of all fractions of the primary magma converge to a point. as its This point has coordinates the ~ime when differsn~iat1on occu&xed and the sr87/sr86 ratio of the primary magma at "that tim9. If certain igneous ,rocks w1~ unusual chemical composi~ionB are foxmed by assimila~ion of crustal material into part of ~he parent magma, ~eir development converge lines will not 'to a point w1~h the other fractions. if a given association differentiation Moreover, of igneous rocks is the result of of more than one parent magma, may also be discovered. Two magmobodies this fact fonned by par- tial melting of source material having dlfferen~ Rb/Sr ratios, will l:'howa corresponding sr87/sr86 ratios. difference in their In genera~ the strontium development lines of associated volcanic or plutonic rocks, trea~ed as possible "arrays., can be used to s~udythe differentia~ion of primary magmas under different physical conditions. The method can also be applied to determine relationships ~he of lamprophyres, pegmat1tes and caZ'bonatitos to the igneous rocks wi th YR1.~1ch 'they" are assoc1a'ted. , instance, those pagmatltles products of residual by other For which are crystallization magmawill fit into 'the array formed co-magmatlc fractions of 'the paxent magma. On 'the other hartd, pegmatites fo:gned as the result of solid diffusion of ions (Ramberg, 1956) will not fit into any array. Their sr87/sr86 ratios will instead resemble that .of 'the country rock •. Carbonat1tes limestone have bsen 1nte~J:et:ed as xenoliths (Shand •.1947), as crystallization carbonatic liquid of magmatic origin and as products of hydrothe~al of products of (Pecora, 1956) solutions (Bowen, 1945). All three theories may be applicable to certain carbona1:1tea. By determ1ning in!t.tal sr87/sr86 carbonati1:e, ~he associated limestone occurrences, "& igneous ratios of t:he rocks and possible choice bet~~en the various hypothesis may be made. If a carbona'tlte can be shown to font an "array" with 'theassociated igneous rocks "in an area whe~e the s~87/sr86 ratio of the limestone is measurably different, a co-magmat:icrelationship between the carbonatite and tile igneous rocks 1s demonstrated. This 1n~erpret:at1~n is consistent. with the results obtained in Chapter 8 for 'the intrusives and 'the carbonati1:e of the Monteregian hills a1: Oka, Quebec. -93- The Composition of the Upper Mantla~ The abundance of radiogenic isotopes in rocks of known sub-crustal origin can be used to calculate o~ the parent to daughter eleme~1: in ~e .For instance, 2Sbaaalt1c according ratios upper mantle • to the average sr87/sr86 ratio in rocks ~be average Rb/Sr ratio of the source material in the upper mantle 1s 0.039 and ranges from 0.022 ~o 0.051 (Chapter 6). Slm11ar irafoxmation could be obtained for the RICa ratio as well as the .U/Pb and Th/Pb ratios of the upper mantle from measurements prlate radiogenic ~e of the abundances of the appro- isotopes. possibility of using the abundance of Ca40 in ~ Recent basalts to calcula~e ~~a ra~os mantle merits. particular OU~ attention. for the upper. In order to carry such a calculation the primord1alabundance must be known. analyzing Pasamonte. of Backus (1952) a'ttempted to do this the calcium in theachondrit1c ca40 by meteorite Al~ough he did report a slightly lower abundance than is found in calcium reagents, his measure- ment was of poor quality and 1s not conclusive. Prom measurements of the abundances of sr87 and Ca40 in basalts 1~ is at least theoretically possible ~o calcula~ Rb/$r and RICa ratios for the upper mantle. other hand, it bas been demonstrat.ed ratios are ~la't1valy On the 'that K/1U3and Ca/Sr constant for each rock type. Since the upper mantle can. be composed of only per1do~lte, eclogite or dun1te for various geophysical and geochemical reasons,.1t of the should be possible to make reasonable es~ima~es it/1Q) and ca/sr ratios for the upper mant:le. these assumptions concentrations of Ca, Sr, « and Wi th Rb could be calculated. Evidence is presen~ed in Chapter 6 'that the lavas in Hawaii may be drawn from ~wo extruded by volcanoes sou:ces differing respec:t1va sr87/sr86 in. ~ei:. in ~e slightly, but perhaps 8bund~n~ of vertical of sr87 could variatlons ratios. .s1gni.flc:ant:ly, Such difference be explained as evidence in the rubidium concentrations of the mantle. Pu'tUX"e work may show that are unfou~ed or impractical. Bome of these sugges~:1.ons '!'hey are recorded here 'to sttmulate adiscusslon of 'the many possible inte~retatlons of the variations of ~he abundance of radiogenic isotopes 1n rocks. :fHE__qOi:1CE~1~~~!;IOt:~~ 9? At-ill 9TJt0~~~IY~ Xt~ IGt~OtJS AtID SEDlti]Ei;l1TARY ROcKs ~~~~'"'--"= ~ AND I 'J Tf<m CRUS'!" ODI THE EARTfi" ~ . !ltJBJ])1.~ ' tnedJncentrationf:f sedimentary '{' '~select.ion' . ,made accord1tig 1., 2. some 250 refeX'encea rockS. 'this search. " of rubidiUm arid, strontium ," in igneous wer,;e conaulted of the available and in analyses 'Was .' 't~) the f~llowin~ criteria: Only'analyses having precis'ion errors 'better were chose'no of '1; 15% or All analyses which give no ,indication of p,recisicn calibration of the mathod were discarded. erxors~r for the concert1:rationa of rub!d'ium and s.'tront1um war~ ~alcul!lted in te;ms of parte per million ,3,. "Average values by, .weight.' The' ',analyses, we&'e we.ighted in the average :'according' to the number ,of different samples ~epresented. ' . ' , , and the values which were finally chosen are f~om analyses publis~ed mostly since about'1~50. case' ofaeep with precision s'ea' sediments and ul't:rabcus1c ' rocks, er&'ors greater for lack of better data. Occasionally, as in the anal~~es' than t: 15% had to be included ~96C3 .I~ •. ""'."'."~------------------~~-------------~- the concent~ations aedin~ntary of ~ubidium zacks is essential the abundance and strontium and to a ~eliable evaluation of sr8? in the crusto culated will be combined in igneous of The averages h~re cal- in certain propo~t1ons according to the crustal model of POltlervasrt in ordez to estimate the RbISr ratio of the continental crusto (See page 118) The sr87/sr86 ra~io of igneous rocks of sub-crustal origin can be used with certain assumptio~s to calculate Rb/Sr ratio of the magmatic source material. Comparison the of this calculated value to average Rb/Sr ratios of various types of igneous rocks eervea to identify the nature of the sub-crustal source material. The Concen~~a~1ons of R~1di~~ 6~d1~ntarv In the foll~~ing and S~rontium in Igneous Rocke~ pages, Table 2.1, the super1o~ and rubidium and st.rontium analyses for igneous rocks are tabulated. ~able includes the author~ year of publication, method, the number of samples represented, of rubidiwn and strontium in parts The the analytical the concentrations per million by weight (ppm) as well as remarks about the nature of the material analyzed. Weighted averages are calculated for each rock type. summary of the results The presen~ation A appears in Table 2.2 on page 106. of the data on sedimentary rocks is e~actly analogouse n a~ma~ and general conclusions are found on page 13:1 , Table 2 e .130 Table 2.1 Analyses of Rubj.diUfi'l and Strontium in Igneous Rocke and K9teoritea Superior Aul(:hor and Method of No. of Reference'Analys~.s Granite Ahrensat Samples-pPM 198 150 New England granite correct1on,applied. O .. Sp. 50 aoo Calibration by I.D. on G-l and tAJ-l Turekian and O.Sp Kulp,19S6,p O.Sp. 100 Grani'te Ca 0.1 - 1.0% 85 4 534 6 219 6 209 ai, 1956 .. .. " 332 HoX'stman, pp~ 29 1955b, p532 Taylor et Remarks O.Sp. ai, 1952 Turek1an, Rb, Sr 9~anite,St. ~ustell, Cornwall Granophyre Slieve Gullion porph.felsite Slieve Gullion Granite Wes~ Mourne Mountain F"Ph 66 170 granite OoSp. 90 257 granite I.D. 27 I.D. so 133 348 Composite of Finnish granite and 9rani~e gneisses. 125 283 Precambrian granitic 1951 Sazhina, 1958 Gast, 1960 • and 9~~issic rocks from Western U.S.A. Table 2.1 continued Author and l>~thod of No. of Sr ppm Rema&"lts Reference Analysis Sall!lplea Dem1n and Kh11:arov F.Ph 4 138 .. 4 164 Ccux:sa 2 mica a::3d ~crocline. g~ani~e leucoeratic 9gaYd~e 187 Caucasus, Alaski te dilte 1958, p. 124 .. 2 Caucasus Kat.chenkovtat a1a.9.Sp 1958, p.226 0 457 4 Granite, Eastern Russian Platfo~ 196 (290) weighted average for granite 197 (245) Granod1orit;~ Tayler at al, 1956 O.Sp. 7 'l'urekiat;1 and O.Sp. as granodiorite Slieve Gullion Armagh 126 440 Ca 1.0 - 5.0% KUlp,,_ 1956 Demin and P.Ph. 108 granodiorit.e Khltarov 195~, p.724 weigh'ted ave~age for 9%ancd1orl~ Por'phyro1a i22 440 (9) (85) Table 2.1 continued Author and )llefefence AS..ethcd of Ana~ys1s No 9 of Rb sampl~~. ppm w Sr p~m ~venite O.Sp • .Taylor 1 105 Syenite 8 147 Trachyta 3 110 Syen:tte, C.,Sp. 953 1 150 Z.D. 1 140 et ai, 1956 A p.225 Horstm~n', 1957 Wager and ~U.tehello p. 218 >' ' .. Faux-sand Hurley 250 "L'racbyte , (in press), Weighted average for syenite 136 156 (14)- (2) 'Andeeitea and Diorites Tayloret O.Sp. 7 12 .nndssite, New zealand Ph 9 88 TOhali~e from th~ 'al, 1956:, , p~22S Ttlrekian and I(ulp, , 1956 F f) Mal~ya,Laba massifj.::, Caucasus Horstman, 1957, p.ll F.Ph. Weighted average for'andesites and diorites 5 'diorite' 110~ lS 88 (21), 500. (13) Table 2.1 continued Author and Refe~ence l~ie'thod of i\n~tX81B Na of o Rb Sr Remarks ppm Sam2!.es Basalt.ic Rocks Ahrens at aL, 1952 OoSp. Fairbairn et f).Sp. 8 49 (40) 1953, p. 43 al., Gabbro. Co~~ection applied 41 (57) Ontario diabase Correction applied. 183 TUrek1an, 1955b, po532. a.sp. 50 500 Baealt Tureltian O.Spo 244 467 Basalt O.Sp. 13 & Kulp, 1956, po267 Taylor at al., 1 1956, 34 9 Basalt Gabbro 30t:. 18 G~bro p.225-226. Horstman0 F.Ph. 15 19570 Cabell and R.Act. 1 O.Sp. 8 Skaezgaard, fayali1;e ferrogabbiro 21.9 Smaleso 1957, p.40). Katchenkovet; 430 Gabbro - norite East.Russoplatfo~ al., 1958, p. 227. Gas~, 1960, 83 p. 1290. 83 83 • Paure, 1961, Ph.D. Thesis I.D. .. at M.I.T. Weighted average for basaltic rocks 2 1 1 26 30 32 460 469 454 Composite basalt I 35 273 Tholeei~ic basalt 7.0 8.0 346 262 Duluth Gabbro NQrite, Bushveld Complex 32 440 (331) (612) .. It It .. II III Autho&" and Refa 1:'ence l<iathcd of 1:010 u of An~YElis Saru~les Gast, I.De RemarltS 1 0.9S 1 6~45 312 Olivinebasalt Hawaii 9 21.5 Olivine basalts 102 1960, p.1290e .. C) Faure, 1961, IoD. Ph.D. Thesis, 492 Olivine basalt ~d-Atlantic Ridge from oceanic islands. M.I. '1'. Weigllted average for olivine basalt 18 (11) 440 (11) 6.9' 149 Eclocri'te Paure, 1961, I.D. 1 Ph.D. thesis, M.l.T. Eclogite xenolith from k!~.berlite pipe, Robart Vict.or Mine , S. ld!rica. pltrabasic Rocks Ahrens e~ OeSp. al., 1952. R All values corrected by factorl 0.45. 13 1 Ultrabasic rocks. M1ca augite peridotite. Murfrees- 0.9 495 boro, • Arltaneas Kimberlite, 171 S. Africa a.sp. 4 0.9 p.254 .. All values corrected by factor of 0.61. dunite, coX'rection applied Serpentine 2 1.2 Websterite and Pyroxenite Lherzolite Rlmberlita, S.Africa Pinson et al., 1953, .. 1 1 12 170 Aui:hor Refeg'~nce ~ iiO. of Sr -~~l?1&13 p~ !~ra'tbod.of and l\na]J!~F! :RIIio~'"""'T'''- Pinson et al., cont•. O.Sp. Remarks 90 1 ea' 1 F.Ph. 5 lO:t 10 Faure, :1961, I.D. Ph.D. Thesis, 1 0.•6 379 M.l'. '1'. 1 0.5 49 .lD Horstman, Mica augite peridotite Murfreesboro, Arkansas Anorthosite Split Rock, Minn. 117 Ultramafics 1957. .. Anorthosi~e Bushveld; Complex ey'roxenlt:e . 'Busbv~ld C~ple:( ~~syntbe-~.i~ ..is..att~mpted ."""-"'." ..,.~~"~-"'_ ..... _.,~...... _ _,-~..:. _._ _"--- _-~.",.~~,._.. .. ... .. ... ....... -- ~ ........... "'~.... for ultraba~ic rocks in general';'" ., AnOri:hosite O~6 .171 495 170 QO. .49 0.5 -------~_. -----.;..,_ ....--- .K1mbezo11 te , S. Africa . Mica aug.i te pe:tldoti'te .pyroxan1te ~~dr1t~ J~teori~es. Meteo.r1'te MethOd 'of Analysist. Forest. City ". " .. ... 280 I .• Do I.D. N.Act. , X.D. N.Act:• I.p•. Average Porest C~ty Analyst. Rb Sr PPJR ppm 3.5 '. 3.91' . 9«»8 ~rZ09 et al., 1956,p.558 • 11.9 Schumache~, 19'56, p. 5460 webster et a.l •• 1957, p.544" 3.04 2.90 . 10.2 3.04 . (6) ... Cabell and Smales. 1957, . p.404~ Gaa~, ..1960, p.2 • 2.75 3.19 . " 10.6 (3) Table 2.1 continued Sr Me~eorita Method of Analysis Chondr1tic Meteorites I.D. HOmestead Analyst. p~ continued. 3.6 10.6 Herzog and Pinson, 1956, p. 5S8. Cabell and Smales, p. 404. R.Act. 3$15 Average Homestea.d 3.37 10.6 (2.) (1) II Modoc, N.Act. 2,,97 I.D., 3.45 Cabell and Smales, 1957, p. 404. Gast, 1960~ P. 20 Scoi:~ Co•• Kansas. Average Modoc 19574 3.,21 (2) Ness Co. f N.Act. Cabell and Smalea, 2.90 p. 404. Kansas N.Act. 2.28 Long Island,N.Act. 2.11 Faba, Co. .. 1'1 Limexick It Phillips Co., Kansas. Beardsley I.D. 4.90t. .06 Ninlnger I.D. 4.83:1:: .06 Richardton I.D. 2.96 Holbrook I.D. 2.22:t .04 AtmM 1162 :t.D• 2.3 Gast, 1960, p. 2. •• 1349 oj; .035 n .. 19574: Table 2.1 continued l~teori te ~%thod of Analvs~s Analyst lro) 0 PI!;lD Chondr1tic ~~teo~1tes continued. 2.9 3.5 Quoted from the 11tezostura by Gast, 1960, 3 ..9 P. 2. 3.8 Average .fit\1HM (3) weighted average for 3.17 chondrit1c meteorites (12) 10.6 (2) Aehondr1t1c ~~teo~ites Pasamonlte gxey phase I.D. white phase X.D • white ~hase IoD. Average Pasamoni:e e9.5 0065 94.7 0.50 O.21(~) 0.31 grey phase I.D. bo'th combined 0065 '0048 Ruevo Laredo I.D. Oc 37 (2) Sioux co. 0.18 X.D • • tt C...ast. 1960. D. 2 94.7 89.5 92.1 (RbISr:: 0.005) Gast, 1960, p. 2. • N1nlngsr 298 I.D. 0.29 n If.ooreCounty I .Do 0.16 n Johnstown .. I.D. N.1\c1:. Average Johnstown 1956, p. 546. " phase I.D. ~ite Schumacher, 0.105 0.04 0.72 2.07 Webster et ale , 1957. P. 544 • 2.07 -105- Table 2.1 continued Meteorite Ma~hcd of Analysis Sr Analysi: ppm Achondritic Me~eorltes continued *Bluff, Faye'ttG Cabell and Smales, 1957~ P. 404. 1.01 R.Act. Co., Texas. Weighted average for achondritic meteorites 0.37 (6) 92.1°\"1 (1) eOm1~ted from average. ~~Gast, 1960, p. 1290, on the basis of previously unpublished data gives value of 74ppm. I.D. O.Sp. - Isotops D~lution Optical Spec~roscopy F.Ph. N.~ct.- Flame Photcm3try ~eutron Activation Table 2.2 suw~ary of Rubidium and.S~rontium Concentrations in Igneous Recks. Rock Rb PPiU. TvJ')e RbISr Sr p~ 197 (245) 1.00 440 (8S) 0.28 (14) 156 (2) 0.87 (21) 500 ('13) 0.18 440 (612) 0.07 (11) 0.04 Granite 196 (290) Granoa:Lorlte 122 (9) Syen1t.e 136 Diorite & Andesite .8S Gabb:t'o & Basalt 32 (331) . 011vlne'Basal~ 19 (11) 440 BClogite 6.9 (1) 149 (1) 0.05 Anorthosite 0.6 (1) 280 (2) 0.002 171 (1) 170 (1) 1.00 IUca-auglte peridotite 495 (1) 80 (1) 6.20 Pyzoxeni t.e 0.5 (1) 49 (1) 0.01 0.9 (4) 10.6 (2) 0.30' ..... -. (1) ) K1~rlite, So Africa Dunl'te --_.--"-'~.. ... ~ - 3.17 (12) ./Chondrites ! 0.31 ... (6) .. ~,___ ~chondr1 'tea Figure in brackets indicates sented in the average. 92.1 _~~"'"''''''''' __ '''.'_'_''' 0.004" ../ _._ .• _ ....... '4-~, the number of specimens repre- '/ Cl ~'j t { ~wmna&Y. The data foE' syenites and vazoious types of ultrabasic rocks are not as p~GC1Be and xepresentat1ve as those for granite and basalt. A plot of the concentrations Sr, K, and Ca in cert.ain _Figure 2.1. Syeni tee -quality -of tbe data. of Rb, types of igneous rocks appears in WSE'e om1 toted because of the poor arhe concentrations of K and Ca we~a taken from Ture1d.anand ~Jedepohl (1961) w1i:h the exception of 'the values for dio~lte and andesite which are from Poldervaart: (1955, po 134). The pattern resembles many of the variation diagrams publ1sbed Nockolds and Allen (1953, p. 105). confirmation 1s quite normal and by This 1s a qualitative that the average rubidium and strontium centrations here calculatsd are of the eor~ct con- order of magnitude and bear the proper ~elat1onship to ~he major constit.uents. The concGn~rat1ons of Rb and ~ increase un1fo~ly the more acid rock types. high in the basaltic ~ocks. in em decreases steadily from a Stron1:ium, however, increases at flrs~ and ~ssumas a maximum value in andesites and diorites. patterns Nockolds and Allen (1953) repol:ted similar for rocks from East. Central Sierra Nev!1C1a(Pig. 13d, -108- ... 0 0 (f) 0 0 0 0 '" It) 0 0 It) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 V rt) ", 10 C\I 0 0 0 0 0 0 0 0 CD 0 0 0 0 0 U) CD 10 10 10 V 0 0 0 0 0 0 .. 0 0 0 0 en 0 en co 10 0 (f) ~ 0 0 ~ Q. Q. tS 0 0 ~ 0 0 (f) 0 0 "..a .,,, 0:: .l( .. .... .... CD ", .... d ,, ...... ~/ ./ ........... ~ ",. ,....... I d / , ............ ..... ........ •, ,, / / / ;I' .... , / /• I a: C!) , \ z a: \ C!) , ,, W .... \ ~ 0 W Z \ \ \ 4 \ \ .,..' 01\ -, \ \ ~\ / 0 .... <t .... ,, , Z LLI \ \ • • z (f) 0:: ~\ , \ \ \ / 0 C/) \ \ ..0 0:: lJ.. \ \ \ / .. .. 0 0 ~ 4 (f) ,, / 0/ tS 52 ~ ,, ,, tS b z ~ Z c \ I (!) w Z ....a: <t I • 'J( / /\, I \ .... 0 ............. / \ 0\ w I / \ Z • 'y' / ILl 0 0 ~ \ / \ 0 I "" I I ~ I ........... x I ", / ....... ,, "/ V (f) ::) 0 C\I ............ ", ./ V 0 d 0 d q q, 0 ", 0:: ~ !J 4 C/) 4 m 0 z 0 0 t\I ~ ..a 0:: 0 0 0 0 0 ", C\I 0 0 0 V U) ~ ~ 0 0 0 0 0 0 0 0 0 0 0 LLI 0:: ::) (!) 0 LL 0 0 N 0 ex> 0 CD 0 0 V N 0 0 0 0 CO CD 0 V 0 N po134), the Sco~tish Caledonian (Fig... 13c,p.135), the Scottish Tertiary alkali basalts (1954,F1g.28a,p.272), the Hawaiian alkali basalt-t.rachyte sexies (1954"F1g.28b,p.272), polynesian alkali basalt-trachyte and the series (1954,Plg.28c,p.273)o On the other hand, a steady decrease of calcium as well as strontim~ as a ,function of (~81 K)-(Ca Mg) was observ~d rocks from the Southezn California Batholith p.133) and the lava series at Lassen Pe~t p.133). in (1953,F1g.13a, (1953,Fig.13b, The maximumin 'the strontium concen~ration is also lacking in the d1fferentlationproducts of thqlee1t1c basalt magma (Nockolds and Allen, 1956). The last. mentioned tion for the t\~O authors give 'the following types of behaviors accauodated in ~e Strontium can be feldf3par st.nctur8 as a replacement of calcium in plagioclase and potassium.in S~ront1um cannot, however, mineralS explana- replace calcium in ferromagneslan (Nockolds and Mitchell, crystallizes early, ~a potash feldspar. 1948). residual li~1d If plagioclase iseteadl1y depleted in strontium and the lat.er acid fractions will contain progressively lower concentrations of strontium. that ferromagnas1an 1s concentrated minerals crystallize in ~e In the case first, strontium residual liquid and thus attains a maximum value b0fozoe it is g>emoved by inco~oZ'ation inl:o the feldspars. The Rblsr ratio incr~ases smoothly from basalts to granodiorites but rises ahazoplyin granites. result of ths extreme differentiation tium in the more acid rocks. This is the of rub1d1u~ and st~on- t~ile strontium 1s deple~ed, rUbidium becomes concentrated in the gesidual magma~ic liquias. Thus acid volcanics ratios as high as 35~ (obsidians) may have Rblsr (Professor W. H. Pinson, oral commun1~ cation). Table 2.3 Superior Author and Refsrence MethOd Analyses of Rubid1um. and Strontium in . Sedimentary Rocks of Analysis No. of Saq&ples !lb Sr }:)pm TJJX[l Remarks - Shale Heide and Shale 227 O.SpCl 3 O.Sp. 35 245 69 .300 ~:r1st., 1953, pCl327 Turekian and Kulp. 1956, • Non-calc. shale r.veJ:a9~ shale p.287. Horstman, 1957.P.Ph Weighted average for shale 26 140:t50 149 (29) Sbale 300 (69) -111- Table 2.3 continued !-1ethoo of Author and Reference Analysis t~o. of S2.&~pw-lea lRb Sr p~m ppm RemaX'ke Limestone Heide ana o.sp. 5 60S Muechelkalk O.Sp. 155 610 Pure limest.one Christ, 1953. 'J.'Urekian and Kulp, 1956, p.290. HOrstman, 1957.F.Ph. 7 <5 <5 We19h~ed average for limestone Limestone 610 (7) (160) Sandstone HOrstman, 1957.F.Ph. 'l'urekianand Kulp, 1956, po 284. 4 60t 61 Sandstone < 20 O.Sp. Weigbted average for sandstone 60 (4) Estimated from Sr constant of ~Aa~~2. <20 Dee~ Sea Sedimants Clarke, 1924. Ch~An. Hevesyat X.Pluor. Sl 470 Red cla¥ compos:tta. 1 760 Red clay Challenger st..3S3 1 254 al., 1934, p.313. Horstman, 1957. q P.Ph. II 4 160 4 60 .. It 253 Composite red clay. II II Table 2.3 cont1nl:~ed Author and Refex-ence Pl~~hoo of !:Go. of S:c Analvsia S.C1mp}..es pmn litemarks Deep Sea Sediments 'con~inued Turskian and O.Sp. 2075 10 Kulp, '1956, p. 289. Goldberg et. al., 1958, Pleis~ocene sadiment from Atlant.i.c Core A 180.... 74 0 oSp. 710 35 Pecif':tc Pelagic sediments p. 170. 110 weighted aV6%aga for deep sea clay Horstman, (8) F .. Ph. 6 a.sp. 3 720 (98) 10 Globigerina oozeu 1957. Goldberg et 800, al., 1958. 0 Weignted average for 91objge~ina ooze Ocean ~ac1£ic foram. & globiger1na 00Z9 10 800 (6) (3) t"lat:e%' Heide et O.Sp. 1 13t"l Sea water al., 1953, p. 327. Smales and R.Act. "'-.Salmon, 1955. Smales and ~lebster, p. 139. 1957, I.D. 1 O.117.!:.Ol 4 o .11St.OOl r ('. Nor~h l1tlantic 49002IN,lSo19°t-;', depth 400 m. Table 2.3 con'tinuetl Autl'lO&' and Reference R<2ei;1!oo. of .Analvsis Remax:ks No~ of SamDl~s ......... . ,.:~ Ocean Water continued Pinson at al., 1957. p. 1781. 1 . I.DIt ~115 7.2 .019 8117 h~lghted avegage fog .6 Surfa.ce water: 66018 ow, 39001°N 7.2 (3) (1) sea water Qamitted from tile average. Ipotope Dilution Optical Spactroscopy Neutron Activation Flame Photomatry X-ray Fluoraac~nce I.D. O.Sp. R.Act. If.Ph. XeFluor. Ch • .nn. ~ Che~cal Analysis The ~esul~s of the compilation for sedimentary rocks a~e summarized in Table 2 4. 0 Compilations of analyses of trace elements literature such as this one have been undertaken investigators. from the by other The most recent of these are by Vinogradov (1956) and by 'rure1t1an and wedepohl (1961). Table 2.5 is a comparison of the results obtained here to those repor~ed in the othe% t~o surveys. Table 2 4 Summary of Rub!di~~ and St~ontium Concen~xations 0 in Sedimentary Rocks. Rb~pm Reck Tvpe Sr ~Qm nb/sr 0.50 Shale 149 (29) 300 (69) Limestone <5 (7) 610(160) Sandstone 60 (4) 110 (8) 720(96) 10 (6) 800 (3) 0.012 (3) 7.2 (1) 0.016 Dee~.sea clay ~lQb~96~1na ooze Sea water 0.117 Turekian and Wedepohl < < 0.008 20 ...v 3 0.15 (1961) degived most of their data for igneous and sediinentary rocks from HoX'stman (1957), Turelc1anand Rulp (1956), Gast (1960), an~ Goldberg and 1\rrhenius Their values agree w1tl1 ~ose (1958). in this s'tudy wl-th only a few exceptions. obtained The concent.ration of strontium in ultrabas1c zacks, for instance, is certainly greater than 1 ppmo ratio A value of 6 ppmwould give a Rb/Sr of 0.03 \sJb1chis about righ't for magmatic source material in the upper mantle. V1nogradovOs compilation includes results from many different sources without any selection of the more reliable analyses. His values aX'etberefore thoso of Turekian and Wedepohl O~ lass truat-worthy this autho&". t.han -115- Table 2~S Concentra~ions of Rubidi~~ and Strontium in Igneous and sedimentary Rocke. 'l'his author Rb ppm Sr ppm Rblsr Granite 196 197 Grano- 122 Turekian et Vinogradov, ale • 1961 1956 RbISr Rb Sr ppm pptn 1.00 170 100 1.7 4.9~O 0.28 110 440 0.25 136 156 0.87 110 200 0.55 Diorite &: Andesite 88 500 0.18 Basaltic 32 440 0.07 Rock Type Rb/Sr Rb Sr ppro ppm 400 300 1.2 70 800 0.09 45 440 0.10 diorite Syenite 30 465 0.065 rocks 0.2 Ultra- 1 0.2 2 27 0 ..07 5.S 20 0.28 maf1cs Chondrites 3.17 10.6 0.30 Shale 149 300 Limestone <5 610 <:,00008 Sandstone 60 < 20 Deep sea 110 720 0.15 10 800 0.012 0.111 7.2 0.016 0.50 A'3 140 300 0.47 3 610 0.005 60 20 110 180 3 0.61 clay Deep sea carbonate Sea water 10 2000 0.005 Zhe Conc~nt~~1o~s of RubiditIm and S~~on~1um in the $g-uat of ~he Ea~th ~In~roduc'tion. --- In orda~ to calculate the concentration of the~ements in the crust of the eagth estimates must be made of the p~oportions of the var~ous types of igneous and sedimentary rocks in the crust. Knowing the concentrations of ~e ele- ments in these rock types ene can then arrive at a fi9u~e fog the concentration of each element in the cruste This 1s a problem of considerable complexity and gsochemical 1nteres~. Contributions been made by many geochamdsts. approaches t:cywardsits solution have Basically ~bree different have been 'triads 1. Geochemical balance calculations. 20 Bstimates based on ~e proportion and composition of roclcB exposed at ~he surface. 3. Detailed analysis of the surface of the earth and definition of certain regions having characteristic reclc assemblages. Geochemical balance calculations are based on 'the assum- pt10n that the amount. of any eleman~ released by the weatheg-ing of igneous rocks is equal to 'the amounts incorporat.ed sediment.s and dissolved in the oceans. Calculations in of t;b1e .3 .nat:ure have been made by Goldschm1dt (193~), Correne (1948), Kuenen (1941), Wi~~an (19580 (1954) and Goldberg. and Ar~henius Each authoIl:' suggested ceztain refinements basic pre~seo Kuenen (1941) first drew attention to the importance of deep sea sediments particularly chemical balance of calcium. set limits of uncertainty Wickman deep sea sedi~nts Goldberg and their analytical zesuits for into the calculations. tion of several simultaneous igneous rocks weathered to the 980- (1954) att.empi:ed to to the results. ~~rhen1us (1956) inco~ozated be obtained. of "the From the solu- linear equations and sedimentary amounts of rocks formed can The method has the following difficultiesl 10 No allowance is mads fo~ erosion and ~acyclin9 of sediments. 2. 'l'he e1.emen'te now dissolved to be derived ent1xely in the oceana are assumed from weathering of igneous roclcs. 3. all~Jance material. 4. The propoxtions of igneous rocks and sediments so calculated apply only .to the rocks having the part1aula: composition used in the calculation. No is made for the addition The second met.hoa ployed frequently is moxoearbitrary because of its chemists have made and used their example, Mead (1914), Clarke, of volcanic: but has been em- simplicit.y. Many990- ownestimates. See, (1924,p.29), HOrstman for (1957), dtXX8- Turekian and Kulp (1956). Many of the la~er estimates go back to Cla~ke (1924) who p~oposed the following composition for a crust 10 miles in thickness: 95% igneous rocks made up of granite and basalt in the ratio of 2:1D 5% sedimentary rocks \'lhich are .An average ratio ao~ shale, 15% sande'tone andS" igneous Z'oclccomposed of granite of 2:1 is equivalent therefore take granodiorite limsstone and basalt to granodiorite. G in the Somegsoch(!m!s1;s as the most representative the average ccmpoai~lon of the continental crust. of (Turekian and Kulp, 1956). The third approach 1~ the most satisfactory and is used here. The besi; crustal (1955). model is that proposed by Polde&"vaa~t In the following sec~1on this model will be described in soma detail and the concentrations t1um in the crust \'l}111be calculated poldervaart°s of rubidium and st~onfrom it., l(odel. of the Crua~. ~troduction. POldervaart (1955) p~oposed ~at the surface of the earth can be subdivided into five regions each of which consists of igneous and sedimentary sitions and in certain proportions rocks of certain compoappropriate for the particula~ envi~onmento He assumed that metamorphic sedi- ments can be included with the igneous rocks in such a model. 'Ene five r~gions of the earth are: 1. Deep oceanic Continental shield 20 3. 4. 5. Young folded belt Suboceanic Volcanic island por each of these ~eg1ons Poldervaart estimated surface azoea, depth 'to 'the Moho, proport1on rocks and their compositions. publlshad information of igneous These estimates are based on in the geochemical \'1S11documented in POldervaartOs to aadimantaE'y paper. by him need no. additional justification literatu3:e and azoe The values adopted and are accepted for the presen~ purpose. Deep Oceanic Reqion. Thi~ region includes the deep ocean basins which are 268 x lOG km2 in area. ~km, The average aep'th of water is foll~~ed by 0.6 km of unconsolidatea (0.3 km solid) sedtmen~s and 5.75 km of a rock whose compressional wave velocity is 6.5 lan/sec. The to~al depth to the Moho1s 10.75 km on the average. The total volume of the sediments (solld) 1s 80.4 x 106 km3 with a mass of 217 x 1015 tons at a density of 2.7. -120- This mass of sediments consists of 104 x 10 calca&'sous sand and ooze 17 15 ~ona of 82.5 x 1015 t.ons of red clay and 30.5 x 1015 tons of siliceous ooze. The layer below the sediments 1s assumed 1:0 bs an olivine basal~ wi~h a de~s1ty of 3.00 1540 x 106 kin3 with Its V01UWB is a mass of 4620 x 1015 tons. The ~otal mass of the oceanic crust is 4837 x 1015 tons. The calculations centrationB in ~e 2.6 of the rubidium and s~rontium con- oceanic crust are summarized ~ne concentrations calculations 1n ~gble used here and in allsUbse~ent are those derived from the survey of the literature. Table 2.6 Concsntra~1ona of Rubidium and Strontium in the Oceanic Crus~. Rbx':' 100 Sr x ~ " ~Jt .. !U)ppm 96 18 440 17.2 421 Calc. ooze 2.15 10 800 0.2 17 Red clay 1.71 110 720 1.9 12 Sil. ooze 0.63 60 <20 0.4 Roek TYPe Olivine Basalt Totals J)pm 1907 100.49 Best estimates Sr I Rb :g: 20 ppm Sr =450 ppm =-0.045 Rb/Sr 100 450 Contingntal Sh1eldReqion. ~~e a~ea of this region is es~imated at 105 x lO~2~ its elevation above sea level is 0.75 km, the averaga thickness of sed1mantagy cover is 0.5 km (solid) and the depth 'to the Mohoi a 35 lon be lO\fJ sea leve 1. The volume of the igneous rocltS is 3700 x lO~3. Their composition 1s exp~essed in terms of g~anod1o~ite, diorite and basalt in the proportions 2300 x l06km3 9ranodior1~e, 300 x 1100 x lO~3 basalt. Ass~n9 and 3.0, respectively, 1015 ~ons granod1or1te x 1015 tons basalt. 7.67:113.67 lo~an3 diorite and densities of 2.7, 2.85 tbe mass distribution 6 O~ is 6200 x 850 x 101S' tons diorite and 3300 The total mass of the igneous rocks is 10~350x 1015 tons. The volums of tbe sediments weight of 140 x 1015 tons. 1s 52.5 x lO~3 They are constituted 41% (51.4x 1015 tons) shale, 4~ of (60.0 x 1015 tons) sandstone and 16% (22.4 x 1015 tons) limestone. combined weight of igneous and sedimentary 10,490 x 1015 ~ons. with The rocks 1s a Table 207 of R~u>idiTh~and St~ontiThu in Concen~~ations the Continental Shield • Rb ~ ~ .€,r x % Roclt ~ % Wt. GE'ancdioz:-ite 59.2 122 4-40 8.1 88 500 1 40 31.5 32 440 10 139 Shale 0 ..5 149 300 0.7 105 Sandstone 006 . 20 0.4 - Limestone 0.2 Diorite Basalt Rb ppm Sr -PPffi < 60 100 72 260 610 <5 1.2 100.1 TOtal 100 90.1 Best es'timate: Rb Sr RbISr = 90 ppm :: 442 ppm 0.20 - 441.7 ~oung Folded Belte. The young folded belts are assumed to occupy an area of 42 . g lO~. 2 Their average elevation above sea. level 1.25 km with the Hoho at a depth of 37 laD below sea level. The prism of geosynclinal sediments is est~ated to extend to a depth of 5 km (5.5 if porespaces are included) and 1s ~haught to include 40%of igneous rocks. of granodioritic:: composition. The volume of sediments its mass is 340 x 1015 tons. is therefore 126 x lO~<m3 and The sediments consist of 1a 17605 x 1015 tons (52") shale, 4401 2" 1015 'tons (13%) sandstone, 74.7 x 1015 tons (2~) limestone, (5"" graY'"Jacke,20.4 x 1015 tons 10 15 17x 1015 ~onB (6%) anaesite and 6.8 2t tons (2%) rhyolite. The total thickness of the igneous rO~te below the 6 3 sedimen~s is 32.8 km giving a volth~ of 1370 x 10 km • To this must be added 84 x lO~an3 of igneous roclts intruded into the sedimsnta:y rocks. lO~3. The ~n The total 1s theZ'eforee 1454 )t mass of igneous rocks is composed of 9ranod1o~ite, diorite and basalt in the proportions 8 47 0 a 1 I 2.85 or, 1n te~s of volume, 9.33 x lO~~3, 111.3 x lO~Km3 and 316 x l06km3• 9ranodiori~a is 1017 x l06km3• ties (see p. 121 The total voluma of Taking app~opriate dena1- ) the mass distribution is as follows: 2750 x 1015 tons granodiorite, 317 x 1015 tons of diorite and .948 x 1015 tons of basalt. The total igneous rocks is 4015 x 1015 ta~s. igneous and sedimentary to/eight of the The combined mass of rocks is 4355 x 1015 ~onB. Table of Rubidium and Strontium in the Young Folded Belt. ~L.8 Concent~atione ~ Ss: ppm Fob ppm Sr x % ~t ~ 100 100 77 277 ~oclt TYPe % t~7t. Granodiorite 63.1 122 440 Diorit.e 7.3 88 500 6.4 36.5 Basalt 21.8 32 440 7 95.7 Shale 4.5 149 300 6.7 1305 Sandstone 100 60 0,,6 0.2 Limast;one 107 Graywacke 004 SS 500 0.4 2.0 Andesite 005 88 500 0.4 2.5 Rhyo11'te 0.2 196 197 0.4 0.4 98.9 438.2 Total < <:S 20 610 10.4 10001 Best eatiw.ate: Rb Sr Rb/sr =- 99 = 0.23 ppm = 438 ppm Suboceanic Region This- is the region of the continental slopes occupying platforms and an area of 93 x lOGtan2• The average depth of water 1s 1.75 lan, tho sediments are 4 km (5 km including pore spaces) thick follc\"Jed by 11 'km of igneous rocks. The Moho is about 18 km below sea level. The sed1men~s consist of two typesl mente which ai:e 252 :c 10~3 t~pela91c in volume and shelf sed1~ sediments \'lh1ch occupy 120 x lO~~3 ~ T'ne composition. of the is approximately that of the young folded belts. lattaX' The mass of the shelf sediments is 324 x 1015 tons and they consist accordingly of 168.5 x 1015 tons of shale; 42 x 1015 tons of sands~one. 11 x 1015 tons of limestone, 16.2 x 1015 ~onB of graywacke, 19.4 x 1015 tens of andesi~e and 6.5 x 1015 tons of rhyoli~e. 'l'he hemipelagic sediments amount.to 680 x 10 15 tons and consist of 519 x l015.~ona of ~erriganous mud, 129.5 x 1015 tons of corcal mud and 32.4 x 1015 tons of volcanic mud. In the calcula~ions shale is substituted for terrigsnous mud, 91o~ger1na ooze for coral mud and andesite for the volcanics. The igneous rocks occupy a volume of 1025 x lO~3 areccmposed of 372 xlO~3 of diorite, 186 x 10~l3 tholeei~ic'basalt and 465 x lO~an3 of olivine basalto and of The masses of these types of igneous rock are lOGO x 1015 tons of diorite, 530 x 1015 tons of tholee1t1c basal~ and 1325 x 1015 tons of olivine basalt. The total mass of igneous rocks is 2915 x 1015 tons. rocks in the suboceanic region js the The weight of all 3919 x 1015 1:ons. the -126- Table Concentrations 2.9 of nubidiuM and Strontium the SUboceanic % 100 Rb x Rock ~me % w'tos ~b Ppm in Region Sr ppm x % Sg: 100 Diorite 27.1 88 500 23.8 135.5 Thol. 13.5 32 440 4.3 59.3 33.8 18 440 6.1 Shale 4.3 149 300 6.4 12.9 Sandstone 1.1 60 0.7 0.2 Limestone 1.8 <S 610 GX"a~lacke 0.4 88 500 0.4 2.0 }\nde8~te 0.5 8S 500 0.4 2.5 Rhyolite 0.2 196 197 0.4 0.4 13.2 149 300 19.7 39.6 Coral mud 3.3 10 800 0.3 26.4 Volcanic mud 0.8 88 500 0.7 4.0 63.2 442.8 Basal t 01. Basalt Terri 9. mud Totals < 11.0 100.0 Best estimatet Rb Sr Rb/Sr yolcanic 20 - 63 = 443 149 ppm ppm = 0.14 Islands. This region occupies the remaining 2 x lO~tm2, rises 0.5 km above sea level and con~1sts 4.5 km of tholeeitic basalt of 4 km of andesite, and 6 km of olivine basalt. -127- The Moho is 14 km below sea levelo The volumes of the diffexen~ ~ock types are: a x lO~3 of anaesi~e, 9 x lO~Jm3 of thcleeite and 12 x l06km3 of olivine basalt. The total volume is 29 x lO~tm3. andesite 22.8 x 1015 tons# tribution of massas is as followss tholeeite 15 27 x 10 tons 6 olivine The d1s- 36 x 1015 tons, totalling 85.8 x 1015 tons. Table 2.10 Concentrations of Rvbid1um and StrontiUm in Ocsanic Islands Rb x ~ Roclt ~3 % t~t. Rbppm Sf.' 'P-Pm Sr x~ 100 100 Andesite 26.6 8a 500 23.4 133 . Tholee1te 31.5 32 440 10.1 138.5 Olivine Basalt 42.0 18 44-0 7.6 Total 41.1 100.1 Best estima'tel Rb Sr Rb/Sr .... - :. 185 45605 41 ppm 457 PPD 0.09 The Crust of the EaX'th. The weigbt of all 'the rocks in the crust above the Mohois 23,687 x 1015 t.ons. of the earth The sedimentary rocks amount to 1701 x 1015 tons (7.17") whereas the igneous rocks accoun~ for 21,986 x 1015 tons or 92.8% of the total. Table 2.11 Concent~at1onE of Rubidium and Etrontium in the Total C&ust Rb,x~ SZ x % 100 Region % Wt .. Deep Oceans 20.4 20 450 4.8 Cont. Shield 44~3 90. 442 39.9 Young fold belts 18.4 99 438 18.2 80.5 Sub Oceanic 16.5 63 443 10.4 10.9. 0.4 41 45.7 0.2 1.8 73.5 440.8 Vole. Islands Total Ilb ppm Sr ppm 100 100.0 Best estimate, Rb SIr: RbIsX' -- 91.6 196 7~ ppm 441 ppm 0.17 In addi~1on to the Rb/Sr ratio for the entire crust values of this ratio were also calculated environments. for certain restr1c~ed The value of 0.17 obtained for the whole crust includes the basalts and calcareous oozes of the deep sea region which in fact is not accessible and 1s not involved in geological processes. Therefore it is more realistic to exclude the deep sea environment of the and to accept the value Rblsr ratio for the continental crust as most repre- sentat1ve. The results of these additional calculations a~e tabulated in Table 2.12. Table 2.12 Concen~ra~iona of Rubidium and Stron~ium in Various Par~s of the Crust Geological Environment ~ Total Crust above Moho ppm SZ' PPnl Rb/s~ 74 441 0.17 .87 442 0.20 Sediments in young fold belts. 100 351 0.29 Sediments in sub oceanic regions 114 388 0.29 shields 88 229 0.39 5S 660 0.08 90 443 0.20 Crust only~ Continental Sediments on continental Deep sea sediments Igneous rocks in continental shields '* Excluding deep oceanic and volcani~ islands region. The 1tb/Sr ratio of the continental that of ~e sediments as 1s that of ~e and slopes. continental in young folded mountains is sediments 0.29 on the continental mar91ns The value of the Rb/sr ratio for the entire crust is lowered by inclusion assumed to exist pr~arily crust: is 0.20, at the base of the CNst. of basaltic rO~ts Because we are concerned w1~h the upper parts of the con~nental crust, a more realistic estimate would place the value of the "effective" Rb/Sr ratio of the crust between 0.20 and 0029 at about 0.25. Estimates of ~le concentrations tium have baen i made ln recent years of ~Ubidium and at~cnby Turelt1an and Kulp (1956~ ~~rstman (1957)and Gast (1960) on the basis of new analytical results. Previous estimates of crus~al abundances of the elements ware compiled by Ple1scher (1953). Almost all of the latter ove~eatimated the abundance of rubidium while underestimating crustal RbiSX' magnitude. As a result the ratios are high by as muchas an order of Goldschmidt value of 2.07. 1.03. that of strontium. (1937t for example, suggested a Rankama and Bahama (1950) still reported Even Vinogradov (1956) arrived at. RbISr - 0.8. These high estimates led to the eugg0s~ion by Wickman ~at (1948) sr87;sr86 ratios in limeatonscould be used for age determination pu~ses. Summary and Conclusions. Average concentrations various types of igneous of rubidium and strontium in and sedimentary t'CCks were com- plied from the most reliable analyses pres~ntly available The zesulte obtained here agree with 1n the literature. those just published are more complete by 'l'ure1d.an and Wedepohl (1961) but and thus judged to be more representative. n brief summary of the results is given in Table 20130 Table 2.13 Concentrations of Rubidium and StrontiQ~ in Ignecus and Sedimentary Rocks Rb ~pm lRoclt Type Rbls.Z" Sr ppm Gran1-ce 196 197 1.0 Granodiorite 122 440 0.28 Diorite and Andesite 88 500 0.18 Gabbro and Basalt 32 440 0.07 Olivine Basalt 18 440 0.04 149 0.05 Eclogite 6.9 Chondrites 3.17 10.6 149 300 sandstone 60 < 20 Limestone <.5 610 Shale These values were used to calculate 0.30 0.50 ""'3 < 0.•008 average concentrations of rubidium. and atront1u..ro in various parts of the earthOs crust according to POldervaartOs model. of the results investigators A recapitulation and a comparison to estimates appears in Table 2.14. by earlier Table 2 14 0 The Concent~ations of RUbidium and StrontiUM in the Crust Reference This author 74 44:1 0.17 87 442 0.20 Continental Crust 100 351 0.29 Geosynclinal sed1men~ 114 388 0.29 Shelf sediments Turek1an & Kulp, 1956. 450 crust. above r~~h.o Basalt & granodiorite. = 0.27 Horstman, 1957. 120 Gast, 1960. Bntirs 74 Average igneous rock 450 0.16 Baaaltsgranodior1ta '!'hepZ'sferxoed value for the RbISr ratio of ~he upper part of the continental crust 1s 0.25. ~ lnl _ Chap~er: --..-... - - .---._-_. -~3 .•.. THE ISOTOPIC COf4.POSIT!ON OF STRO~"TIYM. IN crBE CRUST OF TIm EP,RTri The best geochemical evidence available at the p~esent tAms leads to the conclusion" that the rocks of the continental c~ust have a Rb/sr ratio of approximately the otber hand, basal~s the isotopic of sub-crustal composition origin demands 00250 of s~rontium On in a Rb/Sr ratio of the magmatic source regions in the upper mantle of about 0.040. Because the ratio sr87/sr86 is a function of ~he age and ~be Rb/Sr ratio of the environment, i~ is expected that average " crustal strontium has a significantly than mantle-strontium. higher Sr 87 /srBG ~atio The actual value of this ratio depends on the average age of the crust and its Rbis);' ratio. range of values great variety is large because the crust is c~pcsed of a of rock tYBes of d1ffeX'ent ayes \'ihose RbISr ra~ios vary through several orders of magnitude Table :a.2). The In spite (Chapter 2, of 'this inhomogeneity meaningful age values can be obtained by analyzing those geological environments aver- the strontium "in which are composed of or con- geosynclinal basinsG Othe~ such envirOi~nts ate meta- morphic sedimentary rocks and para-gneisses# glacial till and river water o Because. geosynclinal generally subjected to metcmo~hic sediments a~e processes du~1ng orogenies and may become Mg~anitized~ as a result~ the .isotopic co~oosition .of ths.stron~ium in this particular environment is oc~st meaningful. In the following sections available analyses of 'the isotopic composition of strontium in chemical reagents. sea water and a variety of rocks will b-~ ~evie~~edin order to arrive at the best est1ma~e for the value of the sr87;srS6 ratio in ~h~ czus~. The Xsot,<mic cam~G~ion of Stroni;ium Chemical. ~eaaents eU-tO in The definitive determination sitton of.strontium strontium of 99oS~ 95~an in Water ft of ~he isotopic compo- was made by tiler (1938) using .matall1c purity marketed by Eimer and [aend. His measurements of the abundances of the non-radiOgenic isotopes have remained unchallenged since that time. N1eros value for the ratio sx87/sr86 ~as 0.712. 1\ldrich E!t ale (1953) and Herzog et repeated Nier's measurements al •. (1953) each on a strontium carbonate almost iden'tical in Table 3.1. strontium xesul ts'.., l~ Slb1:tmax-y of 'the data en,,;J..Eonmeni: frem \.ihich the The geological in 'ths EimaJ:and Amendzeagents never bean established t'11ith appeaA:s certainty. ~~asder1 ved has 'the ccmpO\nyis no longer in business. Tabie 381 Isotopic composition of" Str~nt1um in Ch~miQ~l ~eagents 87/86 84L88 ~LS8' Nie~,1938, 0.712 Osl194 p. 277. :t .007 t:. 0012 Ald~!ch at al.Q.711 :t 1953, p. 458. 0.0068 :!:. .00014 0.1195 0.0067 .0004 ..~ .0003 ~ 000005 Herzog e~ al 0.712 0.1196 Remarks s~ m9tal 99.9% puxe Eims&' and 1lmend. srCC'3 Eimer & A...~nd average of six analY8es # o Oe0010 o 1953, p. 462. Sra? ISr Schumacher, 1956, po 21~. Tats work. = 0.0703 r 0.0002 S&,C03,"Gen~Chemo, c.P., 0.712 Ocl195 :!: O.0003:tOe0002 0.0066 ::t 0.0001 let. 10 .S&"C03~ Eimer: & P~ndfl lot 492327, ave~age of eight analyses in nine months. Errors aX'S"'the si:anC!ard deviations of 'the 10000. It is evident fr!CJm Table 3.1 tha't thaprec1s1on of the detezminatione has imp~oved steadily but that N1eroe orig- inal values are confi~ed by all invest1gatozse. precise analyses are those reported hareo The mos~ of the 1eo~@p1c cc~po3i~iouof st~onti~~ in ocem1 wa~ex = seem to indicate that it too has a ~a~io of s~a7/sr86 stront~ ..wa as an adequate of croa1:al sample s~goon't1u..?Q and to conclude that ~he sr87/sr86 ~atio in the ent1ge ccntinental crust has a,value of 0.712. However, it gives meaningful , mateg-1al1s B'trongly age values only 'When the enriched in value of tl1S initial abundance Table 3.:2 Isotopi~ 87186 Author Aldrich and ~11s value has bean Composition sr87 so that the exact of Bra? is not 1mpcr~ant. of Sea l'ia~eE" Stg-ontiu..va 86Le.,E~ 0.712 0.1196 0.711 0.1201 sea \tJater lbrzog, 195a~ Barzog at al., 1954~!. ~lald at 0.712 0.1189 Pinson et al., 1957. Z 00002 :to.0003 1782 0, Gast, 1960. ~Referred saa water sea ~at:e:r 0.718 0.1195 :to.003 :t 0.0004 al., 1956. po 0.0067 0.0061 :to.OOOl Oceanic surface wa'ter. 66018 oW. 39001 N 0.112 .:to.002 to by Pinson at al., 1957, p. 17820 It can easily DS demons~zat@d that a value of 0.112 fo~ ~11eavezage crustal s~67/s~B6 xa~io is incompa~ib10 with an average iib/sx- ~atio of O.25~ If -cheaverage agC) of the crust is 2 billion years and its s~87/sr86 ratio at thm1: tirn~ 'Was0.704, the valuC3 of tbis would 'be 0.725. ~veg'5ga If the ra'cio to-day s~e7/sx86ratio 1:oday is 0.112 and the ava~ag@ Rb/SL ratiofoz ~he C1~et is 00250 the age of the crust w~uld have to be about 600 million years. This is cleaxly not so. Calculations of this nature indicate very s~~ongly tllat.the ratio czustal sr87/sr86 must be 9~ea:ter 1:'han0.112 .. If this is tile case, then the p~oblem arises that the isotopic campos1~ion of saa wateg strontium is not in equ11ibrilli~ wi~ crustal stE"ontiumo Whe~er thiS appa~ent aiscEepancy cannot be decided at this representative data. evidence is real or not time because of 'the laclt of If it is r0al, it could be taken as that much of .tl1e.strontium in solution oceans is ofsub-crus~al, volcanic o~1g1n. in 'the Because rubid- ium is enriched in the micas which are not affected by chemical ~eathe%1ng to any great extentc ~at it is possible ordinary waa~!e~ing processes on the continents neve~ If ~is i:ru~1 much of the zadiogen1c s~87 is continental~ocks in existing may never enter ~he oceans but is imme- dia~ely trapped in sedimentary basinso ~le Rb/Sr ratio of sea water itself is too l~~ .to develop any radiogenic Sr87 of its own. An \ (See Chapter 2.) .. altez:na'tive explanation is to postulatei:ha't sr87isr86 ratio in sea. \'later 15 ..v~~j.~le 'the depending on tbe natuze and age of -the rocits in tbe nearest: lamd-mass or ."to 1n the source of the ocean water'. In tbis case a more value of the srS7/srS6 ratio for the en~ire repre~entatlve ocean may \-Jell be higher than O.712 and t.he discrepancy may disappear. A c:1Ucal .3.2 sh01l}s inspection that the isotopic not be as uniform cf t.he da1;ap:resranted in Tabla compos1tion of sea water may as it seems. Whenthe sr:87/srSG ratios aX'snormalized to sreGIsX'es.,;;;;0.1194', the corrected values range from 0.7097 (Pinson at al., 1957) to 0.7145 (uer2og et at., 1954). The s~87/sra6 ratio (0.118) reported~bY Ewald et al." 1956, :1s suspect because their o1:her ma'terials evidence are unreasonable. resul~s for There is 'therefore soma in suppor~ of tbe hypctha~16 ,that.geographic variations in the isotopic composition of sea water-strontium for ~e future. Only a faw dete~na~!ona of strontium of th3 isotopic composi~ions , in rocks ha.ve been repo:tted. in the literat.ure. Gaat (1960) analyzed ten granitic zacks ran~1ng in age from 200 to 9~aater than 2650 tUlllion years and found sr87/sr86 .. :atioB ranging fram O.720tc rrean of ttle :Rb/Sr ratios average age 1s at least 1.003, averag1nq'O.830. The for 'these X'ocks is 1.64 and theix; 1750 million years'. On the other band four limestones analyzed by Gast (1960) show a variation from 0.704 to 0.113. Ewald e~ ale (1956) :epoxted sx:87/sre6 ratios fo: stroni:1anites, s'tonea celestites and lime- ;'Jh1ch xoange frOm a low of O.692~ O~;002 for a Cambrian l1mas1:one 'to a high Qf O.710:t. 0.002 for an 'Upper Cretaceous s'tront1an1te~ The .low value is almos1: certainly in Grror. It is clear that existing data are not adequate define' ~e c:ompoa1t:lon of crus~al merely to demons~ra~ s1:rontlnm but serve the extreme varia.tions 'l'he best estimate must therefore reasonable ~ge for the continents to which exist. be made by asswning a and':calculating the An upp,ego11m! t 'to 'the abundance of 5r87 can be set by assum1ng that the crust is 4.5 billion years old and has had a tlb/sr ratio of 0.25 throU9~Qut ~his timeo Tal<1ng the initial sra7/sr86 ratio as 0.7004 (Achondr1tesPasamontso Gast, 1960), the preBen~ sr~7/sr86 ra~10 should be 0.748 • . . It 1s hlghly pxobable thai: the coni;inen1:al masses are youngeX' 'than 4.5 billion. yea:r and the ave'rage Sr87/sr~6 ratio sh~uld therefoX'e be less.thal'l'O~748. 1\ lower 11m!t. is obtained wi. th the assump't1on that the age of the continents is only 1 billion years and that the .. initial ratio was 0.706. mate of ~he ~11s value is a reasonable es~1- sr87/sra~ ga~io of ~be upper man~le 1 billion years ago which has a Pb/Sr ratio o~ 0.04 and a prGsGn~ value fox the sr87jsr86 ratio of 0.109. t.1onsthe present The possible ~e sza7/srS6 r~ss With these assump- ra1:10 in the crust 18 0.717. foZ" the !}Yara9~ Sr87/srSu ra.tio in contin~ntal masse,sis t'herefor~ from O~717to 0.748. Gas~ (1960) used. an initial, ratiocf 0.714; a RbISr ratio of 0.33 and an average age of 2 l>1111c;»nyesrs'to propose 'that. 'the pres~n1:Sra?/sr86 ratio of 'the silicic O.742~ crust 1s -141Perhaps the moat meaningful culating the average sr87/sr86 blage of sedimentary estimate can be made by cal- ratio of a geosynclinal and volcanic rocks. of such a geosynclinal assemblage assem- The eomposi~ion is approximated by the proportions of sedimen~s and volcanics in POldervaartOs sedimentary rocks of young fOldGd' belts (See Chapter 2). The calculations environment Table 3.3 leading to the final estimate in such an are summarized in Table 3.3. The sr87/sr86 Ratio in a HYPothetical Assemblage. Wi:.." x Sr Rock Type wt.% Sr.ppm 100 S~ Shale 52 300 156 44.3 Sandst.one 13 < 20 ,3 Limestone. 22 610 Graywacke 5 ~ndes1te Rhyolite sr87/sr86 Geosynclinal 87/86 x Srtb 100 O.733tz 0.324 0.9 0.733 134 38.1 0.712 0.007 0.271 500 25 7.1 0.708 0.050 6 500 30 8.5 0.708 0.060 2 197 4 1.1 0.708 0.008 0.720 352 100.0 o Pure shale and sands~one derived from wea~ber1n9 of granodiorite 2 billion years in age, RbISr 0.29, TOtals (sr87/sr86)o = = 0.708. The sr87/sr86 ratio of such an assemblage approximately 0.720. is therefore 'this is considered to be a reasonable estima~e of the initial szS7/s~86 &atio of a pazagneis6 tion"of gsosynclinal The isotopic sediments. compositions of stZ'ontium in two com- posl~es of ~al~ozo1c shale from the east and west coa8~S of No:rth J.UDezo1ca were determined as v10us estimates. .:JUlian "reiGS Dennen. 'l'"'necompos11:es check on tl1e pre,. px-epared \t1SX'0 and were made available All psrtin(l9nt analytical a by ~ • by Professor data W'. H. are campi-led in 'J:able 3.4. The average value of the corrected for the t.wo shale cOIGposi'tes sr87/sr86 ratio 1s O.721St 0.001. This is remarkably close agream9n~ wi~h ~he predicted value I of 0.720. It should be borne in mind 'that these are no~ samples of pure shale but coni;a1n calcium carbonate alld volcanic mateZ'ial. They 'therefore resemble i:he average geosynclinal sediment mo:rethan tbe pure sh'ale. other two analyses completeness. Schnetzler is The are included here for the sake of The sandy clay. analyzed by Pinson and clOSG to the ahale composites. e m e· r. ~ ro €I) ro .... 0 U ~ ~. -!J i4 ! to ~ ~ ... & 0 Q2 po't3 e!-S m e, U ro 0 0 to M 0 ~ M to tU C~ 0 m J!. (J ij) a: ~ k t>1 ~ «JfI4 ~ ~ Td ~ ~ 0 .0\ 0 \0 <' \D \0 0 0 trJ fj ~~ ~ Q..4 0 (f1 • cG r: i en CJ &: ~ 0 0 ~ ~ • 0 0\ ~ ~ &on \,0 0 \0 0 0 0 • • , c .,e ~ CO ~ 0 • ~ t • ttl P) ...~en; ~ v-1 • 0 Ol 0\ ... .-t 0 J.& C M 0 ...a to N 0'\ rs4 cat Q\ P-I .a.J .. c: 0 0 ...a .. lR o r\1 ew ~ .eJ 0 ~ 0 ~fJ ~ \0 \0 N N 0 N ~ ~ l' 0) "at# ~ • • 0\ tn tn "...., po( N ,CO I' 0 • fD4 0\ .... 0 Q, i' - 0 0 CJ U) \0 .0 ..-f ~ r-.. ~ .e.J CO 0 m H ~ (U .... ic... ~ ~ N C"'" t") • • ....CO \0 e-V CO 03 r-• ,•• 1 N .r-- • '.:I ...0 .s 0 .... At n1 ~ ~ u ~~ ~(J) ...OJ '- N +' (l) g fJ) (lJ [i o&J I k~ Q) ~~ en as <11 -rot G) >to '0 :>t t') w r::: a~ CJ ~ ' '5 ~ '8 ((J c ~ • c.. 0\ OJ • ~ • ..l\lI ,., pot cP2 ,.,tU tf • C") ~ 0 ~1 0 CD ~ roe ~ U\ CO M ~ ~ N ..., (t) ~ 0\ 0 ~ 9:4' s:: .t Oc ~ R0paated measu~am~nts by save~al invest1ga~ors seem t~ 1ndicat~ ~a~ ~ha sr67/sr86 ratios of stront11Im carbonate reagent (Eimer .and Aiuand) al'ld saa .'Watez-stx-ontium tical a~ O.712~ It has become customary are iden- in age de~exminat1on \40rk using ~he Rb-Sr m{?thod to assume a crustal average sr87/sr86 ratio of 0.712 on the grounds that the st~ontium in the oceans is an adequate sample of strontiu.'11in the crust. The ~/sr ratio of the c~uot and its p~obable age make a higher value more likely. to "a predicted r~ge 0.117 to 0.748. E,t'trem3' asswnpt:1ons for the average Br81/sx86 14aldng realistic assumptions si~lon and types of s~ronti~~ present obtained for rat10 of of the COMpo~ a value'of a geosync11n~1assemblage. 0.720 is !fBasu~ements of the.sr87/sr86 rat.io of two composites. of'palaozo1c from and "\.fesot coaatsof the east 0.7215 thus conf1mdnq shalas A'OOz-1ca avezoage the earlier estimate. The beat information crUstal Korth lead noW available points to an average sx87/sr86 rat1.o of "about O.72S:tOoOOS. sive study is planned to confi:m this estimate. An exten- Intrcduci:ion. ' All determinations of the concentrations of rubidiUm and-stx:oni:ium were made by tbe mei:hod of isotope dilution , (Webster. 1960, 'p.203). of The method in\'olves the -addition a known amount of "spike" to a known welgl1t of the sample andtha - isot.ope ratios subsequent measurement on a mass' spectromet.er. a solui=ion of the element of the resulting Thea spike " is CO to be analyzed w'hose isotopic campos1~1on baa been greatly altered by enrichment one of th~.1sotopes of this element. The procedures used in 'the preparation Qf rock' samples for~alys1s are identical to those given b~ Hart: (1960, p.182) dried and poWdered 0 Briefly ..' a knO't!Jn weight of the rock sample is s~1ked and then digested in '10;1 mixture of hydrofl~oric acid and sulfuric ~1d Jon-a large Pi: crucible on a bo1-1ing water bath. of Aft-er the rock is digested t:he hydro- flu@ric acid is d~iven off by evapo~a~ion and the zesi~ due is disEolved in abou~ 25 ml of 2N vycor-d1stilled hyd~cCbloric ae~d. Rub1di~~ and strontium are separated. fro~ eaoh o~her and most of the other elemen~e by passing tbe solution through a cat1onexcb~nge (DO~x 50, 300 mesh, l~ progress column cross-linked resin). The of t.'he Rb ions in the column,is men!'tored by a Pt-wire flame tee~o Strontium is detected by use of a radioactive sr85 tracer which 1s added to the solution before it i's pui; on the col~. 'llle strontium and rubidium frae~1ons are evapo~ated ~o dryness, dissolved in a small amount of demineralized wa~er and stored in clean 1 oz. pOlyethylene bottles until they can be run on 'the' mass spectrometer. Strontium samples are placed on ~he filamen~ as strontium oxalate 'tJh1chis conve~ted 'to t:he me~t. briefly to ~ed heat. oxide by heating the fila- RUbidium samples are con- verted ~o nitrates before being put on the filament. /19 a rule from 42 to 54 scan's were recozded in "each analysis •. The measurement of the desired a standazd ev1at.lon error isotope ratio of 1;he mean of. less than has Systemai:ic Oo2<',bo errors caused by contemJination s~~ple during the che~cal using vycor-distilled alized watero of ~he precessing were reduced by hydrcchlo~ic acid and deminer- The amount of contamination was deter- mined by_ making a ~otal of four blank analyses thgough~ . .out the course of tIle investigation ~1'f1eresults a.re e discussed in moxe detail on page I60a for rubidium (O~061~#I'S) and strontium . were applied to all Blank corzactions (Oo249/",#"~) analyses. One of the advantages of the isotope dilution technique is that after the spike and t.he noxmal Glemen~e are thoroughly mdxed quantitative unnecessary. Provided the isotopes x~covery is have equilibrated, any part of tbe solution will g1va ~he desired result. Moreover, the isotopic ratios can be measured with a precision of 001% pre~is1on standard deviation of ~he mean, or better e Assuming 'that: the calibrated and contamination spikes are propeX'ly is caKefully controlled~ the absolute accuracy of isotop~ dilution analyses can be high 0 . Radioactive sr85 is prepazed by irzadi~~n9 .RbCl with 15 Mev deuterons The sr8S is fo~e~ sr8S• in the ~oI.T. cyclotron. by 'thenuclear reac~ion Rb85(d, 2n) It decays by It electx-on capture .half life to Rb85 ~J!th a of 6S days and em:1:tsa delayed O.Sl Mev. solid The irradiation I' ray of t.ime is 1 to 2 hours depend- ~n9 on tbe amount of ar8S aesired. After ir~adiat1o~ the sample is s~ozed in a lead container 'three days to allow short-lived fo~ ~wo or activity to decay. Bas1des the desiged sraS, a number of other isotopes are produced by nuclear reactions affecting ~e rubidium and 'the chlorine present as ltJell. as the impur1ties such as potassium and sodium. types of nuclear reac~ions (d, 2n), (d, 2p) and (d, 0<.) can occur: G The following (d, p), (d, n), 'Theproducts of ihes0 reactions either decay rapidly, a%e noble gases or can be separated from sr~S by ~ation exchange techniques. After.the cooling period the sample is dissolved in abou~ 20 .ml of 2N vycor-dist111ed hydrochlo~ic filt~red, and put on a clean cation exchange It is advisable to keep the column inside acid, column. a fume hood and to use lead bricks.~o pro~ect the operator. sample is e~uted in the normal fashion using 2N The hyd~ochloric acid., The rubidiu~ fzacticn is collec~ed and s:t,ored in a lead ccnt.~ ..ner ~cause it contains 1800 day,Rb~~ p~aduced by ,Rb8S(d, p) Rb86. The fzaction containing sr8~ is easily located by its activity. It is evaporated to dryness, zed!ssolved and ~aased througb the column a second tima. tium'frac'tion , :polyethylene is diluted bot~le ~e final concentrated wi'th 2N Hel t"1l1ich is 'CO fill stron- a 500 ml ~,ept in a metal conta1neX'• .O~ly, 2 or 3 milliliters of ~his solution axe ,added per sample to g1vG an activity of ~ 15 cts/mj.n which is easily detected by a Geiger counte~o The purity of the 'tracer solution with, respect to rubidium and strontium was checked by adding the trace~ 'to-the spike nU.xture used to dei;ezmine blanks. these contamination exper1ments,aescr1bed Since in detail on psge160 ~ indicated very l~J levels ,of contamination, sufficient purity of the ~racor was demonstrated. The small number .of s!:es ,by 'the mass spec1:rOlOOter atoms added is not detectable 0 Calibration of tbe "Spike-, Solutions~ IntrOt.~ucti()n • Thepzeparation of the "sp~ke" ,and the "shelf" .solutions used in the calibration of the "spikesn is a -ISOjoint ventuze by seve~al.~~mbe~s of the Project s~aff of the Geochronol0'3,YL3.bo~atogy at 1 1.I. T. ~'U,deg'. the 1 supervision of Professor Wo H. Pinson. Much detailed information was given b~ Pinson (1960, p.237). For ~his zeaaon these ~opics will be considered only briefly he~e. ~ne actual analy~ical data conce~ned with the .spike calibration experiments are t.zeated in more detail ~o preserve them for the record. The ~spikean used in this investigation in Rb81and sra6, r~apeetively. made using a sr84 spike. in 'the form of salts tory. The salts aknoWDvolume correct.ions are enriched A few analyses were They ~~re originally obtained from ~he Oaut Ridge National Labora- weE'e carcefu1.1yweighed and dissolved of de~neralized in water•. Temperature tiare made as needed. Before these solutions can be used for analytical pu~oses spike the iso- topic campo~it1on of the element and its concentration in the solution must be confimed with tbe greatest. accuracy and precision ErroX's j"n 'the cali- possible. bration of the spikes are systematic ezoz:orsand detract from th~ accuracy of i:he analyses. A nsh~lf" solution is a solution containing a known amount of an element of normal iso~opic campositiono The concentrations of rubidium and stronti~~ in the Rb81 and ~r86 spikes. respectivelywexe determined by ieo~ope dilu't1on uSing such Ctshelf" solutions. Ths rUbidium shelf solution was prepared by P~ofesso~ w. H. Pinson by weighing in a known vol~~ purified Rbcl and dissolving it of demineralized water. A temperature corrGction and a corgection for the pressnoa:of 0.52% ~~re applied. tion:ls It The calculated concentration of ~h1e solu- 140.~.9r~l\b/mlo . 'lWO 1ndepende~~ gravimetric deteg'CD mdnations using ru~1dium perchlorate pzecipitatio~ averaged 142.;Ug~ Rb/ml. This was considered agreement and the calculated used in the calibration The strontium value experiments to be satisfactory of 14009,)'fgrRb/ml "'Jas (Pinson, 1960, p.242). shelf solution was o~19inal1y prepared by R. P •.Cormier on 3Uly 2. 1956 by weigblng and dissolving strontium carbonate reagent (Eimer and Amend, lot 492327). The concentration of ~his solu~1on is 28~9~sr/ml. portion of 1~ was diluted by Pxofsssor W. 8 June 24, 1960 to 14~O~9~sr/ml. 0 A Pinson on Cormier's solution was mentB {(;hapter 5. Table 502). The isotopic composition of the rubidium spilte and concentration of rUbidium in the solution were each Analytical Data for aba7 Spike Calibration. Table 4.1 Shelf spike ~rlmlo , Date 85/87 Qual 2/20/60 .01760 Fair 3&10 2/25/61 .01789 Good lal 1.0158 64007 2/28/61 .01787 Good 1:;1 1.0329 62.30 ~veraga .01179 .000094 ff .::t E =:t Data 0 2/26/61 . 85/87 .4320 63,,30 :t 0 ..52 .t;. .82% .526" :2 d2-.. ) n(n-I) E 'eT )( 100 ) t1 ::: rnePl'} 11 The abundances of Rb8S and RbB7 ~~re calculated the average measured 85/87 ratio as follo'tr~s; 85/87 87/87 Total' = = = O.01779Z 63.49 0.000094 1.00000 ~ 0.0 1.01179:t 0.000094, .E d: O.,009~ from -E% F~actional atomic abundance :t: 0.000092 65 ~ Olt01747 , Z 87 := 0098252 The ~~'k 85 = 000171 = 0.0923. are therefore identical experimental 00527 :t. 0.0092 0000009 Ridge analysi.s of this and 87 ;!;; spike ~Jas The resul~s obtained at Mo!oTo to the Oak Ridge analysis within elt':£"oZ'o The at.oudc 'i;leight (physical scale) dium was calculated}disragarding of the spike-rubi- the packing fraction for tbe two nuclides. 8S X 87 X O.01747Z 0.000092 0096252: OoOOO~9 Total atomic ~~igbt If Nand 87 a ,8 1.4857t. 0.00782 85.479a t. 0.00783 8609649:= 0.011 of S are the ~otal numbers and spike-rubidium, ro,85/~ = respectively, in am1xtura (0.13%) atoms of no~al than the resulting ratio of spike and normal rubidium is equal to x. = o. '215M + O.2785N+ O..0175~ '\\lhere the atomic abundances for normal rub1diumare Equation (1) O.982SS the isotopes of aseumsd to be Rb85 = 0.7215, Rb87 = O.2785D (1) is solved for the ratio.! which is t:rans S formed into a wei9ht ratio through multiplication by a cu ~~'Pffi~1 .. :flu Atomic: 'W"'eight of s~1ke: Rb A'tom.:tC \1Jeight :; F :: 85.557 ~ --A:.~ , 860965 Th~ t-Jeight ratio is then solved for N, the ~Jeight of normal rubidium in the 'sarnple,b~: substituting ,values for S, the weight ofsp1ke-Z'ubid11J.ID addedo In order to minimizae~ro~s samples in much a way that ~e it is desirable. to spike of rubidium, as/a7 resulting be ~loBe to un! ty •.. Because basal ts 'can'trat1on~ appropriate ratio will cont:C:~iin. very a il'~re dilute lOt~ c:on- sI>ike was prepared and was usao in. almost all analyses' gepozted here • . ,. 'rhe dilute ~87 ep!ltO was prepared ap1k0 containing as follm-Js: 63029~$rRblml 10 rol of were p1pe~ted in~o a clean volumetric flask and diluted ~o 100 rol with demdne~alized water at 29° Co A tempsrature correction both to th.e .10 ml volume a~ corrections the final w8smade solutiono The we~e taken 'from the Handbook of Chemistry P~YSiC8 (~951-1952" po 1791). Actual volume p1petted at 200 C _~, D J 10 1..002275 :: 9.9773 ml. and ~mount of Rb delivered = This amoun't'\~asdissolved Pinal concentration The concsntxat1on = B 631.46~gr$ in -l&~2 ... :: 99.773 ml of 1.00 75 water 6~1~46;U.9!:S.:;: 6. 33 ~j7Rb/ml. 99.173 mt /. of this spike solution was checked by an isotope dilution measurement solution. 63.29 x 9.9773 "lhe result: using the nOkTaal rubidium shelf obtained was Gc. 581".9?'Ym'~.(ml Wllich is in satisfactory agreement with ~he calculated value. Never- ~hsless. the calculated value was prefer~ed. A further chec~cwas made by analyzing the rub1d1~ content of R1292 using both the dilute. and the concentrated spike. The results age identical within experimental Calibration of sraG spike Solutla~. The 1so~opic composition and the concentra~1on of strontium 1n the .Sr86 spike solution were determined triplicate isotope ratio and isotOpe dilutiOn runs. respectively. Table 4.2. 'Ehe analytical data are presen1:ea in by com~~~ l)~~rsptopie Date . ion,,86L88 87~8 Pair 80014 72 GOod 1=?3765 .. 0077 90 Good 12,,4216 .0076 1.4176 12v4168 7/25/60 1.40.02 12 4715 2/27/61 1..4002- Average 1.4060 - ::t -E :t 0 :t. 00275 .0058 0.413% . ..to.221% Shelf:a~ike 2/26/61 2/26/61 Qual.,.4!.. 10 6/22/60 c:r . Soans 8!r~8 :t..OOOO9 :tl.16~ sr~PfJl:Imi. 86/88 2:2 1.5110 21.54 . '2:1 0.8537 21 44 281 0.8577 21.,56 6 .~.51 . Z 0.037 :AveX'a~~ .0=- - E ) .-E. - - t. O.17~ 0- t1 Shelf solution contained 14.03~~rsr/mlo . ~le atOK~c \~19ht (physical scale) was calculated disregaxding the packing fractions for the different nuclides: 88/88:: 1 ~OOOO::t0.00 87/88:; 1.4060 :t 0.0058 86/88 = 84/88 = Total 12.42,16:t 0.0275 0.0076 :!:. 0000009 14.8352 t 0.281, E a: O.189~' Atomic abundance = 0.06741 :t 0.00013 ;t 0.193 87 '::: 0.09471 ~ 0.00043 't. 0.455 86 84 t 0'.00243 % 0.290 1:. 0.000006 :! 1.20 88 III iii 0.83731 0.00051 Atomic weight (phys~cal scale) = x 0.06741% 0.00013 S.932oat 0.0114 87 x 0.09477 % 0.00043 • 8.24499:t 0.0374 96 x 0.83730t 0.00243 II 72000780t 0.209 84 x a.OOOSl.:! 0.000006 , 0004284 t 0.000504 S8 TO'tul' • 86.22771 = 86.228:t 0.213 (0.247%) The ratio of sr86/sr88 of a mixture of N atoms o~ normal strontium and . to S at:omsof spike strontium 1s equal \1: O.,?986N O.8373S' J O.B256N 0.06745 The weight factor \~~ichconver~s the atomic ~atio of N/s ). to a weight x-atio is Atow,j.c \I:ei~bj; of ft0zma1Sr Atomic \ie1ght of spike Sx: ...... :: ~ 86 ..228Q . 1... 0112 that. the st.rontium in the unltncwn has The assumption a "no~al" composition is usually not strictly tgue for geological mate:rla8n. Normal stron't1um is sr8'/Sr86 the foll~~in9 isotope ~atios: sr86/sr8S = 0.1194 and s~a~/sr8S = assumed to 1'1a<'''ia = 0.712, 0.0068 (N1er, 1938). Many samples ~nalyzed for ~he purpose of age aetermination by 'the Rb-Sr method aze enziched in the Sr87 isotope. Por bast results it is 'therefo:re desirable stitute the proper1sotopic' abundances in the sample into equation (2). fogosamples which ere strongly sr87• AlIDost all samples 'to sub- for the stron~lum This is part1Cl.l1arly enzoiched in radiog~n1c analyzed hereaiffered very s11ghtly from eJnormal~ strontium ~d only all concentra- ~1ons could therefore safely.be calculated using the . equation as given above • . 84 Calibration of the Sr . SRike • .I Only a few analyses :7" t;rue r \\7ere made ust'ng tb1s8p11te and reco%tl in Table 4.30 Table 4.3 Atomic Abunaances'of Oak Rid99 the Isotopes. M.I.'!'" 84 .543 .002 .5356 86 .137 .002 .1401 81 0038 .002 .0428 'ss .262 0003 .2808 The M.l.T. values are averages' of two independent detexminations. An ieo~cpe dilution measurement gave SX'= 19. ?o/i.fr/m1. The calcul&ted value is 19o~.9rsr/ml indicating 1.0253. very' good a.greement. The weight factor 1s !reciaion o~ Rtiliidiuman~ ~t~ontium Analj[~as~ J. The rep~oducib111ty of isotope dilution analyses of rubidium and strontium was detexmdned by making triplicate analyses of several samples containing t~ations of the 1:t'1O different concen- elements. 1\11systematic errors were removed as far as possible by ca~eful spike calibra~ion~ and by m~~ing a blank correc~1on to all determinations. -A ~emperature correction not made because it is Rubidium to the volume of spike added was inaignlficant.. C),nd Strontium. Blan~~ The blanks were dai:e:rm1ned by mixing of 'fd)87and SrS6 spike in a p~ crucible i:hen subjected to the eni:ire procedure prepare X'oc:ksamples Compaxable amouni;s of 0 known ar.~nts The mixture was o:c'dinarily used.to for isotope dilution analysis. reagen~s were use~ 'throughou't and the blanks were put: through an lon exchange colwun which had been cleaned ~av1ously in the .customary way. the resulting isotopic compositions of ~e Finally rubidium and tbe strontium were dei:erm1ned on the mass spectrometer. The amount of contamina~ion of each element was calcu- lat.ed by as~umin9 it. to have'nonual isotopic compos1tionQ During the course of this i~vasti9ation four such rubidium and st?ontium blanks were determined Rb~r. srp£ O.173'Cl 0.425 ,9'/60 0.0826 2..;75* 11/60 0.0560 0.138 2/51 0.0636 00183 Dai:e T (Table 4.4). "i '8/60 " b~cause they ar~ unusually high and above the general level of the ~thers. corrf9ctionsare :Rb The values used to make blank = O.~67S~$r, Sr = 0.249/9Y The ma9ni~ude'of the blank co~rsc~ions to 0.5 gram Table 4.5 Ma9n1~ude of RUbidium ana Concentrat,ion of Rb & srcorrecUons. Blank correction S~ in rock (pmn) 1 10 100 Sample slse Stron~1um Blank Rb~ -50 -13.5' - 5 - 1.35 .S = 0.5 grams. - 0.135 made on four saraples. by ~out1ne procedures Bach of 'these s£b."\1plea\t.Jasanalyzed used in tl1e Geochronology Laborato~;y at M.I.T. Therefore the spread of tho da~a at various centration lave].s is a ~ea11st;.1c indication duc1b111ty (Tabla 4.6). ~Table 406 Summaryof Triplicate of the repzooc," Rb a.nd Sr Analyses. Sr p~ 394.9';'t,; 402.9 397.1 398.6 :t 2 0 0 :/:.o. 5~" :t 3.1 ~ 0.93% 131.7*11<11 148.3 149.7 149.0 :t 0.7 :t 0.47% 'i: 0.99 :!:.: O. 66S~ lll' Using concentratl!dRb97 spike (63.2~..9'- 1m!). t/. USing f3r8~ spike. G** Spiking in doUbt. C011c:> Om1tted from. the average. (continued) -163- 'fable 4.6 continued -, Saniple ~Yo. P3111 pyroxene concantr. Average ,c- Rb ppm Sr J:mm 12.7 272.7 13.2 13.0 257.9 254.9 13'.0 ' 261.7 E t 0.147 :f: l.l~ c:r :t E. % 1.96% 0.255 ;t 5.6 i: 2.14" 9.6 z' 3.67% :t: 17.50 0.92 1.41 03111 Garnet concentr. Average 1.03 21.80 17.70 1012 19.00 Cf - 1. B :t 13.~ rr z 0.148 0.257 1: 22.9% E _ 0-= 2.~ Z dfl. :t ~ ~ 1.40 7.37% 2.~2. !: 12.7% x ) 100 'n(n-I) por ~he purpose of evaluating of an individual a single analysis the precision error analysis the standard deviations «(/) is most applicable. In general the precision of rUbidium analyses in material lngN10 ppm Rb is of the order of ~ 2%. tion decreases for contain- As the concantra- to 1 ppm the error increases rapidly to material containing 150 ppmsr or more is of the order of .; 1%, but may be as higll as::t4%. At 'the 20 ppm leyel . the .precision appzoach~s t. 15%. From the evidence 'pgsoo sented here a conse~vative est~ate precision erxoor of a single analysis robidium and strontium of ~e probable ist 2% for both whe:ce Rb;? 10 ppm and sr:; 150 ppm. It is of interest to noi:e that the rubidium and strontium concentrations of R1292 which were made using .. t~econcen~r&ted Rb87 spike and the. Sr84 spike, respec~ t1ve~Ytlare in perfect This gives confidence . Rb87 spike agreement \'11:'th the other valusso in the calibration and the 8%,86.spike of the dilute which \\'ere \lsed on' almOst .all the other analyses. The accugacy of rUbid1um analysss is indicated by a comparison of the r~sults obtained 'here for. aRbCl soiut1on prepared by G. R. T111;on:to ~ose of several other laboratories (Pinson, 1960. p.244). was analyzed in triplicate different' by Professor ~s solution W. H. Pinson using pgooportlons'of sp11te and shelf solution. resul~s given here ha~e been.recalcula~eduslng the The la~est calib~a~ion data fox the ROB7 spike solution T~le 4.1 Analytical Datafor ..Tiltont.s Shelf Solution. Spike: shelf 1:1 0.6355 . 69.,41 1.009 68.17 67.69 68.18 68.66 1.263 .1.254 AveragG Concentration: 6a.79~er/ml. if = t: 0.36(0.5%) a- = t; 0.62(0.9%) The reproducibility in te~s of tho standa~d deviation for a single analysis is Z. 1%. This is a. m1n1mum "slue because these analyses did not.'include the chemical cessing used, fo~, analysis pro- of rock G&~ples~ The aver.age of t.he M.l.T. analyses is identical w1thin experimental erxoor to ~e Table 4.8 Interlaboratory resul1:s obtained elsewhere. Results forT11:ton's.Rubidlum Shalf Solution -- It!!JI' Iml .. Laboratory ~ M.loT. 66 8:t 004 0 Carne91(~,r;.Insti tut.ion GastD University Minnesot~ .. , of 6705 6805 . Chapt.e&- S. fgEASUREMEf.\lTOF THE ISO'rOPIC COf.iPOSITION OF STROIG~UM~ ~ntroduet1on •. The cbemical procedures ~nd instrumental used in this investigation techni~es are not new but were developed f' over a pe:e1od of years by members of the ~eoch!:onology Laboratory bave Vag-laus"aspects 81: M.IoT~ been described in 'the Annual 'as,in the geochemical literature of these procedures Pro~lress Reports as wel:". (Hart, 1960, p.169, :Pinson, 1960, p.237J sexzog et al., 1960, Herzog et al., 1958, Herzog and Pinson, 1956). In this chapt:er these analytlcalt:echniques are briefly summarized foll~ by a discussion of.measurement precision ,and absolute accura~ ey. Chemical Procedures 'It For strontium isotope ra1:io determ1na~ions roek. samples were ground to a'grain size of less than 250 m1c~onsin ,a steel, percussion mortar. Approximately 0.5 grams of the powder, which was well mixed by rOlling it on a sheet of paper, w~s placed in a Pt crucible digss'ted :i:nabout 2S rols of a 10:~. mixture and of:HP and on a bOilin9-waterba~1n a fume hcod~. ~ilic~ 4 . was volat.ilized as SiP" by a reaction.with Fions and a 4 ... H S0 - 2 . . solut1c~ of soluble sulfa~es wa~ obtained (Vincent, 1960, p.41) .• '1'he decomposi'tion process Z'equ1red from 3 to 4 hours. The resiClue:was dissolved demineralized water. in about. 50 mls of "the solution was stirred clean Teflon stirring. rod, if necessary. of the. residue. all of HF the almost to dryness. 1Ina~ely 2S.mle of 2N vycor-dist111ed thus converting to dissolve In ordaZ" to remove all traces water solution was evapora~ed w11:ha Approx- HCl were then added, the salts to chlorides. The solution was'.cooled and f11'tered into a pyrex beaker. for t~mporary storage. " Strontium was separated fx-omthe other elements by .cat~on e2tchange t:echn:1quea using 12~.cross-linked eluan't. ~esin and 29 vycor-d1st111ed The exchange columns ~X'e successive ,distilled Dowex. SO, 300 mesh, samples by duplicate cleaned washings Rei as between with 6N vycor- Bel" followed by 21fHCl or dew.nex-alized water. Before the solution was put on the column it was filtered a second time ~o remove p~ecipitatee which usually . ). formed on standing (Rei, ~aC12)e A few mdlliliters of.a solution contain1~9 radioaciive Bras were added .to give su~fic1ent activity so that the location of . 'the si:.rontium en the ion exchange column could be detected "lith a .Geiger Counter. was .then levelled column.by . T11etop of the z:esin . ana the solution means .of a pipette. 'Was pl,acea on t!-:~e Care \'Jas taken not 'to lJ disturb the surface of. the. resin to obt.ain the best possible separa~1on of strontium diuman~ potassium. The order and alkaline earths are eluted Ll, .N-a, K, Rb, Ca, Sr (Conn1er',. .fraction containing from calcium, rub1~ in \tJh!ch the allta11s .fhOi'D 'the colUmn is 1952, p.7.2). The the stron~1um waG collected. in 25 ml po~t1ona 1n 3 t:o 4- pyX'ex beakers which we:r~ 'then evaporated 1:0 dryness. the greatest. The beakers containing amount of ~lct1v1ty were selected residues were d1ssolved1n and the a few ~11i11texsof demin. , erallzed water. This.stron~ium concentrate was ~tored in a clean 1 polyethylene OZ .. bottle until it could be analyzed on.the mass spectrometer. Whenanalyzing e'trontiu..1U in basal~s it \i8S usually all soluble oxalates. capillary Next, it was picked up in the tube' and deposited in the cent.er of a cl@)an tantalum filament in the source of the massspectromet.er. 'rhe liquid was evapora'ted by passing a small current. '(NO.5 amperes) through the filament until sampla forme(! a small wh!te cake. Th~ filament. the ~emperaC) ture was raised for an instance ~o red hea~ ~o expel occluded gases fram the sa~ple and to convert the o~ala'te1:0 the oxide .. and/placed in the tube of the mass. spectrometer preparation '.me source was now. re-assembled in for the analys1sG Mass s~ctrCmet%ic Teehn1croes. The instrument. 1s a 60° sector, 6 inch radius, solid source, single collector mass spectrcme~er which t~as built at N.I.Te lot employs a magnet: sweeip and the ion current 16 amplified ing reed electromet.er cOZ"porat.ion, Pasadena, by means of a vibrat.- (Model 30, Applled.Physics california) Cl Samples were mounted on a tantalum ribben (0.001 x 0.030" by Fansteal Metallurgical co~ra~1on" Nor~h'Chicago6 IllinoiS} spot welded to posts in 'the source. Operating 'p~easures ranging fr~"'l1 3 rom Hg waze obtained,by,means x 6 to 10.... :l' 5 10-7 of a high capacity mex:cury d1'ffusion' pump using a large cold ~rap cooled t'.1i th liquid nitrogen coupled'with Scientific (i-1 8. Ma~t1n Co." .Evanstcn#\, Illinois,>" 0 a fore-pump (Duo-Seal Pumps, W.,M. Welch Co., Chicago).' \1ere recorded on a The ~alts :Brown Electric Strip Chart. Recorder (Er~;n Instrument Co., tiphiladelphia). After 'the 'sample l1ad been applied to the filament tbe~source waS returne~to the maeB, spectrometer bOi:ted into',place. ' To ensure a tight.. seal a Aluminum,gasket was used .for each run. mass .spectrometer was than evacuated. "measured, by means of.an and l'leW The tube of ,the Pressures ~re ion gauge (CJ.lfpe DPA 38 ,Consoli- .dated. Elect.rodynamics Co., Pasadena, California, D1st. Prod. corp.). Whent;he pressure formerly had. fallen .about 2 x 10-5 mm Hg (usually in less ~hanone t.o hour) ~e filament current was turned on and the sample was con,ditlonod ~or 2 ~o 3:hours at.a temper~ture just belOw ..t;hat"at which strontium ion emssion' begins. ing the sample in this Condition- ""Jay allows r\.widium conta1lt)ina'tion to be burned off, gives abetter operating vacuum, and imp~ove9 the aenBitivi~yof ion emission du~ing ~he run. filamsnt,curxent was ~nc~eased alowl~~ s~le p~event the from falling off 'the filamentJuntil Of strontium ions began to increase the peak heights \fJ6X'eJ the emission spontaneously When D great.'enough to be in t}~erange of 1;he 100 (81;87, sr86, srS4) .and 1000 milll~~o;Lt (sr88) scales of the V.R.E., from SO to 100 consecutive scans t'} were reco~ded wltho~t further changes in 'the. filamen<t In this 1nves~i9ation an ins~rumen~al precision 'for 'the sr87/sr86 ratio of better than 0.1%was desired. Experience .sho~~d tha~' such precision by recordingbetwean bfusing could be achieved 80 and 100 conseQJu'tive the 100 an4 1000 mv scales on the V.R.Eo twq'scales are least sensitive and small irregularities to hi.gh';. 84: to' 88, is molecules lit (These ~requency noise in the ion emission.) The.re~olutlon of.1:he instrumen~,1nthe mm,8<) •. scans arid mass range adequate at prestSu&"es.' less than 5 x .10-6 higher pressures causes broadening i~ overlap of the pe'aks. collisions of ions with gas of the ion beam-resulting The Sr88 peak i'n particular intezfs~es with the smaller sr81 peak with the ~esult that the measured values of the sr87/sr86 ratios are too high. In this \'Jork pressures o~3leas than 2 x 10-6 rom of Hg were prefer&-ed for a reliable the Sr 87. ISr 86 measu&"ement of ratio. After s~rontium isotope ratio and isotope dilution runs the filament was cleaned by heating at maximum tem~rature until no trace of strontium or rubidium could be detected at a filament current higher than that: employed during a run.' In this ~lay up to six or eight consecutive samples ~~re run on the same filament without danga~ of contamination. It was found to be safe to interspace strontium iso~cpe dilution and isotope ratio, runs although 'this was avoided as possible in' this project. as much After making a 1~b1dium analysis the filament must be discarded because rubidium is difficult to clean off the filament. After completion of the run the chart was analyzed as follows. pencil Baselines 'tiara drawn wi1:h a sha11:pNo. 4 and peak height.s were measured to the closest 0.01 inch uSing an engineering ruler. The peak heights ~~re averaged in se~s of six and isotopic ra~ios ~~ge calculated for each set. valueS foE:'the entire ratios obtained The final average run \~re calculated for all 'the sets. from t.he The number of scans. i~cluded in each set should be even in order ~o remove the effact.of increasing or decreasing peak haignt:s from the isotope ratios. An instrumental aevia.tion the form of the standard culated for the average sr87/sr86 precis10n error in of the' mean was cal- ratio of each run. This ,erEor was used t;o judge -the qualit.y of the measuremente It will be discussed in more. detail in 'the next section. p~scussion of Errors. In'trcduct1on • . . In the following discussion bet~en m$asurement pxecis10n 'the absolute ,accuracy. or a distinction reproducibility The formsr lethe errors arising from several sources. is made and sum'of randcm The magnitudes of these rand~~ errors in a mass spectrometer cannot be predici:ed wii:h certainty. In thissi:udy the x-eproduc1 b111ty of the sr87/sr86 ratio was dete~lnad from a number of analyses of the same sample and was e~ressed Ct as the standagd deviation for a single analysis calculated,fzam these results. 0-== ~f 2,', d::' of a nUiuberof measurements ) I'J -I average' is M implies tbat an, additional the a8lT~quantity,has the band M .Z The standard deviation) whose measurement of a 66" probabilit.y of falling inside 0- • The absolute accuracy 1s. tIle algsbraic sumof all systematic. errors affecting a series of measurements. Again such e~rors arise from many sources and are difficul~/to evaluate in ~he. lengthy and complex procedu~e used to dete~ne isotope ratios. Because the ~rUe value of the quanti'ty to be measured is usually no~ 'known. lt 1s ,customary t~ analyz9 inter-labora.tory II standards to ensure 'that at least, a certain degree consistency is maintained. / The standard used here is a S,X'C03 by Eimer and AmendttJhich has previously been analyzed by Aldrich et ale (1953) and Herzog et &l~ (1953) .' The p~~c1sion errox or xeproduciil11ty measuzoement of the B-;:87/sr86 of ratio of a 1s 1:he s~ of -seveX'al random errorso Several of these elrZ"ors, \ih1ch are listed below, may be systemat.ic for a single measurement but are random for a series of repeated maasurements. They therefore affec~ the reproducibility and are included These ares in this discumsion w1~ the truly random erxors. 1. 20 Contamination of the sample with variable amounts of C:normal" or: I'8spikeco stront1wn during chemical process ing or mass spectrometric ana~y~is. Variable effects evaporation from t1on, deflection 't1on of 1cnso 30 of fractionation the fil~nt, ~y.the magnetic Pluctua~lonein~s of isotopes lD auring ioniza~lon, accelera~ analyzer and collec- rate of e~esion of.ions from the filament caused by inadequate temperatuxe cont!:'ol of the hot filament, chang8~ in. the resistance of the filament, excessive bulk or near-exbaust:ion of the sample and other reasonso 4. High frequency electronic noise as ~el1 as instrumental drift due 'to 'temperature changes, interference from 'the operation of other electronic equipment,etco s.: ///Lack of sufficient resolution of peaks due 'to bJ:oaden1ng of the ion beam as a resul~ of collisions of ions wi 1:h gas at.emv:; in the mass spectrometer tUbe. The resolution is also affected when 'the ions are not mono-energet1c because of improper alignmen~ of the fil&~nt, the sample and the slit of the accelerating. p~a'ts •. 6. Instabill~y of ~he magnetic field and of the accelerating voltage. Low frequency noise in i:he st;~ength of the magnetic field results in corresponding 1ns'tab111ty of peak heights •. Var1at:ions in the accelera'ting.voli:age affect 'the spacing of peaks but may also affect t:he peak"height.if such changes occur just before the recorder reaches the top of a peak. 7. Uncertainties in drawing baselines and measu~in9 pealt heights with a roler. Of these sou~ces of errors contamination of the Ba~ple, and poor resolution may be systema- isotopic fractionation tic for a single run but differ from one analysis to~he next ~hus affecting the reproducibility. An isotopic fractionation all meaauredsr87/sr86 correction was applied to ratios on the assumption that "the sr86/s~88 ratio is constant and equal to 001194 (Nter, 1938). Any deviation from this value was asc~ibed to the effects of isotopic fractionat1ono Only one-half of the correction needed to xeadjust the-sraG/SraS ratio was applied 1;0 the sr87/sr86 ratio because "the difference in 'the masses 86 and 87 is aa only one-half' tha.t between 86 and Por example, 91ven that. 0 s%86jsrSa s?:87/sx86 = = 0.1187 0.112 then, (sr87/sr86) cOrr - 0.712 00.1194 = 0.7099 0.11905 "Even though the exact nature of this tionatlon in ~he mass spectrometer isotopic frac- is not understooQ, -Xi'2- lanonnalization Q of the sr87/S~86 ratios is justified the resulting improvement in the precision and by the fact 'that the mean of several ,standard (Table ~able 5.1 by of the data analyses is not 5.2). Xsotop1c composition of S~rot:!tium in Basalt fX'~ Ki lauea. Hawaii (lR1292) ES7 No.of .. 86 (87/86~orr 86/88 84/88 scans Qual. e7068 .1196 .0010 138 Fair .067 .7012 .1194 .0063 84 Good .7090 .054 .7069 .1187 ~OO66 84 Good 4) 1/25/61 .7040 .13 .7058 .1200 104 Pair 5) 2/8/61 07091 .073 .7061 .1184 90 Good Average .7077 No. Date 87/86 1) 6/1:/60 .7062 0091 2) 8/31/60 ~7102 3) 9/3/60 -- .0065 -- :t 0.00114 .0066 .1190 :t 0.00026 :t 0.00033 t 0.00015 :t 0.16% :t 0.037" a- t 0000254 ;t 0.00059 :t 0.00074 :!: 0.00029 E :to 0.36" ~ O.OB~ 0=E/' / .7066. ~ Z 0.28% t: 2.23% 0.62% ~ 4.45% 'the results in Table 5.1 show that the reproduc1b111t.y of ~he sr87/sr86 ratio, expressed as the standardd3~1atlan for a single analysis. is improved by about a factor of four by the fractionation correc't1on. In th1s set of measurements a ~eproduc1b111~y of Z 0.0006 (0.08%) has been achieved for the corrected sr87/sr86 ratio. The column headed E87/86 givas the inst~umental p~e~ ciaion er:~or of thesr81/srB6 not identical with ~e' but indicates Th1~ 'er&"o&" £s for eacb run. rep~oducib11ity of the measurement only the extent of varia1:ion of truly randam erro~s affecting the particular mass spectromstzic measure- ,mente The quality' of a run was determined partly on the G---. basis of this instrument precision erro~ of the sr87/sr86 ratloand partly from oi:her criteria of ion emission, t:he the run 'and evidence of excessive frac- of scans recorded, pressure during t;1onai:ion. t)GoOdIll, 'the stability As a. x:ule me'asurements having an instrument error of 0.08% w1th errors or bei:ter ate of "Gooa" quality. caFa1r"~ or apoor" on the basis of the other criRuns having an error greater than 0.25' were considered ~e Those between 0.087£ and 0.2% a&"Sjudged to be terici mentioned above. discarded such as the' number to be "'PoorIt and were geneX'ally or.repea~ed. xeproduc1b11ity of the s~86/sr8a ratio in Table 5.1 1~t;O.6~ whereas ~bat ratio 1stO.36~. the sr87/sr86 of the u~corrected sr87/Sz86 This supports tbe earlier assumption th~~ ratio is. subject to only one.... half of the -ISOisotopic f~actionation effect reco~Qed in the s%66;SrBS ratio. The ~eproduc1bility of th~ sr84/sr88 ratio as . . . indicated in Table 5.1 ist4.4S~• . AccuraCY... ~ The absolu~e accuracy of isotope ratio measurements is diff1cult to assess. demonstX'a~e ..consistency Ir-. made elsewhere. The best that' can be done.1s to of the measurements For 'this purpose with ot.hers 'the strontium of a S~C03 reagen~ (Elmer and Amend, lot 492327, Co~ier's shelf ~olut1on) was analyzed at intervals throughout course of the investigation. Table 5.2. ~e the results are given in -181- Table 5.2 No. Strontium Carbonate Reagent, Eimer and }\.m.end lot 492327 (81/86) 87/86 Date 86/88 corr 84/88 Qual. Good .7114 .7116 .7117 .1200 .1195 .0068 .0066 Good 6/30/60 .7133 .7130 .1193 .0064 C-ood 4 7/20/60 .7133 .7127 .1192 .0062 Good 5 8/21/60 .7136 .7127 .1191 .0069 Good 10/17/60 .7128 .7116 .1190 .0069 Good 12/1/60 .7111 .7102 .1191 .0067 Pair .0064 Goed ........ Good .7098 4- 5/3/60 5/15/60 3 1 r#. 6 7 v- ".-1 8 2/13/61 .7083 .7077 .1192 9 2/27/61 .7093 •7123 .1200 . 0.7114 Average "" - :!: 0.00064 j; a- Z. 0.090% 0.0018 x :t 0.077" t:. 0.0016 B ! "!; 0- 'E 0.25% 0.00055 spread ,than the uncorgected are .zuns #7 and #8. 1:. 0.0066 0.00016 :C 0.00008 ;t 0.134% :t 1.23% Z 0.00045 :to.00023 +' 0.38% 0.22% sr87/sr86 The corrected 0.1194 0.7115 ratios The values ra't1os of these two runs differ of :t 3.48" The only ~e Run #8 is outside therefore rejeoted. ana Sr87/sr86 from 'the mean by 0.8 will occur in a ;normal population (Dixon exceptions correc~ed and 2.4 (J. Ths probab111:t1es that these or 0.8%, respectively smaller have a markedly ratios. and whose standard.dev1~'tion.cr - lO\«3X' a- values whose mean is 0.7115 = 0.0016 Massey, are 21.2% and 1957., Table A4 ). 'the cus1:amary 5" confidence limit and is with the ~ssion of run #8 the isotope ratios assume c-r average valueSe the following r:r sr87/sr86 - 0.7119 ,z 0.0006 , ~ 0.0016 (S,r87 /s~86) - 007120 Z 0.0003 , Z 0.0009 0.1195 :t 0.0002 , :t 0.0005 0.0066 :t 0.0001 , ~ O~OOO3 corr -- sr86/sr8a COlt sr85/sr88 It is apparent tha~ the average sra7/sr8G rat.io is identical v- to the corrected value of this 'ratio. of the latter is Z. 0.0009 or Oel~. The results of this, series.of favoura'hly with those previously for similar gatora The reproducibilit.y measurements reported strontium compounds. work, is given in Table 5.3 (Also chapter Tabla ,5.3 Isotopic compare by other 1nvesi:1- A summary of 'this 3. Table 3.1). composition of St.rontium in Chem1cal Reagents. Aut.hor 87/86 86/88 N1ez:, '1938, O~712 0.1194 0.0068 Sr met~l 99.9~'pu%o, p.277. Aldrich,' Z 0.007 et af.,1953, 00711 ":t 000004 8A/a9 ,Remarks %0.0012 :to.00014' E~mer and Amend. 0.1195' 0.0067 :to.0003 %0.0005 SXC03' Average Eimer & Amend,' of 6 analyses. p.458. Herzog at 0.712 al.,1953,p.462. sr87/sr Schumacher, 0.1196 0.0070 sreo3' Eimer & A~nd, lot 492327. = O.0703t. 0.0002 1956,p.210. This work. srC03' Gen. Chem., C.P.,lot 10. 0.712 :t 000003 ~.1195 0.0066 %0.0002 :to.OOOl Srco3, Eimer & Amend, lot 492327, aver. of 8 . analyses in nine monti~s. Errors are the'st:andard deviations of the mean. ~ne agreement between the results of all of ~e analyses of 'the S!:'CO:;wit:h those previously ctbe~ 1nvest19a~ors gives confidence other isotope ratio measu~eme~~s. necessarily an indication 'stratesconsisi:ency. accepted w1~ eight reported by in the ~ality of Such agreement is not of accu~acy bu~ merely demon~ A single measurement. cannot be confidence unless' it 1e part of a set of lr- similar ~asuJ:emen~s with which it: can be compared. It is well 1:0 :rememberthai: one out of the nine analyses of the srco] bad to be rejec~ed even thou9h the run seemed 'to be of good qual! toy. Conc:luslons. The reproducib111~y of measurements is improvac1by as much as a factor ratio a fractionation of the sr87/sr86 of j!our by making correction based on the assumption that the sr86/srS8 ratio is ~. const.ant equal to 0.1194. The reproducibility of an oceanic basalt .. :t O~OO~6 (O.O~) by repeated (R1292) and a srC03standard analyses is and':t 0.0009 (0.12%), respect1vely. on these results of good quality demonstrated the reproduc1b111~y is aBs~d to be Based of a single analysis t:. 0.001 or 0.14%0 The instINman~ precision sr87/sr86 ratios is a quality of a run. error for the useful crt'terion re...eaeureri!.snt for judging of the Because it: is sma.ller than the repro- duc1bl11~y it is unrealistic to claim a precision error of -0.001 (0.14") for the correc1:ed sr87/sr86 !:at;1c 'iFJhenthe instrumont error alone has a value of 0.14%0 ~e isotope ratios obtained for the are in good agreement Aldrichet w1~ srco3 standard resul~s reported by Nler (1938), al~ (1953). and Herzog et ale (195.3) and give confidence in ~he accuracy of ,~he isotopic analyses of s~ront1um in zacks reported in this thesis. -lSSca Tim ISO'lOP1C COMPOSITION OlF S'1'RONTlm~ !N OCEANIC AND CONTINENTAL BASALTS. Introduction ... Before the ratio of sr87/sr86 in igneous ~ocks can' be used as acritex-1on for a crustal or subcrus'tal o:ri9in~ the range of variation of this ratio in'rocks of known sub.crustal origin must be determined •. For this purpose a survey was made of the isotopic in a number of.oceanic composi't1onof strontium and continental basalts widely scattered geographic localities. distribution from many The geographic of the sample localities tests the lateral an~/or vertical homogeneity of.t~e subc.~stalsource ... ' . regions of basalt magmawith respect to the abundance of sr87• All conclusions.and calculations in this chap~erare based on corrected ,values of the sr87/sr86 ra~ioo The nature of the correction it are and the reasons for'making described 'in Chapter 5, page /77. 1ng the apparent homogeneity ~e conclusions regard- of ~he upper mantle are not affected by the use of the corrected sr87/srB6 ratios. as 1~4%.between success1ve analyses of ~he same sample (R1292,Chapter 5, Table 5.1~ P.I7S). Th~s variation is the result of isotope fractionation in the mass spec~rameteg. Because of Jot a reliable determination of the s&,86/8:("88' ratio of a sample should be based on several measurement.s spaced over an extended pariod independent of time .• Ev1.... de~ce that the effects of isotope fractionation .1nthemasa , spee~%om.eteraxe random is preasnted in Chapter 9 along with a d1sc:~s~1on of the.- numerical. value of the sr86/sr88 ./ ratio.in terrestr1almaterial and tektites • .The p~e~1Sionof the sr84/sr88 ratios (r5"). Prequently .n more'detailed account is glv~n in.Chapter 9. of. nib1dium and st.J:onUum in a number of basai~s were dete~ned method of isotope dilution in order. to cOmpare lev only 12 to 18S.X'84 peaks were scanned • The concentrations representative is very (Chapte:r: 4) 0 by the This was done the Rb/~r ra~los. of basalts to ~e calcula:ted .values of. ~he 'Rb/sr ratios in their. source regions. The RbISr J:a~ios of t~e magmatic calculated from the sr67/sr86 source mate&"1al W91:e ratio' of Recent basalts by aSBumi~g that the initial Sr81/sr86 "'z'O.C02 (Gas~. 19(0),and'thatthe 4.5biliion years (~apter ratio was 097004 age of the earth 1). is If,basalt or andesite magmai!oms by part1.~l melting, of rocks in 'the upper man~le, then the 'difference between the Rblsr ratios in bas~li;s ancI'the' calculated values of the source rcck,:is ~vlde~ce of magmatic'differentiation Q~ of fractional \r melting' of the source rocks to produce Another p~rpose of these analyses tbe magma. was to outline the r~nge of variat1on'ofthe concentrations of rubidium and strontium in oceanic basalts fer geochemical in1:eres't. Real variations "'- frOm of the sr87/Sx86 ratios in basal~s ~he same, or from d1fferentlocaliUes are an indi- , cation tha~ magmawas derived different Rb/~r rati9s. calculated from, several Acamparison of these ratios, w11;h the assumptions,out:l1ned found in different souX"ceshaving above, to those types of ,ultrahas1c and basic rocks 'may serve to give an,',indica'tion of the' nature of the source material. Analytical Resultso All experimental data pertaining to the basalts are compiled in Tabla 601. ~'de6cription of the epeci- mens as well as in.fomat!on c;.bouttheir .locali~ies is given in cnapter'lO. Oceanic Basal'ts. The localities an~lysJ.s are .Hawaii, Saiuoa, Ascension. Island, the Mid- and the. Azores •. In the 'following sections AtlanticR1dge .. from which specimens ware chosen for \.r the results will' be. discussed fo;g eachof.theae loca11't1es. Hawaiian Islands. The Hawaiian. Is~ands aX's a 9ro~~ of shield 'formed by flows-of basal~s'and primary tholeeit.1c either fozmed crystallization' of tholee1tic basalt magma or represent a ..different' . inatlng fromot1)er depth bf.th~s~ earthquake . ~J 1960,p. judging On the bas1sof the base of the. crust. (Turner 219). The isotopic compositions Bb/Sr r~t1oB of'five basalts were determined. 'l'he t~ be be.tween 48 and 56 is: estima~ which 1sWellb~low and'Verhagen, m.a~1a series. or19- . soui:.ce 8:'egions in the mantle. source. regions, focl, lava •. Th.ec11vlne derivatives their alkali-rich ';as,<a r~sult of fractional volcanoes. of strontium and the from the Hawa1ianIslanda The sr87/sr86 ratio for one of these, III k ~ :a-, en ~ ~ N .'0 11 tlt. ~ ~ Q • e 0 \0 a 0 en 0 <:> 0 0 (tI • '0\ f.D P) M " \0 I' C") Q) G'-f ....... • pj fI'J Pl 0 0\ ~I: to 0 \0 ". CO U'\' '. (;) • ~ lot 0 ,~ 0\ qt N '.-2 . M • 0 ,c-I '~ afl ~ ~ eo r-~ 0 fn", 0 ~; m A. \D 0\ C'\ tn • k "S Ii4 '8 8, l J 0) 0 CD 0 qe 0 "'t, 0 e '\0 CfJJ,4 fJ} • rat to ~ fW8 o &'"Q <EI ., ~ CO 0 V1 ~ ""'tdCO" ftS ~ ./ ~ faCJ ~ to \OP'tU\ m S OJ 8 0 \D Q(W)r-- ~ Q) ~ O\QCO PIC'" .... • o~ .... ~ ~ • 0 ~ C\ 0'\ to P4 p1 0 CO ....• pf O)pfN ~ ....00 pO F4 0 0 moo Ploa • • • M ...... \0 , • • .... l' Q ~ Cl 0 r• • 0\~1l\ \fIOO C"'t . 0 l"'" 0 ~'Q • G ~ an " ~0 000 .0 ' G 0 0 ..., +1 .. , +I 0\ P4 ~ O~'~ '0.0 0 CCLn '00 ' \.0 0 • 0 0.. ~ r:- ~ • • tI tl \!)o to' • M 000 .,.... 0 I' r- ....... 0 0 ..... \!)~C'l ~ \1) 0)' \0 00 00 0'0 0 lit .~ \0\.0 000 0 en ~~ ~P4N \000 0 PI PI 0 \0 0 • CO CO 0 '\0 '0 0 0 +. ~ @) 0 CD (X) k J.1 PI• N \0 0 000 Q .0 0 ~ Q') .-4 .~ \Oow ex> .,..u 0\ '(1) ... 1• ... ,• P4 ~~~ m or=f 00&4 00 00 • 0 oLn +, 101 w ~ \,(i lnf'l'lQ) 0 0 0 " ,QOga, '.~ I' ~ o ..... ~ cons 1ie +, B • ft:II fJ) r...;. S I'or-a 0 ~ ".... ' r-. ' .... '0 to AS U P4 fj cn~ .0 tD C tIS Q) S c-I .(JJ H ~ ........ co ~ tt: N ""0\ N ....PC a ,..." .~ G\ :s f""f N CD N "'" '" at ns cu ~ ~: . 0\ (000 &'00 • 0 IJ) of'f .,..e (D 0 0 t;Q k • ~ ~ tU .t ~ z :s to ..... PI 0 ....en' ~. CJ'\ rr.: OJ S'»-0 d) ~ &n 0 ....... • 0 ~ t--tnm O~ en 0 00 , 0001 m +I+,~,..., , .e,.I ~~ ';d It m! N~ a~ .... N Q 0 ~ ~ .s ~ pf 0) H .... N 0 ftJ N s: o 9:t ~ Ol\b b~ fG tV . ;> en >0.4 0;04 q,.s (l) tW f.e .,.,fa t2nJ~ ! Es CD ~ • (i) • C"'4 ~&! 3 GJ r- 0 ....t to ~ '0\ \0 ..... to ra 0 • +. ~. .... .~ • • • 1'1 ~ r-- r- 4J PI .C N .Q , ...co Ib b ns 0 ~ 000 0 \0 ,~ '\0 N cnLnO'\ r- Q 0 r-- 1-1 CD S Q) 0 00 CD G) .-ten l' r- 4\t 4lCI' \0 t' ~ ~~ .., 'M 0 ~ M <U ~ e ~ ,t>4 en oqs U') f'l M " \0 ~ ~ 0 ~. 0 • f) e an \f) • 0 k CO 8, Q, f) fi1 s: ./ CD 0 co ~ • ~ ~ • 0 an • in \0 .pO N • .c-f CD ) CO ~ 1-t tn 0 PI ~• • pt a- tt) +, 0 to 0'\ ."• 0\ '" • Ln fit • 0 pot c::I .. Con • Q) ~ P"4 0 1-t 0 0 an en 0 pf • 0 0 • ..... 0 41 +1" CD ~ 0 0 Q • 0 ~ ~ N N 1-' ~) .. co ~ \0 CD r-. (i\ ~ r£ ('. ~ i" CD ... "'03 prof. f'I4 • ,... 0) " 0 • • r- CO 0\ 0 ..... • Q .. 0 ~ \.0 0 0 CD .CO p-f .... ..... • ~a c;t 0,0 00 00 In \0 0 \0 t.n \0 \0 0 0 \0 \0 Q • • • ..... • (;) • 0 f) CO CD 00 • .... 1-1 N 0 .... 00 ('I) ...... • • .... ~ ~ en .-4 (0 H ~ II i co • \D 0\ 0 I' Q I"- 0 \0 • r- 0 .e0 (J\ t.n 0 0 C7\ 0\ 0 N N ~ .... ~ \0 0 • P) N ~ <:> • co f-f .... ..... .... ... • CD CD pt 0 o O. •• ...., +1 an .... • 0 ~fa k 0 Q) ~U) 11 l' 1'1' • • cnr- tnG\ ,00 f' ~. <:) r-- • • • & res '~ pot .... -0 (/) ~ .6J c 0 co C 4» u (I) ~ m~ • .• Ct '. O)~ P4&n c o 0 ((i ....0 ..... ~~ eeof ~ P'"I ~ P'a . "I 0 'd. N ~ i! 0 co 0 00 • ."• "• G) l' \D 0 co ...• It\ N .... I «d' ~ "' ... I""- \D 0 00 ~ f' • ~ 0\ 1 .... ON 00 Ib b • G) ~ .,.. C (0 :> f.Q "' .... ....ell ~ .m .eJ 18 m UJ IE f&.I l'tl O,Q ~ 0 \D "" ..... nI ~ : > u ~ 0 • ... 0'\ N -= . 1-1 +1 <U~ (Q • • -H "1-, • • • \D ~ OJ ~ • • .~,+, 00 0 r2 00 00 00 "• 0) 0 tot 0 +1 ~\O N ,.... 0 Pot " In N .... ~ ~ 0 "1-\ ....co P)pf ~~ l.O ON 00 CD o "" 0 0""' 00 a~ \.0. CSto CD 0 .... \0\0 0 ~ N ~ N \D 0 00 l' OJ aJ \011) f¥l fW\ • • 1-1 1-' :5~.... s.a ns c2~A I' <:) 0 00 00 OO ~ alan 0 00 00 Ib b ".... " ~ • Q. \0 .... "0\ .~ l' '" r-'" ....ftt 1 "S., 5 0 <:) C"'- l' 0 Q 0\ r- ON l' an \0 CO ......... • '-S) \0 1'0 ~cn 0\ 00 00 an ,.." • \0 Q S i:1t CD Q .5RS ....L' ....ns 0 ~ 1-\ tl CD ~ Eat N 0 \0 0 I"- GJ • 0 \0 \0 0 ... ~ ..-4 en \0 \D 0 o-t \0 • N Q ~ \0 0'\ tI I""- ~co \0 ~ 0) i' f' ro- .-4 .0'\ \.0 0 0\ .to ~ CO ~ ~ s:: 0 u \0 0 ~ 0 '0 0 co ..., ('I) Ul CIl m ~ ~t:: ra • M ~ r.s, J ~ ~ CD 'tf 0 ~ U /~ .... 'l;:;J' .... • PI .v8 (1) 0 \0 ...m 0 N U') ~I• tt..4 N (\oJ l' \0 0 pO CD 0 C\~ ... to cu CI> ~ ~ CD z.. to <C ". 't3 C ffJ • a basalt f~am Kilauea (R1292), wasm~asured The average S'i:S1/sr86ratio --+ , . ~ 0-- ..,;.00026, fo~ this .' five~imeBo roclts ,is 007066) , ' <7"== - .00059 (Chapter 5, Table Sol, po 178 ) 0 The sr87/sr86 ratios of the other basal~$ from Hawaii Island (R1282, R2001, and R1993) fall inside the band O.1066.:t 0.0012(20-) • They therefoK'edo not. differ cantly from R1292 at the 95% confidence signifi- limit although the possibility tha~ they belong to a,sllgbtly diffegent population 1s not ex~luded. :t O. 7069 :.t ~. 00025 .' srB7/sr86 All four basalts average fa: ) • \) The sr87/sr86 ratio for a basalt from Maul Island (R2002) is O.7~47%O.OOl calculated:from duplicate analyses. 'this' basalt 'therefore seems ~o differ the"other foUX'~ Ii: is customary 'to reject .. that. 'two means are identical .. them exceeds its own.error (Evans, 1955"p. D = 0.0022 'deviations 746). and.1-ts the hypothesis when the difference ai: the 95% confidence between limit In this case thed1fference 'eZ'J:ol:, ~., of the two means" the evidence from considerably calcUlated is% .00106. fram i:he standard Since D ;;- z.~, 1s suffic1sn1: to conclude that the two means ,are significantly different. of at least two source regions have been derived. t~ therefore have evidence from wh1cbHawai1an magmas The calculated RbISr ratios for the ~wo source regions are: the Iaiand of Hawaii P03:' Maul Island. (RbISr) 11~ts source (Rb/Sr) eOUICce = 0.022 Z 0.012. 0.034:t .011, C The errore are of uncertainty and do not necessarily reflect on the p~ec1sion of these values. ~ ~ampar1son ~o the RbISr ratios of igneous ~ocks (Chapter 2, Table 2.2) '-r material shows 'thai: t:he magmatic: souzce resembles .mos~ closely the basic: and ultrabasic 19neousrocks. Moreo'ver" it. is evident t~at neither chondx-1tes nor ~he achondri~es listed in Table 2.2 are a suitable source material for the Hawaiianbasalts. The 'Rb/$r ratio of the chondrites is ~oo high whereas that of the achondrites is 1:00 low. I't is unfortunate that: the data"on the concentra~ions of rubidium and strontium in ~l 'trabasic igneous rocks are so fragment.ary. survey of these rocks 1s desirable values of ~e systematic if the calculated RbISr ra~10 ~re to be used to identify the .nature of the ma~at1c eougce ma~e%ia16 The mean RbiSI.' ra~o 1~ O.024SZ:0.0005. for the three basalts The rubidium and strontium are remarkably unifoftl :t 6.2 ppm, A respec~lvely. from JCilauea concen~ra~ions and avez&ge 9. 56:!:0.31 ppm and 386 R1993, a basalt from Mauna. Rea, contains 46.7 ppm rubidium and 1249 ppm stron~ium but has basa~t8 areB = and RbISr Rb m lB.at' 9,,3 ppm, 8r aD 601091: 209. ppm O.031:t 00019 •. RbIsX' ra,1:ioof the basal1:s from the Island of The Hawaii .1s identical # wi thin experimental e1:ror, t~ the RbISr ratio of their source region calculated average~sr87/sr86 page from'~he with ~he a~sump~1onB ou~lined on. • The rocks of the Samoan islands belong to the typical alkali-rich olivine basalt association of ~he 'Pacific petrographic province. Two of the three rocks analyzed . are olivine basal~8, the third (RiggS) 1s an olivine gabbro f!Ccm a volcanic lr.ieckon. Ofu Island. • sr87/sr86 ratios fom The corrac~ed I a homogeneous set of values averageD . + .. , log 0.7077 - 0.00009. The Samoan rocks differ significantly on Maui .Island in their sr87/s:r86 r'atioso between atem and 'the rocks from the island however, may net be significant. from the basalt The difference of Hawaii, The average concentrations nb/Sr and the Rb = ratio 2S.Si:a.3 of rubidium and stgont1um Samoan'&"ocks aure of 'the three ppm, Sr :.419.8+105 ppm, Rblsr = 0.061-:- The large standard deviations of 'tbemeans' O.025c a fairly wlde range of values which for rubidium indicate to 41.6 ppm and for strontium from 22205 to is from-14.1 567.6 ppm • . ~e V" calcula~edvalue of t~e region' ~s O.03St ().Oll. Rblsr ratio of ~e source This value is less than the pre-' sent Rb/Sr ra~10of the basal~s and indicates enrichment of the basalt (or depletion of stl:ont1um) magma in rubidium relative to the souJ:ce ma'terial. Mid-Atlantic Rid<;le. and t:he Azores. AscensiOn Island. \ 87- /sr The Sr 86 ratios .' of 'the specimens from Ascension Island and the M1a-Atl~tlc) axe 0.7070 and 0.7062 0.00065 respect1 vely «(f), 0 The value for ~ba boulder f:om ~e 141d-AtlantlcRidge is based on duplicate an~lyses The sr87/SJ:86 is 0.7088. zoat10 of the oll,vine basalt from 'the Azores This is higher than observed in any of 'the Concentrations of rubidium and strontium waxe ob~a1ned 0 -195- for the basalts from ~scensio~ Island and the ~dA'tlantic .~dge.o Rb. = 31.9 p~, 'the latt~r Rb The resuli:s for the fozmer are Sr e 643.0 ppm, Rb/sr = 9.75 = 000496 and for ppm, Sr callB.3 ppm, RbIs%' :D 0.08240 The calculated Rb/Sr %atios .of the source material , of the b~salts a~e as follows •. Ascension 0.035, M1d-At:lant1c Rblsr = 0.044. Island RbISr Ridge. RbISr ::0.030, and 'the = Azores 'l"he.value. for the Azores is the highest .0£ all oceanic basalts. I..ummary. All averages discussed ~n the previcu~ section are summa~1zed \ \ .. in Table 6.2. Table 602 Summary-of Data on Oce.anic Basalts. Noo of (87/86) colt'r Locality samples Hawaii 4 0.7069 (0)' + 0000025 Rb ppm 18.8 (Rb/Sr) .. so~;X'ce Sr ppm RbIsX' 601.9 0.031 0.034 :to.019 :to. Oil .t 9 3 :t 209 0 .. 1 MaW.. 0.7041 (2) 0.022 :t 0.012 :!: 0.001 Samoa v- Ascension Mid-'. 3 :!: 0.00009 " 1 1 r j .Averages 0.7062(2) 1 0.7088 11 0.7072 t 0.00033 \\ NOUI 0.7010(1) 25.5 419.8 0.061 0.038 :t 8.3 :tlOS :t 0.025 :tOoOll 3'1.9 643.0 9.75 118.3 0.050 0.035 00082 0;.030 :t 0.00065 Atlantic, Ridge Azores 0.7077(3) (1) A.ll errors 0.044 21.5 492.0 ~ S.O:tl08.S 0.044 0.036 ~ 0.014 ;to.Oil are standard deviations of t.he mean or are calculated from them. (2) The average RbIsX' ra1:ios were calculated from the average concentrations of Rb and Sr. (3) ~r in b~acket8 indicates number of analyses. -197ContinentalBasalts~ sr87/sr86 t:atlos were dete1ro1ined for 16 volcanic rocks from eight continental localities. This part of the invest:1gat1on 1s an important phase of the test of the la1:eral homogeneity of the upper mantle. toplccompQ!$it.1on of s~ron~1um entering the mantle is of pagt1cular the The 180- crusi: f:rom significance for any inter- pretation of the origin' of crust;al igneous rocks. 'Specimens'of volcanic were chosen. rocks from tbe following loca11ties Japan, Deccan 'lateau, Indiar Columbia " River area, Qregonf Di&ba~efrom New Jersey,Iceland, Mt. Vesuvius, Italy.,., Yellowstone' park,'and Squaw~reekf Montana • .The experimental \' data are compiled .. !nTable 6.3. " Japan. Tb3:ee ~ocks f:ro:a two Japanese localit:les were analyzed •. Two of .these are andes1 tesfrom. the~1:rd 1s.a dac1t.e from Banda! were e.X11pted ~e 0== in historic San. All Sakuraj lma, three rocks 'time. average corrected sr97/sr86 ratio1s 0.7081, 1:0.00013. c:r-=:J: 0.00023. sidered to l1e ou~side Japan is. generally con- the Pacific petrographic' province - -198lot en ~ ~ .. ~ft: ~ ...fO 0) e - 1..( 0 I 0 < u ...t c co' u ... g .... ftI ..., c s:: ....., 1 C Q ~ 0 to ...\0 \0 (1'\ a.n \D \0 0 \0 0 0 0 \D 8 0 e . 0 co N pf co CD ~ • ...., iEot ...0 • CD ... a.e \C """ \0 • ,..., • CD l' ex) ~ ~ 0 a). co 0 0 r-. • ~ rto .... \0 r- 0 \0 CD .... l"'- co 0 I' 0 • '"• 0 ... " ~ A ~ ~ CJ\ ..... ..:s ns a ~!I UJ ...... J s: ..,"fi .... \D ..... ~ ~ ...• Pol +1 ",""N 00 00 t.n co 0 r--• 0,0 • • ~ +1 ,., \0 0 P'fCJ\ ... ..... """ .... 00 r- oc Q oft ... ~ • +, 'b b ~ C't- • • • 0 .. to '0 t: H .. ~ ~ CU co • • G) ~ l' 00 00 00 • 0\ R1 ~ ~ ~ N ~ ... cu CO N qf ~ 0 i= c CO .... N Z~V " J.4 o ... tU G) CO 2'! CD ~ ~~ -c:r ~ BR1 ....,:a. fiU U ~ \D ~ eN ~ U') 0\ \0 0 \D 0 0 Q ~ N Con co 0) • ",\0 CD 0 I" ....\0 \0.... \D Bco ii • • """ r-. • co r- +\ +1 • 0 ftIDr.. 0 0 00 ,...... 0 CD ~ N ~ rCO GJ ... co J \0 00 • 14 ,.., N~ 00 tl 0 fntz ('11) N 0 0 0 • 0 ~ ,..'"• .......• .... P4 ~ 0 C'\ ~ 10 Ccc' 0\ CO OJ r: CD 0 0 ~ )~ a J Q) CQ ~ ~ C't) N U) a UJ Q) ...... -s... • m u ~ f6.4 ... 0 ....0 en ..t • PI It$ :J ..., 0 ...• s:: 0" en 0 • 8: en an \D • e J.4 an \0 0 ct.) ........ • ....en <:) r- • .... ....l"- N • 0 \0 \D ~ CD ...• ~ ...s 0 0' 0 Q\ 0 • • c:-t 0' ~ ftS co r- r-- '" G) \0 0 ....~ C' t-.. • • C a : an ~ \0 ~ " N 0 "0 0 M 0 0 0 0 0 N P) 0 0 0 0 0 0 0 . +1 ... l' 0 0 , tW') ....0 0 0 • +l • 1-, 0 \0 .... "b b ~ ... res co 0\ m : 0 0 0 +t 'H • :> ~ • it• +1• • a ... "N M 0 0 0 eEl a fa U u a 8 CD fG ~ tat \D Q) ....CO CO .... ..... p; ' OJ ~ ' - 0199 • \D ..0 r- tQ f"'i r-pf f 0 0 \0 \0' N • 0, • 0 ... I; (I). • 8' ""o co o~cit • 1i rig ~COCD ~r- ~ 00 00 \D NN \D 00 \C)\O Q ',1' \D an C \DO\~ \0 \0 \D, 0'0 Q ~ CO ..... U ,.,. S • \D . CD 'fD flit M OJ i II E4 ~ • 0\ 00 0 00'\ N .... ........ • • • 1-1 -.. ,.... ,.,"" ....... ........ ""'N \0 00 r: 8. 0 (O\DO\ 000 r-r--c-• • • ~ .(1 \0\0 ..., 1-' co• CD opt ... ~ :3 pf cO .. 'Sk~ '2: N ~ ~ td ... CD ... CD .... ~E-t ..1 .... ... N ~Q 8 " en "" ......• • • .., +1 r-- .-IN 00. Q r- OO • • 0 ....... 00 +, 0 1:4 QJ ~ ~lbb ctJ co • GJ G i~ •• ., :1 ~cG 10 .... ,"'fI», . ~~ ~e-t ~ ~" COlll'f i8 CD c-f ..... e It ~ ....u ~ ! C' f' • 0 0 0 PI I'0 ,... 0 \C <:) flllo • • c:rt U CD c-f ...~'l', , ..., qj". 0 ' • 00' 00 g . co=, P4 OOU • • • R1 0'\ 0 r- 0 ~ 0 ...s roaN CD Oo-J 0 00 r- OC • 0 t'i1 • ..., +. ~. ~~~ Ul 'It I' I'0 CQ~ • orI ... ,., ..... • 0 00 >t • +1 1-1 n: ~ '14 Ib b : 8 Ao q.e $' ! 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'nl \D "• ~ ...,0 .... u ~ en ~ u ~j f~ '"... r: cu CD CJ H • 0 \0 0" Q ..-4 ~ 0 \0 • ~ tn '0\ ... : If) ~ cu .tJ H ~ N 4t;St ~ U1 GJ .,.. ....(0IU 'a CD to ~ :::~ C ~ .... •. 1 1-1 +1 \0 ~ ~ Jp Q ....U -. fa ~ ~ U ~'f)' ~ ~. C ~ an In 00 OQ r4 II I' ~\O . 0 .... \0 ~~ ... ... ~ ~ .&J ~ ... 0 CD • " co (X) N \0 lid' N : u ........ ... 0 ca > ........ o &'0. ........ s: & Z. "'.... ....... ~m GJ c. 0 ~82 and to fall inside 'the -andesir;;eline. It is therefore of interest 1:0 ~cmpalte the averagesr87/sr86 ratio of the Japanese :rocks totbose 'test for the difference for Hawaii and Samoa. of two-means reveals Japanese rocks differ significantly and Samoa, witb1ng experimental The th-at 1;he from those of Hawaii error. The calcula~ed RbISr rai:10 of the source z-Eg1onis 0.040 t.O.Oll wb1chis not too different: from the value Hof _thls_~at10 in olivine ~~salt8. The concent~a1:1ons of rubidium -'. andes11:es and Deccan Plateau. and strontium in 'the 'the dac1 te were not detem1ned. Ind:1a.- -'the t:holeelt1c basalts of the DeccanPlateau in ) western_ India toClay occupy an area ..of 200,000 . miles \ and naacb a max1mum~1c1tness of Bombay. This. immense -quanti t.y -of ~asalt in Cretaceous square .. EOcene 'time. 6000 feet near was extruded 'the rocks resemble closely in chemical. compoSition and volume similar o~currences . in 1:be-Columbia - .snake River area of WaShington, ana Idaho, ~he Ke~enawanlavas S~rcmber9 ..1avaB of _South Africa, of Lake Superior, the Triassic Oregon the diabases - -202- of New Jersey, and ~he Paranao basalts of South .»meriea (Turner and Verh~gen, 1960, p •.205). The 1s0'top1c composi1:ion of strontium of three basalt spec~mens m1ned. from the Deccan Pla~eau were deter- The .average correC1:edSr87 O.7089.~= ~ O.00019,~~ has .of .solidification. The. (R1864) is 0.069. ratio of °If the age tha increased by 0.0003 since the time The initial ratio would therefore average sr87/sr86 ratio for 'theDeccan basalts is very slightly ese volcanics. i!!i is taken .i:o be 100 1Qi111on years, sr87/sr86 ra~10 be 0.7086. ratio * 0.00032. The Rb/Sr one of the. three basal~s of the basalts /sr86 higher than that of 'the Japan.... The .calculated Rb/Sr ~at10 of the source region is 0.043%0.011. Columbia River Plateau. Oreqon.. Three specimens of basalt were selected Unfortunately the resul'tsobta1ned for analysis. on two of these (R1435, R1443) are of inferior: quality because of instrumental difficulties spec1ment (R1428) (poor vacuum). was.analyzed in triplicate 8:87/sr86 = O.7067:t The ~ixd and the average of 'these, 0.0001, . is considered to be 'the best. estimate of the t"roe value. Specimens. R1435 and R1443 average 0.7113. concentrations of rubidium and strontium of ~e R1435 aze Rb a 54.8 ppm, Sr = = 310.6 ppm, and RbISr 0.176. Since the .age ,of the basalts is Miocene ~llion years) an age correction of 0.0002 is applied ~o the average srS7/8:r:86 ratio. 'lbe best: estimate .a~10 1s therafore s~7/sr86 initial (25 The'calcula~ed RbISr ratio'of ~e of 'the 0.7065:to.'OOOl. sour~e region is , '+ 0.011. ' 0.032,'Triassic Diabmses',of Mew Jersev and Connect:1cut • . TwO specimens of diabase from the Palisade sill (~4209, tt4210) avorage s:87/sz86 14 1:hird speciman of diabase cut. was. ratio = 0.7094 bebieen Since analyzed The average Connecti- srB7/s:r86 The test for the difference two means shows that D~ 2 ~, from Nt.. Carmel, in' duplicate. 0.00065. = O.7075z 0.00045. the a1fferenc:e D• is - ,0.0019,00.=:1: ' 0.00079. significant, However, because the age of these diabases is apprOximately200 mill:l.on years, ~ed1fference in the ,amoun~s of r~logen1c solidification may be'due to differences 8%87 accumulated of the magma. Since analyses since for rub1dlWl1 and stxont1um axe not available, 'ra'tio for all three the aVGxage sr87/sr86 rocks is calculated corZ'sction is applied and an age to the average on 'the assump't1on that RbISr :. 0.1 and the age is 200 million years. best est~ate = O.7072,cr= is therefore sr87/sr86 =0.7081 The - 0.0009 ~ O.0007~cr= ~ 0.0012 •. The calculated Rb/Sr zoatio of the source region 11' 0.036%0.011. Miscellaneous Localities. Single 1so~pe rat1~ analyses were perfcxmed on basalts from four continental ratios of these area localities. The sr87/Sr86 IcelCUlda 0.7101, Mt. Vesuvius, I~alYI 0.7082, Yellowstone Park& 0.7084, Squaw Creek, Montana& 0.7096. The olivine baBal~ f~om Iceland is pOBt-Plels~ocene in age and ha~the highest abundance of Sr87 of any ~asal't encountered 1n i:he survey. sOurce region has a calculated Its sUb-crustal Jtb/Sr ratio ~ 0.051 :t 0.012. The .ar87/sJ:86 ratios of tite o~eZ' X"ocke are well wititin the range of what may be considered afJeraqe ' . .sub-czustal of Tertiary s'trontiUJJl. The basal't from SquawCreek is age. Assuming an age of 50 million, -years and a Rb/sx ~at10 '= by 0.,0002 to 0'.7094. 0.1, its sr87/sr86 ratio is reduced The RbISr source :regions, are as follows: ratios calculated for the Mt. Vesuvius 0.041, Yellowstone Park 0.042, squaw Creek, Montana, 0.047.' Summary. The results'discussed summarizad1n 1n~epxey1ous'sect1on age Table 6.4. 'Table 6.4 Summary of Data for Continental Basalts a~ the Tima of Their Extrusion. Locality lfo. of Itb Sr Spleso(87/86~orr ppm ppm Japan 3 (Rb/Sr1 ~b/sr .7081 + 0.00013 ~ccan 3 'Plat •• India .70a6~ :t 16.1 234.9 00069 . 0.00019 Columhial 0.70650(3)54.8 '310.60.176. Riv., 0.0001 Oregon Tr1asslcD1a- base, 1 0.040 0.011 0.043 :to.Oll 0.032 r 0.011 0.036 30.7072* ':!: 0'.011 .New Je1esey Xcelilnd . ~ource 0.051 O~7101 :t 0.012 Nt.Vesuvius 1 0.041 0.7082 ~ 0.012 I~aly Yellows~one 0.042 1 0.7084 :t'O.012 'Park s~aw'Cxeek 1 0.7094* 0.047 :t 0.012 Montana Average 14 0.7083 z.,O.0004 * 35.5 272.8 :t19 ~)4 ;t 38 small age correct1~n applied. 0.13 t.. 0.07 0.041 .:to.Oll Conclusions. The, average volcan1cr~ks continental 0.0012. s~87/sr86 ra:tio for of BUb-c~stal 'locall ties is = 25 basalts and other origin f~om oceanic and O.7078J 0= = :f: 0.0002, f:r = :r: The values range from a. low of 0.7047 for a basalt. from Maul Island to 0.7101 for an olivine basalt from Iceland. The average localities Sr87/sr86 for eleven basalts .1s 0.7072 :tOo0003. from continental localities from oceanic Pourteen volcanic rocks averags O~7083%0.0004 0 The difference between oceanic and continental ba~alt3 . ' is probably not: signific:an1:. Although tbere is a tencisney for continental basalts to ,have slightly higher. srB7/sr~6 ratios than oceanic basalts, exceptions are common. The magma~1csource regions in the upper .mantle are therefore homogeneouswithin narrow limits .. The limits. of homogeneity 0.7078 + e~ressed in teJ.'tltS of t:he Sr87/sr:8~ raUo are 0.002. - 0 ..003 The Rblsx gat10 for the magmatic source regions, cal- culated from the average sr87/sr86 ratio, is 0.039 ranglng from 0.022 to 0.051. The upper mantle is therefore -201homogeneouswith respect i4>/sr limits = to its RbIsX' ratio within 'the 00039+0•012 • . - 00017 source re9~ons have a calculated The Bub~eanlc RbISr ratio of 00036 whereas the sub-continental sources average 0.041. Small but real differences be~weenbasalts from different in the sr87/sr86 ra~!o localities seem to exist. Such differences are evidence that magma was derived from more than one source ,and that small variations in tlle Rblsr ratio of ~e upper man~le ex1B~o Small differences.in sr8'/sr86 ratio are diffi- the cul~ ~o de~erm!ne reliably because. of isotopic fractiona. ' t10n in 'the mass 'spect:rome~er. partly by applying 'this can be overcome a fractionation correction based on a constant value for 'the Si:86/sr88 ratio and by making several independent analyses spaced over a period of tim9. The conc~ntrat1ons of rUbidium and stron~1um ~n the oceanic basal'ts .respectively. average 21.50~ 5.0 ppm and 492% 108.5 ppm, Rubidiumconcentrations range from 9.13 to 46.7 ppa'lwhereas strontium concentrations 118.3 and 1249 ppm. vary between 'lhe ~b/Sr ratio, calculatedftom t:he average concentrations, is' O.044:t 0.014. Concentra~1ons of rubidium and strontium for thole- eitlc basalts were detaxmined average values are Rb =- foz only two rockso + 35.5-1904, The 'm. _ + pP.' Sr - 27208- 38e The DDn7 IT corresponding average RbISr X'atio is 0.13 zOo 07 • Thole- e1~ic basalts ~erefore seem ~o have a higher Kb/sr xa~10 'than oceanic basalts •. of the RbISr ra~1os of .the basal~e A comparison with thoBe calculated for their source regions may give some indication of ~be processes of magma foxmat1on. Generally ~he RbISr ra1:1os of t:he rocks are higher 'than those of ~e1r source material. Rb is enriched (or SE' 1~ depleted) in t:he liquid fraction. ~e only exception This is evidence that to this occurs in ~e basalts from the island of Hawaii where the Rb/Sr ratio of ~he basalt: 1s 1dent:1calto 'that of tbe1r source. This may be 1n'ter- preted as evidence of essentially complete mel~ng the source rock. may bt! identical of If this 1s .true, the lavas of Kilauea 'to 'their sub-crustal source material .. with ~spect 'to the concentrations of major elements .as well. A comparison of the Rb/Sr ratios of ~e source regions w1th tho~e. of certa1~e magmatic of igneous rocks may give same indication mantle. According to Chapta~ 2, Table 2.2 olivine basalts and i:be eclogite.have 0.045. of ~he nature of the upper identical Rb/srrat:ios at abou~ Very similar values are indicated for the source regions in ~e following loca11~iesl Deccan Plateau (0.043), Nt. Vesuvius Park (0.042), S~aw Japan (0.040), (Oo041), Yellowstone Creek (0.047) and the Azores This similarit.y does noi: necessarily above mentioned volcanic melt1ngof (0.044). provetha~ the rocks .\tIGre formed by partial a basal~1c or eclogltic mantle. Other ultra- basic rock 'types such as per.1doti'tes, pyroxen11:es,etc. may have suitable Rb/Sr ratios possible interpretations, concentrations as well. Because of such a comprehensivesurvey of the of rUbidiUM and strontium 1n ultrabaslc igneous racks is highly desirable. Neither chondritic nor achondr1~lc meteorites listed in Table 2.2 are a suitable source material ~or Recent .basalts. The Rb/sr J:a~10 o~ chondrit.ic meteor- ites 1s'too high whereas 'that of the achondri'tic'meteoJ:1~es 1st.oo low. -210- sr87/Sr86 Tim RATIO, IN PRSCAtmRZAt~ BASIC INTRUSIVES. In~roduct1on. The isot.opic material of compos:!. i:10il of the soux:ce. basalt: magmacan be determined by analyzing. Recen't basali:s~ 'The value billion s~ront1um of of 'the 87:87/8r86 ra1:io 405 yeu:s ago is known from measurements of t119 achondrite pasamonte (Chapter 1). With 1:1118 1nfoxmat1on a development line for ~e crustal siC~7/sr86ratio in the! subc:a source regions throughout geologic. 't1me can be drawn (Figure 704). An attemp~ has been made 1:0 determine of this devel~pment 'line 1ndapend~ntof 'the slope any mete~rl~e ,data by obtaining ini tlal .sr87/sr86 ratios' for basic .igneous intrusives of subc:.CruS'talorigin , us known. ~e ' samples chosen foX' 'this study are frcm tile Bushveld Complex 1n South Africa, Duluth whose ages Gabbro in Minnesoi:a. and 'the Introduction. , T - -. The basic rocks of the Busbveld Complex are the prOducts of frac~ional crystallization ~1on of a large volume of basaltic and differen~1a- magma. They are overlain and in~ruded by the so-called -RedGranite" 0% Bushveld, Granite. 'rhe relationship 'to \;he basic rocks is not clear. (1960, p.298)su998st of the gran! te 'l'urne'r and VeJ:'hoog~n that all 19DeOus rocks of'the Complex belong ~o ~he same cycle of igneous activity. Concentrations of rUbidium and strontium as well as sr87/sr86 ratios were deteJ;mined for three basic and ul'trabas1c igneous xocks in an attempt -the io1 tial ar87/sr86 ratio, at; the ,'!'he Age of the Schreiner Bushveld and .three time of emplaeement,. Complex. (1958) :r:epo:r:ted Bra average age of 192~ m11110n yesxs by 'the Rb-S:r: method ites 1:0 measuxe feldspars separated for fCNX' whole from them. gran- W.S data have been recalculated and are presented in F1gure 7.1 in diagrammatic J:oclcs . (921, fOnl. Three of the whole 810. arid B6) form an "aX:l"ay" end conve:r:ge . -212- 1 I ,, 1 ~, I I I {):I 87 86 SrlSr I. 500 (/)1 0' iii'1 Ii. 1 iV' FIGURE 7.1 Sr DEVELOPMENT LINES FOR RED GRANITE, BUSHVELD COMPLEX, SOUTH AFRICA 1.400 llJ,' I I ,, , I ,I 1.300 I I , SCHREINER, 1958, R 114 I I I , 1.200 I ,, I I ,I I ,, 1.100 I ,I I ,I 1.000 I I I I I I I I , I I I I I I I 0.800 I 1 I I .., ...- ;I' 1/ ... "~ ,..." );1' 0.700 3 , 2 TIME IN BI LLIONS OF YEARS 923 probably does not differ. sign1fican1:1y three whola rocks. The feldspaJ:s (B21. 810 and B6) seem 'to d1fferen~ array converging fo~aslightly from i:he otheX' .'t Ii 2225 andsr87/sr86 .:;.740. at about The reason for 'this ence ~s not. ~lear.and .it may not be s19n1fican~. ing to Schreine,r's data .the initial Busbveld Gr~n1t.e 1s""'" O.740. ~ch Accord- sr87/sr86 ratio of the definitely sub-crustal origin for this rocle. ~reover, the underly1n9 differ- excludes a the .9raniteand b&s1c roclcs canno~ ..be co-magmatic •. N1colaysen e~ ale (1958) ~eported a Rb-Sr age ~f 1915 %50. ~111on . mica-pyroxene years fo~ biotite fxam a coarse-grained gabbro at 'the Rustenburg same authors'also ~f The obt~;I,i1edU-Pb arid '111-Pbages on crystals monazite and zlrconfrom cluded that: Platinum Mine. an age the Bushve1d granite. of 19Sof 150 miliion They con- years for both the granite and the basic xocks was con&is~ent with 'their results. B~r1mental RSsul~s. The anal~lcal data are shown in !'able 7.1. and are present.ed graphically raUo determinations *recalculated to A in Figure, 7.2 •. Only single ls0~ope were. made foX' each Z'oc'kand an error :a. i.~41,.Xl0-11 yrs.-1 oftO.OOl is assigned to The erroZ's the ~ld1um SbOit1n .are considered for the'co~rected sr87/sr86 rat1oso to be realistic ana strontium eB~~ates of the reprodu- cj"b111ty at the part1culaJ: concentration T~le 4.6). Descriptions analyses levels (Chap~er 4, of ~he rock specimens and infozmation about ~he sample localities are given in Chap~er 10. Discussion • . ~) sr87/sx86 ratios ls'consls~ent The pattern o~ the with the llb/Sr ratios ment lines 1n the three rocks. conyeE'ge 'toward a point. at about 4.5 billion y~ars an~.a sr87/sr86'ra~1o of 0.707. large er~or.envelopes.make In. fact in~ersecticns.'are ~eBe values meaningless. All intersection at about years 1s tilerefore. not excluded by th~ da1:a. Intersections of ~e 81:J:ont::1umin yeaX's HOwever, the obsezved from zero up to about 14.3. b1111Qnyears. 2 billion 'l'he develop- the1:hree give' (sr87/Sx8~)o development: lines. for 1:he rocks .w1ti1 t: as O.110~ 0.001, and 0.707't, 0.001, respectively. xat10 of the anorthosite = 1915% 50 million 0~708t:.OoOOl Since the arB7/sr86 1s a1mos~ constant in time, .1'ts 1n1Ual raUo of O.707Z0.001 is cons1dexed to,be -215- ~ en i ...... ~ I: {Q 121 ....m• -.8 "') 0m • fa • K , .!l • i ...• ~ ig ....., j . "'S ~~ ~ +1 ..... CV) , 'N~ . ~ ... o .,.., ::~ • J J ~~ oN " ... U') \0 ~ Q m ~ \D CD w ~ ~ ~ r\0 ....... • ... 0\ 00 ... 0 ~ I' CD "r-• CD .... it cnRS ~ N ..t • .-..0 • 1'-. ...0'\'" 00 1'0 • • 1-1 r-- (f) .... f"- Q 0 .... • r- 0 \0 0 \D 0 ~ ~ ~ ~ ..t ~ S.... .... I! • 0 0 ...• .... 1-1 . Q ./ N I" .... . N ... It') oN 0 0 0 ... ~ ~ • CO ..... '"'!l& O\Ul ... • r-o 0 • & ~.8 0..@IO .~ 0'\ ... it ... ...r • I B 1: S cae ~ 1ft ... : en +1 \0 0 0\ • • t") N III \0 CD 00 ,) en 0 0 en ..... (\1 ~~ 0 <:) NO '~Q ......+1 CD ~ ...... . .. • • .(W1 lot \0 .... a. .... ..... 0 NO 00 00 . 00 ~! ...... >ec 0\ .... .\1) : -216- CDaCD(/) ~ m 0 r:: ....... CDa(/) ,, ,, \ ,, \ ,, ,, ,, , ci r- 0 \ \ , I I \ , \ \ \ , -\ \ \ 1 I \ \ \ '\ , , ~ (5 \' \ \ ;;: ci ,, , \ 0 0 t- ~ ,% {l. 0 \ 0 \0 \ \ ::E 0 ox \ " , ,, , a:ii: IL (/)<1 ::l O(/) ...... .. zx (!)I.&.I -...J oQ. -:21 (/)0 «0 m ...... 0 :::r::...J ..-1.&.1 > :::r:: N~ "';01 a::::r:: ::l"~IL ILO , ~a:; ~0') \ I , ,, ,, , , ,, 1, , ,, , , ,, ,, ,, ,, ,, ,, 1 ,,, (J) I \ ,, (\I I \ \ a: « ...... >- \ ,, , \ ,,, \ \ \ \ rt) I \ \ \ I IL I (J) 0 Z 0 \ ''~ \ \ \ :J \ ~, \ , \\ '" ,, \ \ ," \ \ \ \ I , \ \ \ \ \ \ \ \ \ \ I \ I ...... ~ ..-- ,, , ,, ,, ,, \ \ ,, I I m z I I I' \\ ...,. " 'l' " I' ,, ' I '~ \ ...J " '1 \ ...... 1.&.1 ~J (I) \ ,, 0<1 00 \ '- ~ ,, ~ ::E <% 0 It) (!) +1 \ \ >: UJ , r-ci ,, (/) ,, , , ~ ~ en \ ", 0\' ~ ~ ,,, .- UJ \ \ , , \ \ \ ~ 0 0 d \ \ CD 0 b r- 0 r:: It) -217- the best estimate of this ~antity for the basic rocks of the Bushveld Complex. Figure 7.2 111us~rat9s the difficulty o~obtaining p~~c1se age values,and initial srS7/sr86 ra~ios for rocks having such low RblsJ: ratios. 'The precision of the measuremfJntscan be improved a~ the reBut tsmade moxa meaningful by ,making a11'aetem1nai:ions 1n'triplicate. Conclusions. '.rbe age of 1:he basic: igneOUs complex 18 1915'%(~o million rocks of 'the Bushveld years 1958).' An attempt tode~r:m1ne' (N1c:olaysen eot al., the1x srB7/sr86 'ratio ,a~ 'that: t1me me't with 1nd1ffezen't 'success becau~e o~ the low precision of the measurements. for 'the 'in1 Ual sra1/sr86 The best value X'atio is o. 707 t. 0.001. \ '!'he Dului:h Gabbro. Inb:oductlon. \ ~e D\llu~ Gabbro COmplexis an immenses1l1-Uke , , mass extendlnglS0 sota. I~' 1I11es noZ"~' east "from Duluth, was foxraec!by repea~ec1 In'tzuslon .Minne- and crystalli- 'zat.1on of feld~path1c gabbro,. layered gabbro, grano- -218- d~or~te ana 9ranop~yxe. Lack of chilled margins indi~ ~~t~s that the sepa~ate i~tru~ions took place in ~ela- t1vely rapid succession. be explained The variou~ rock ~~s in 'terms of dlffereni:iation can of a common p~rent magma of basal~1c c::~position (Taylor, 1956, p.42)o 1\~ ~xcellent summary of 'the geology of the Duluth Gabbro Complex was given by Goldich et ale (1961). ~e Gold1ch age of theDulu~, 'Gabbro as reported ~ by " et ala (1961) 1s abou't 1100 million years.' 'l1lis is an average value of several 1ndeP.E!ndentdeteminationa by the K~Ar mathod on several samples from different localities. A single specimen of gab~ro was analyzed for its ~ldlum ratio. and str:on~1um.content ~nd 11:8 sx87/sr~6 The stratigraphic position of the specimen 1n the Duluth Complex is \noi: 'known. ~rlmenta1 Results~ . The analyt1ca~da~a areehown in ~able 7~2 and Pigure 7.3 •. 8rr~rs' assigned ~o the corrected sr87/sr86 ratio and.the concentrations of rubidium and strontium axe considered to.be realistic es~1mates of the reprodUC1b111ty. -219- I I s~js~6 I 0.713 I I 0.712 , 1--- - ... ..... ---. _ ..... -- -- I ------------------0.711 0.710 OU\..lJTH GABBRO -.---- -~----------- ------------~ I .---r-~8-;~~~:~-0.70920.001 0.709 0.708 ok 0.707 I I 0.706 0.705 0.704 I I11100, 0.703 M.Y, 0.702 500 1000 AGE IN FIGURE MILLIONS 7.3 DULUTH OF YEARS GABBRO -220Table 7.2 Duluth Gabbro Sample No. of Nu~t9r Date 87/86 (8!./86~orr86/88 R1231 1/~2/61 .7125 ".7101 84/88 Qualit:y scans .1186 .0067 84 'j: 0.001 ,J{b = 6.8.0 ppm :t~, SX' = 34S~7 ppm ~'2%, Rblsr O.0197t:O.0007 s Discussion. The X-Ar .age' of 1100 mil11on' ye~s minimumage"for ,the rock. probably a is The 1nlUalsr87/sz86 'ra~lc ) fox the Duluth Gabbro is therefore . + loss than 0.7092 - O~OOl. Summary and Conclusions. Th~ analytlcal.J:88ults for three basic.and ultrabasic: igneous rocks from ~e BuebveldComplex are not . " .with an age of 1915:' 50 million .. 1n~n81stent years ) (Nlcolaysenet al. 1958)•. 'the best estimate of the sr:87 /s~6 rat:1.oat , A single " t:118t: time '1s O. 707'" o. 003 , gabbro specimen - 0.001. from tbe Dulu'th Gabbro l w~ found,~o bave bad,a 8r87/sr86 ratio of 0.7092 % O.OO~ about 1100 million yeaz:s ag~. Figure 1.418 matle fo~. a sUlIIDIlry of ,the results in ttiagram- ~ck'o' precision of. single detezminat10ns <10221, does not allow much confidence in the numarical values -\ do not disagree 0% invalidate 'the hypothesis tha~ 'the sr87/sr86 ratio of the upper If,lantlehas increased ~rOU9hout geologic time. approx1tn~tely as indicat~d. in Figure 7.4. -The precision GJ:%OZ-S can be decreased and 'the \ results made mote meaningful cate analyses. J' Table 5.1. by duplicate or tr1pli- Compareanalyses of Rl292, Chapt.er S, -222- Nfl) 00 .- q q ' U) LIJ c::[fD fDtn ..... fD'" tn ..J+I (0 ... N '"0 en", «0 £0 ..... CD (0 g 0 '"d 0 0 ~ 0 '"0 N 0 '"d > - 0 0 ": tn ::) 0 a:: IZ - 0 C/) e{ m v 0 0 :x: .- z ct a:: ..... I 0 II ::) (0 ..J fD m ~. , ..... ::) 0 e{ fD 0 LIJ a:: L&J t- Z 0 a.. N ~ Z c:( C/) 0 -I if L&J C/) en 0 l- > :x: - ::) e{ to a:: fI) (O&. fDC/) "" fD'" C/) ..J e{ - IV Z ~ ~ LLJ 0:: ::) (!) LL Part. 1 THE .naB OF .nN ECLOGITE BY THE Rb-Sr r-mTHOD. In1:roduci:ion~ In zoec(!ntyears the hypothesis has 'been debated in the literature whether the MOhorovicic discontinuity a phase change from eclog1te 'to basalt OJ: is whether it is a chemical and physical boundary between a peridotite mantle and the sialic crust. The debate was opened by Lovering g9s'ted t;hat achondrlt1c meteorites mantle of a dif~erentiated 'therefore most. closely (1958) who eug- are derived from the parent meteorite body and resemble the mantle of the earth. Pointing to similarities in the chemical compositions eucrit1c achondrites, basalts, of nor1tes and ~clogites he proposed that the upper mantle is composedof' 'eclogite and that the MOhorovicic discontinuity dary bet.ween eclogit.e 'and basalt. of t.he Moho under the continents from estimates of the p~sBure at the Moho he estimated is a phase boun- Prom the known depth and ocean basins and and temperature conditions the.P - T curve for this ~rans- formation. OU~ In support of ~e hypothesis Lovering po!nt~d tbat the heat flow measu&'Eid at 'the surface ei,lr'th CQuld.be accounted of t:he for if 'the upper 200 - 300 kilometers of the man~le are ~omposed of eclogite and 'the concentrations of U, Th and K decrease ~1~ depth. The occurrence of diamonds in eclogite xenoliths found in k:Lmberl1 te pipes 1ndlca1:es that the source of these ecl~ gites is at a deptb of at least 130 - 190 lan. Similar sugge~t1ons have been made before (1914) and Holmes (1927). Goldschmidt .(1922) by Permor proposed the existence of an eclo91~e layer between levels of 120 to 1000 'km. Birch (1952) on 'the basis eclogite of s~udies of .1ncomca to' a depth of 300 or 400 laD. Kennedy (1959) discussed the application of ~is hypothesis problems to .tbe solution of fou~ impor~ant geologlcal I of continental plateaus. 1. ~11ft 2. Subsidence of geosynclinal sediments 1n'to a denser substratum and their subsequent uplift 20 fom mountains. 3. ~e unifo~ty of heat flow a~ the surface of continents and ocean basins •. 4. The diacxepancy between the rats of erosion and the 11fe~ttme of eont1nen~B.' Kennedy concluded ~hat these ques~ions could be explained qualitatively by assuming phase boundary between eclogite 'that the Moho is a and basalt and that its position is controlle~ by temperature and pressure conditions. MacDonald and Ness,(1960) ~xami~ed the hypothesis in 'the light o$: ex1st1n~ experimantal that altbough the hypothesis evidence. They showed could be' used to explain many problems connected with orogenies and cont1nen~al the expe~mental 'evidence :La insufflcient O!: uplift, to either prove disprove it.., Harris and R0W811,(1960) no theoretical ofmeteox1tes justification pointed out that there 1s fer using the composi~1on in quanti~ative calculations of the composi- tion of tile earth. would be 60':' basaltic An achondr1t1c mantle in particular which cannot be rQconc11ed with heat: flow measurements. Moreover, partial melting of a basal~1c mantle would g1ve rise to rhyolitic or phono11tlc magmaand one should expect these reeks dan't on tile cont:inents and in the oceans are. to be more abun- 'than ~ey actually They favoured a mantle of per1doUt1c composition citing as evidence ~e occurxence of ultrabasic nodules in baa~lt and the e:ustence of.ultrabas1c nonCl:>differentiated or1gine If rocks of pr1~a~y the uppermntle is composed of eclogite, fragments of eclogite should be found in oceanic basalts. In faci: eC1O<Jite xenoliths occur only in .k£mber11te pipes \~1ch are restricted to the 'continents. According ~o Powa~s (1955, p. 93) and Kuno (1959) in Turner and Verhoogan (1960, p. 219) eclogibl xeno11ths are found in lavas of the Honolulu series of Oahu and the Koloa serlsofKaul, of Abundances of St:ront;lum Isot:01)eS in Basal f-:!, ~Dl:lcat1on ~ ~~ Hawaii. Eclogite problem~ Measurements of 'the srB7/s%,86 ratios and continental of bcen~ oceanic b~salt:8 allow an estimate to be m~de of the Rblsr ratios .of their .sou:r:celDate~ial•. '1'bus 'the possib11tt:y .. is here given to compare the 1O)/8r rat:1os of eclogites those of ~e whei:her OJ: source material not eclog1b!s from wbich to derive isotopic compositions of basalts axe a suit.able basalt magma. ana to .de'term1ne soUZ'cema1:er1al By analyzing .1:he of strontium and the concentrations of I\b and Sr in several phases of the eclogite be possible to detexm1ne.the time of its An age of 4.5 billions to it. may alao cJ:ystal11zationo yens would be a strong argumen~ in -227- favour of the emstenca of emeclogi t.e layer 1n 'the mantle. Wi'th 'tllese c:ons1deraUone in mind a series of maaeurements was undertaken on a~ eclogite from a kimberlite p1~ Robe~t Victor Mine in south at ~s Africa. Geological Summary. The diamond-bearing upper Dwyka shales kimberlite pipes intrude the which are of Mesozoic The pipes ~'e. contain abundant xenoliths of all reeks in the stratigraphic section. The emplacslDBnt to have been preceded rock fragments fell lite magmacarrying up• .A detailed of ~e kimberlite by a gas explosion back into 'the cavity diamonds and xenoliths magma1s believed following wh:Lch in which kimberof ecl~1~e walled descrlp~lon of the geology and t:he occurrence of diamonds in the kimberlite was given by Williams (1932). ExPerimental Data. The expezimental resul t8 are 91yen in Table 8.1. a graphical representation .andinterpretation r8$ul1:s see Pigure a..l. detail in Chapter 10. of these 'I'he samples aredescrlbed in For -228- ~ k co \0 ~ 0 ~ i2 • .s ¥4 k CO • er-('I)r-. • • !a. P)~~ P'ic-l.-4 • ...B' U :'i5 I: k 0 .... m 0\ ~. ('J\ 00 .... i-t' '(iI. 0 .OJ r: NON 0 ...CD ~Oa) 0'\' "'I'\S) 0 \D ~.~ • • IJJ ~ 'tJ ~(1)~ B: CJ) ='en • ; ~~g ....ctJ ...,s:: II Q)r"CO co co 000 • • • Q) \D \0 0 ' • • (0 ~ ~ ~ NNN N i-\-"" ..tNfIIO ~~ r-NO 0 PI" N .... Pa N en cn .. O A ~~. '0 ". c;:) ..... 0""' • 0 '""' .~o . co .... • '"'" .... ............ ... .-t..-f • • • .. CO k 0 00\~ emf'- ~ ..,00 ,...r--I'- ~ .... • • • • N ~ ~ .... g • • ~ B rto ft) @) ~ Q .... $ . 1 ...0 i .to ,., '" (X).-4CD 11'"4 .... 1"'1 ....... ~ 00 GJ• 1i +1 0\ CD CD Q) 0 " It an t'3 • 1oI'tS g~ 0 .... ~~ • • ItS .... C\ r--r-r•• • () 0 +1 +f 0 0 \00\0 .. ~~~ N..n .... B ~~~ ...... ""...8' .... ~ co ...OJ u ....a • ~ ~ I»e ... P4 ~ 0 0 • ~Mc-O 1-, +. .., to Ibllu Q.. '2.6JUl I 1~ ~ ,C ~ ....... 0 0 • • • ""0\11\ ttlNCS\ ..tc-lO • • • 00'" ~ ~~~ • ...r- • .... N .... t'\ at a: • &n~ ...... ...... • • CDC$\ 00 Sl1 • • .., ..., ..., c 8 'GJ 0&.1 0'" <:)0 ~lt or- 0'" • • QO lblLU ~ s: C3\ an . ...... r-r~~ • • OpLn \D\ft ...... .. CD SOl • • 0\ fI4 PI • "• fill \0 P4 Ii' 0 •0 ~o ... ,+• g~ 00 • • ~~ , ... S~ ON • • • a~ tI ~~ 'bll~.t • t.nN U III ~~ .C'\ c.? .... r=t 0 c::l' 1-, ... PI... .... • ~ o~ 0 0\0 8' ~ .., ... .... r-~ a r-r- 0 B .... ., '. ~~ ~ Z'Jot I 8l8. .tn 0 ..., +. +1 +1 .. 4l '"0\'" ~ ~< r-. .N .... ..c i... ...... ~ 0\ ~ "a--.~ 14 u ()..t (:)0 u c: ! o Q. • • 0 0 .... +1 r-C"-i' s:: c-t \0 .6J M 8l • • ..0 00 ...• g~ • • \D 1". ~~ co ... .... or-- 0 U .... " ..... , ~ $~ Nton ftJ GJ &.~ ..-.• g)i (;) """ lot @) • ........ 0'\ .fj Q., m ~ co Q) """ • • • 0 ... 0 0 CD fa:I • 0 • pot• • • .... • • 0 .r-- \0 \0 CDmrn 0 <:)Q i . 0 ,en""" ...... i' "Hi-,." 0'\ q, • • • CJ "" tl ~ r-- .... ..-. ra-s . N~'J) \0 rD 000 000 Q .... 0) .." \0 • • • 6 Ds4a. • Q g O'\Q')O' . ,S:> Q :"~~ \C) .. • &neDI' JJ) \0 . g~• ~ CO\D ' 0 • lnN • 101 +1 ~ co co ... .: 0'\ ~ 0 0 opt J.... ~'. ~t . r-4 .pof ~ \0 \O~" \0 000 ~ , 0 e +J'M .... 0 • () N ..... <- N~t') .s O' C)'.O' r-.C\'iJ 0 C 0 .... ~ . .... 1-' 8"' U CfJ \D~ i'&ns.n i-' ta U'\. 0 0 r- • • • • >t en ,.0 .0,1 • • • ....• • • +\ . r--~ • 0 c;:) 0 0 .....mOl '" CJ'\'CO N .~., 0 0 0 t» ~ f:.I t1J > ~ -229- (I) o o ~ "- o o U) o ": o v o "- o C\J o ": o o o "- o VlL o ,, , :J:O: ~O \1'( ..J~ 0° z> LIJ ,, \ X .... (J) Z anO ,, ..J \ 0: \ WW ~m o ...J ,, m \ \ \ (!)o: \ o \ ..Jw 00- \ UJ._ CO .... W::i 0: lL ct o:ffi:c ::)m .... (!)~::) --0 lL~(J) ,, , O-ct o -w U)Z ,, ,, \ , ,, \ \ \ \ \ \ ., I I , 1 \1 1 I' I\ 1 ' . 11 I 1 1 '\ " , ,\ \ , Discussion of the Resul~so The .. average corrected sr87/s:r86 of the eclogite was found to be O.708S.:t0.00075. value of 0.711%0.003 it-yo Rb/sr The 1"especti~ely. first As a for an ~clog1te from the same .local~ ~aUo of th$ concentrations GaSi:,(1960) reported this rock is O.0463.:t.0.0006 while of Rb and Sr are 6.9 and 149.0 ppm, far as 'the author is aware 'this is the and only detexm1naUon of the rubidium and strontium' concentrations in eclogite from the kimberlite The sr87/sr~6 ratios of continental.end range from 0.705 to 0.710 (Chapter 6). pipes. oceanic basalts If 'this variation is ~eal, the calculated Rb/sr ratios of the sub-crus~al magmatic source material range from 0.022 to 0.051. These calculated values are based on the assumpt10n that the .iniUal s~7/sx~6 ratio is O.7004:tO.002 and that of the earth is 4.5 billion years (Chapter 1). gave values of O.014.~o 0.060 fox ~~s ratio of the ecl~1te . The Rbis: This is compatible with the that at least same basalts could have been derived from an. eclo91~ evidence, Gas't (1960) is w1th1~ 'the range of values 1nd1- cated for the upper mant.le. hypothesis ratio. the age layeX' in ~e however, does .no~ prove that actually composed of eclogitesinoe upper mani:le. This the upper mant.le is peridotites or o~er ul~rabas1c rocks may have similar Rb/sr ratios. Assuming that measurements could ~ made suffic1en~ly accurately and tha~ ~he distribution of radiogenie srB7 was not changed during the intruston .of the,kimberlit:e magma, the-time of crYsf::al1iz8t1on of the eclogite inea.from the point of intersection ment lines can be determ- of the B~rontium develop- of the whole ~oc1c, th~ pyroxene, and ~ba garnet (Figure 8.1). Unfortunately, the results are .ra1:her 1ndiffel!- en1:•. The following observations can be madec 1. The general pattern of the abundance of Sr87 1s compa~1ble with the present RbISr ratlos:.of ~he different pbases. 2. The error envelopes for the development lines of strontium in the. pyroxene and the whole rock ov~rlap. The two are therefore indistinguishable w1~1n experi- mental erzoor. 3. Al'thougb the pyroxene is enriched in Rb (13 ppm) 'the garnet has the hi9hes~ Rb/S~ ratio and consequently has the highest 4. ~e 5. If the initial sz:S7/sr86 ratio. .strontium development 11ne of ~he 9arne~ intersects those of the pyroxene and 'thewhole rock at 12.6 and 11.OS'b.y~, respec~ively. sr:87/sr:86raUo is as'sumed t:o be O~700r O~002f the following intersections are obta1necf: Pyz-oxene Gamet: and whole rock. 4.6:t 1.8 b.y. 2.0 5•g+ -1.5 b.y. -232- Conclusions. The Ilb!Sr ra't10 of the eclogite is 000463'1: 0.0006 which is w1tb1~ the range of values of this rat:10 for the souree 9f basaltic magmas. The ~oseib111ty ~bat some baaalts are derived from an eclogite layer in ~e upper mantl~ is therefore not excluded. ar87 developed in The pattern of the radiogenic the whole.rock a~d the garnet and pyroxene phases does not lead to an unambiguous of cry8tal11za~1on. dete~nation The evidence does seem to indicate a very old age for ~he eclogite. .determining low concen~rat1ons causes relatively theposs1b111ty The difficulty of of rubidium and strontium large expeZ'1mentalerrors. 'the intrusion destroying MorC!lover ~xists that a re-d1stribution of radiogenio srB7 between ti1e pbases of the ~clogite during of the time took place of 'the kimbezo11te magma, thus the st:Z'on'tiumdevelopment pattern. -233- Part 2. THE AGE OF ,.THE ELY GREENSTONE AND THE SOUDAN FORMATION IN MINNESOTA. Introduction. 'the Bly greenstone and .the Soudan iron formation .belong to ~he oldest volcanic and sedimentary rocks of .,recambr1an age in Mi.nnasota. Clat:edlnd1r:ect.ly as being These !:ocks have been gran- ol~er than the Saganaga ita which 1n~rudes tbem (Schwartz, 1956, p.l) • . An at:'tempt. was made to cief:e:mu.ne the age of volcanics directly by ~e Rb-St' method. ~e For t~s pu~ose one specimen of Ely 9reenstone.an~ ~oudan shale have been. an,al~zed. In addi~1on the 'rat.io of, c.alelt.e segreg~tlons in ~e bet.ween pillows Sly greenstone was dete~ned. This work is ~e occurring 'sra7/sr86 present not. cons:l.dered to be completed and sUDDary' 1s in the nature of a progress repoX't. Geoloqical Summary. The Bly greenstone consists of an assemblage of lava flows, tuff and I01nor basic intrusives. The xocks axe fxe~ently al~ered to chlorite Bcblst. and -234and h~ve been rec~Jstal1ized contacts. belts near intrusive "igneous The greenstone outcrops in disconnected extend~ng from the Gunflint west\"1ard tb~ough Vermilion district, On~ariO, According to ~ake. Goldich et. al •. (1961) the .Ely greenstone belongs to ~he Keewatin group and has been correlated type Keewat1n~. volcanics first described w1th the by Lawson in 1888 near ~he Lake of the '~s. The .Soudanforma~ion is composed .of al'terna't1ng bands of ~hert and i~on oxides. Ferruginous carbonates, chlorite and sulfides also occur in small amounts. Schwartz (1956) considered an upper member of the Ely greenstone. (1961) prefer to give lot formational The Keewatin volcanics Gold1ch et ale rank. were intruded by the Lau1t'ent1an gran1'tes one of which .i~ 'thfJ Saganaga 11'th on 'the Ontario - Minnesota border. Saganaga grani'te according c1etemi~ed by Golcl1eh at: to X-Ax' and ale (1961)~n because it:s age may actually the Lauren~lan bathe- The ~ge of 'the RbIDSr ages m1ea and feldspar fram pegmat1tes 1s between 2.5 and 2.6 billion ~ver, be the soudan format:1onto be ~reater ~an granites "are believed da~e the Algoman orogeny which tookpl~e years. this to pre- approximately -2352.5 b111ion years agoo AccorCi1ngly, 'the age of the 01.50 R$ewatln volcanics 1nclua1ng the Ely 9reenstone shouldvbs greater than 2.5 billion years. ZXPerirreental Resul1:8. Table 8.2 ,summarizes the analytical reproduced in graphical form in Plgure8.2. assigned t.o the correQt~d rubidium dat:a which are S7:87 /sr86 The errors rat1'os and the and 'stront1um analyses are estimates xeproduc:ibi11ty. Infomation of the regarding the locall1:1es from which the samples ,were colleci:8Q ~e given in' Chap~er 10. Discussion. '1'he,Elygreenstone specimen has a sr87/sr86 of 0.8555 and a Rb/sr ratio of 1.45. ly high for a basic volcanic rock. ratio This is surprisingIts s'tront:lum develop- meni:iine intersects that of 'the upper mantle at '237S.:t 150 million years and sr87/sx86 The calcite pillows = 0.705. occurs in font of narrow bands bei:ween in an outcrop of 'the Ely Greenstone at the C1t:y lim1ts of Bly on 81ghway 21 enroute to, Babbitt. not clear whether these s8gX8gat1ons fo~ Ii: 1s by pxecip1t:a- t.1on from sea water at 'the time of eruption of the lava -236- .... 0\ c-f 0 U) N &n\D ~O ~ ~ • .... 0 e ~ 0 • • 00 +1 . -N 0 c 0 ! a.s ~~ g; U) N .. t ........ 0 'ftl e &2 5 ~ en 8 tN 0 ~ C to 8 r;;. 0 •co ~ en ex) G) an co. \D 0 CD Q • ~ CD s:(lJ cf: or4 ~ CQ .! 0 ... ''8 • au0 .a.I '10 ~, .... -H i-J lIS =' '0 res c m ~~ M +i 0 • .-4 :5 rt) an ;2.1 'tI C) Nlt C'l)N CD ~ \D ~ (X). 0\ ... 0 0' f"'f • pf • COt.4 tL1 ~ ~ .... CD c J.4 0 ~ • CO N NO 1""'0 ..nf"'f ~ COO r-- N CD +1 ~ CD I) "'• !! ... Q (\) ... ... cu 11 U3~ ~ N N .s c Q) 5 ~ f4 r: 0 \D ...s ~ 'M .... ~ ~ r-. an M M' Q4 .. ... r- O) tQ ~ +\ ~ r-- \D ~ t- • 0• • 0• .." ~ ....... an &no ~ .e 0 fa. ~ '80 en ....0U1 s:: G) ~ C ...• 0\ en 0 t- • ...>t ~ Sw cw res CD N ... M r-- • ....\0 ~. ti ~ ~ ~ :1;' ....tJ co ns tI) "r- '" ... C') .....- ~ ~ tJ " QJ C't\ ~ Ul tf\ -237- 0.860 0.840 FIGURE 8.2 ELY GREENSTONE AND SOUDAN FORMATION THE 0.820 0.800 Q780 0.760 UPPER 0.700 2 3 TIME IN BILLIONS OF YEARS or whether they age ths p:toduc't of later metamorphic In any cae~ the al'te~at1ons or hydE"othermal ac:tivi~y. Sr87/sr86 that ratio of the calcite' should be greater of the lava at: the time of its ing that the small, 1: = Assum-' Rblsr ratio in the calcite is negligibly' an intersection stone at foxmatlon. than' 1s ob'tained w1t:h the Bly green- 2300:t 125 million years. The possibility was introd~ced into cannot be excluded that rubidium the Ely 9%eanstone by metasomatism solutions originat1ng fram'the and hydro~he~al Lauren~ian Int:r:usives. In 'this c:asethe age obtained here dates the i:1ma of metasomatism and makes this I'ock unsw.~able for age deteminat.1on purposes. The s~ronUum development linE» for of 'the. Soudan fozmation (1'1gure'S.2). is plotted An lntsJ:sact1on ratios were identical is not. necessarily on 'the same diagram with wl'll have meaningful co-ord1natea the 8pec.1men the B.1y'9reenBto~ only 1£ their a't the tiDa of deposition. true. 0.68S. the value srS7/sr86 ratio at: 1:11epoint. 'of intersection This This The SOUdanf()ZiDation gives an age of 2700:t 250 wi'th the Sly greenstone, of t.he sz:87/sx86 almost: certainly is invalidates 'the age •. On the other hand an intersac1:ion with the development line for the upper mantle gives an age of approximately 1000 million geological This low age is contzoary to years. evidence., These d~screpancies suggest that the analytical data, for the Soudan formation are in eJ:'ror. Conclusions. Theanalyt1cal clusion dat:a,ar8 consistent r. tha't the age of the Ely greenstone 1s 2375 150. This value, h~v,er,. possibility is suspect because of the of metasomatic addition .. Ely' greenst~ne tian granites. would make ~e ,pu~ses. with the con- of rubidium to the at. t.ha time of intrusion of 1;be Lauren- This would lower the age value and rock unsuitable for age dete~nat1on A~cordin9 to Gold1ch et; al. the Bly greenstone is definitely (1961) the age of great.er than2.S'itb:Lllion years. The age of ~he Soudan fo~at1on, as 1nd1ea~ed by the data, is incons1s~nt both with geolog1ca~ evidence and the abundances of stron~ium isotopes as. they are known at "'th,le date •. If . .'the ~ly 9re~nstone was enriched in rubidium after its formation, the Soudan forma't1on apparently was not. For 'this reason it may be more suitable for age determination pu~oses. EVIDENCE OF SUB-CRUSTAL ORIGIN OF ROCXtS FROM THE MONTEREGIAN H1:LLS. In1::r:oduction. of strontium in four igneous Isotop1c compositions rocks from the fttOnteregian bills of calc1'te from a c:arbonatite determined. 'the purpoDeof demonstrate a co-magmatic and in one specimen at Oka, Quebec, were this investigation was to origin for the different igneous rocks and the carbonat1te occurrence at Oka and to test t~e hypothesis that ~he parent magma was de:lv~d from the upper mantle. Geological Summazy. The Monteraglan inent topograpbic hills are a aer1es.of.e1ght prom- features extend~ng fxom Montreal eastward across the St. Lawrencelowlands for a distance of about. 50 miles. 'the centers of these bills composed' of alka11-.rlch igneous. rocks which collectively consti~ute a distinct petrographic one of tile 1nt;rusives ranging in campo~1tlon are province. All but consis~ of 'two or more rock types from ultrabasic and ..yamaski te to lni:ermediate nepheline and basic essex1~e eyeni tea and nordmarkitso A campzehensive of the 9aol~1cal bibliog~aphy and a summary information pertaining to the Mon'teregian hills t1er~ given by Dresser and Danis (1944, po455~482). Geological evidence sU9ges~ that the magma bodies \\iers derived from which ~ormed ~he individual intrusives a common parent whose camposi~1on diori'te~r gabbro. was that of an alkali T"nemagmareservoir to have been inside the Precambr1an \\Jhich 1s believed basement was ~apped 1n several pliic'es and portions of ,:1. t were drawn off througb pipes loc~ted al~ng. a fractu~e systeD?-which may ~ related .1:0 the Appalachian orogeny. s~ructural and Stratigraphic evidence ind1ca~eB a post-Upper Devonian age. Osborne proposed a Tertiary age on the basis of s1mtla:1tles of pleochso1c halos surrounding crystals of zircon and t1~an1~e and those developed 1944). in 'l'erUary intrusives Urry (1936, p.1221) reported 511:.'1.5 million east (Dresser and Denis, years of Mount Royal. (LOwdon, 1960# p.38) foX' a t1nguaite The Geological obtained a Helium age of sheet two miles Survey of Can~da an age of 122::t 9 m.y~ by the K-Ar method using biotite extracteCl from a nord- markite This ~ge be's been con- from Broma Mountain. firmed'by the Geochxonology L~oratory at M.I.T. (Eighth Anno prog_ Rep. 1960)0 rocks The age of 'the igneous of the l-iOntGE"eg1anhills is therefoE"e Cretaceous. ExPer1men~al Results. The experiment~l data are s~~ar1zed and are shown in graphical in ~able 8.3 form in Pigure 8.3. The errore ass1gn~d to the values of the rubidium and strontium concentrations as well as the corrected sr87/sr:86 ratios are reasonable est1mai:es of the reproducibility. of A description the samples appears in Chap~er 10. Discussion. The average value of the corrected arB7/sr86 ratio of the essexi~,. 0.7049. the yamaek1te and the. tinguate is This value is considerably lower than that for However, the nordmarkite which is 0.7156. when the s~~on~um development lines with ~e1r er~or 'envelopes are plotted for 1:het norc1markite and the yamaski te , an intersection and is obtained .(sr8!ts:86)o at t = 11~:t.25 m111~onyears = O.7947~ 0,_001. This time is 1den1:i- cal within exper1men~al ~rror to the age of the biotite from Brame Mountain which was detexmined by 'the K-Ar I -243- If&Q') \00 MO M Ul 00 ~ M , • • • " Ul ~\O NO .... ! ~c,a e: ~~ ~~ \DPI pof 0 C'! .... +1 \0 ..... 0 al' • f/J a pf ::s 0 Ii ore0\' tM o 2 ~ Q) . .&,1 to • fa CD ~ ~ ~ e-. +t ....::s ~ .-:t 0 ~o <Q) ~ \0 \0 co I' O"t "'"" "'"•' . rJ ! Zot. \D J ~ 00 1"-0 r0- C" C" .. cuttS Iii~J. ....... mart ~IG~ MSu a U) , • .... • s:: Iil m M ;M >00\ r- r)CXJ .... 0 ~ .... " qt ~ ~ ~ g .. '0 0 \0'" • ... l' I' ....• ;;.t 0 ~ ~~ ~ ... pot ...G) ~-... . .8 ~ I ,... tUC\e ..., ~f:4"" ~ r-- ... C 4J ~ +1 1"-0 0 U m .go l'tllII fDf ~ • • 00 00 CD ri~ oco CDco , " ... • .... • "'\0 .... Q) 0+1 1'0 ~ ~ ~ ::J ....0 1'0 U\PI ~\O. 0 \D ' an ~ d)~ ~ ~ '8 B .11 s: CD :!1~ "... "'"• co cno ....0 ~-- \D U ...t ~ \DC' 0 0 ~ ... • Nil) \00 00 • • • 0 fo4 tot 0 -1-,• ....0 0 r-.r- CD ~0 u .... ~ 0 -1-1 0 N Q. P-I \DLn 00 1'1"- ~0 ~ ns Ln <P PI ' 00 1'0 ....... 0 \Dln N .... 0 U\ 0 • ClJ 4<1 ....... .... .... ;'1 0 t): 00'\ • • • • +, Q) • CO ,., en • • • 0'\ • • M 00 00 • ~pof 00 1'0 C" 0 0 \0 \0 0 \DO 00 00 ' co ... C" to "-" co .... to-O\ \0\0 \0 en' t¥a ~ " 0 an 0)(7\ 0'\ 0 0 ...• t: s:: ) P'I Cl) to '~tft )1 0 ~ a '8 8 0 0 'm co J OJ CD t2 J ~ C ... ... , Ul ..., ....AS• PI • ~~ en to Q C'I) t!! ~ U .t ~ 8CJ 10 U\ .. III t: lUN aU .s .... tJ PI mU g. Id loa CD ~ a ij~ .... ~ '2o ~ ~ cw0) ',C m 1j ..... .... CD .... ~ -244- to o CX> C\J ":' I"-: I\- I\- o o o o o o \ \ ~ :\ \ , \ \ \ \ o , C\J \ \ \ \ II \ \ \ ~. \~ \ ~ z cS , o CJ) I- ..0 o \\ o V en 0:: UJ \~ \- CJ) \ \ \ \~ \ \ \ , en 0:: l&J \ \ \ \ a::: IZ \ , 0« (OlIJ >- \ \ \ \ \ \ LI- \ o \ \ \ \ ~ \ \~ o CX>en \ \ \~ \ \~ \ I I, \t \ -z, , I \ I \ I1 \ \ \ \ \ \ 0:: l&J Q.. Q.. ::) \ I (!)~ <t \ « \ , o (7) ~ 1\ I \ I \ I - (!) Z o :e z +1 I C\J ~C\J --~~-- --i--- --=---- "}, z « z a: o lIJ t..J ..J UJ>= :I: lIJ o:e- \~ \~ ..J ..J lIJ o:e C\J- \ \ \ t- lIJ a::: :> \ \ (!) LI- ---'"T---\ \ \ \ , o V metl1odo 'l:he ~ime at which the stxont1um development lines 1ntersec.'t is 'therefore also the tirr..~ at t4hich the diff'ereni: rock types wax-s separated magmaand from each o'thar radiogenic from 'the paxen~ and began to accwnula:te strontium 'in accordance with their different Rb/sr xoat1os. There is thus strong evidence that ~e nox:dmarkite is a magmatic. differentiate of tbe same , parent magmawhich gave tise to 'the yamaski te and the o~ber rock type.s. The only way in whlch the nordma&"- ,kite could develop a sr87/sr86 gatio of 007156 i~ 115 million, years is 'to have. had an initial O. 7047. , Had the no~ark1 'tEl ma~ ratio of been contaminated at the i:1me of its 1ntzuslon w1'thold crustalnater1al • . having a s-g87/szS6 ~at10 much ~J:eater'than 0.7047, it could not give an intersection with the'yainas1d.te at a i:ime which is 'known 'to be 'the age of these roclts from .ot:her ev1denceo The 's'rB7/srB6 ratio : O.7047~ 0.001 •. This is ' .. pared 'to 'the aveX'age of the parent. magma was suJ:px1s1ngly'low sr87/sr86 rat100f when com- eleven Recent .oceanic basal'ts which is 0.7012 (Chapter 6). low abundance of. This sr87 indicates 'that the soUrce material -24Gao from which the magma wa~ fo~d ~lt1ng by par~lal or complete had a very low lUl/Sr ratio, source ma~er1al for average oceanic ratio of the scuJ:ce material even lower ~an '!'heRbISr basalts. can be estimated by assum. .' inq an age of the eart:h of 4.5 billion sra7/sr86 initial The RbISr ratio the years and an rat:Lo of O.10~4+ 0.002 (Chapter 1). calculated 1n 'this way 1s O.022:t 0.011. This value is so, low that only an ultrabaalc xock could qua11fy as a source for 1:he p:rimazy Monteregian magma. If ,~e content: of alkali bill ~lements decreases downward in the upper' man~le, 'this would, 8ugges1: that the source xeglon fog the magma was at considerable depth ,in ~he.upper man~le. 'The isotopic concentrated comPosition" of 'st.ront:lum in calcite from Nb ore from the carbonati'te Quebec, was de~em1ned tium 1nthe at Oka, ,foX' cempal"lson with the"st%'on- igneous rocks of 'the Monteregian h1l1so The .relationship ,of i:he carbonat.1te occuX'rences at Oka to the intJ:U81ves of the Mon~ere91anbills clear ana the general quesUon of bonatites independent :t 0.0005. 1s still 1s not. the origin of car- unanswez-ed. The 'average of two detem1naUons gave sr87/sr86 1:1. O. 7062 8,3 As can be seen in Figure -i t:beerrors between -247the carbonat1te value and those of the essexite, and yamask1te .overlap. The sr87/Sr86 ratio of ~he carbona- t1te is noi: feeasurably different igneous rocks. from 'tha~ in the. basic The resul'ts are 'therefore not inconsistent with the conclusion that the calcite of the carbona~ite at Oka is. of magmatic origin. This conclusion :La str~n9thened a s~87/sr86 ratio of O.709~O.002 for Grenville tinguaite by comparison with reported by Gast (1960) l1JQes1:one. An impure Grenville 11mes~ne a small amo~nt o~ mica would give a much containing higher ratio. Gast also reported a value of O.713:t 0.'003 for an Ordovician l1mestone from perry County, Texas. Conclusions. 'rhe' whole-rock Rb-Sr age for a nOJ:dmarkite from Shefford ~ and a yamask1 te irem Ht:. Yamaska was found 115 t 25 11111100.ye~s. detetm1ned by This. age 1s1c!entical the. K-Ar method using to t:o. one b1o~ot.e .'frOm BrCme Mount8iri • .The srS7/s~6 ratio of the paren1: .magma 115~ 2S .. million years' ago was .0.7047 :t..O.OOl 1~caUng ~hat. .the -248- source was sub-crustal. the conclusions ~1e evidence reach$d p:ev1oualy presen~ed confi~B on the basis of geologic evidence that the different rock types are products of magmatic dlfferen~1at1on of a common paren~ magma. The unusually magma in~icates low sr8~/sr86 ratio of this primary that the source region had a lower Rb/Sr ratio than that ~ouna for continental basal~s, suggesting an unusual depth of origin. 'l'be'1so1:op1c. composition of stronUum ,from~e car:bonatlte in calc1~e at Oka.1s compatible w1~ the con- clusion that ~he calc1~e 1s of magmatic, sub-crustal origin and may be rela~ed to the igneous 1ni:ruslves of the Cl:ea. Cb.a'Qter 9. VARIATiONS sr86/sr88 OF THE sr84fsr88 AND r, RATIOS. Introduction. ,During the course of this investigation 96 strontium isotopE! ratio fo~ determinations a total of have been per- in ccoperaUon 'with C.C. Schne'tzler. is composed. of 63 analyses of terrestrial '!'he t;crt.al rocks mos~ of which are bas:Lc igneous rocks of sub-crustal origin, 25 tektite analyses and 8 analyses ofa The opportunity is therefore srco3 standard. given 1:0 compue the ..abundances of the non-radiogenic in the three types of material isotopes of strontium and to calculate average sr86/sr88 and sr84/sr88 ratios. ~rimental Resul~s. Most of the, Bra6/ar8S raUos were calculated frcm . about 80 consecutive ..scans •. The reproduc1bl11 t:y ex- pressed as t:hestandaXu deviation istO.0006 ortO.5~. (flable 9.1). of a s1ngle' analysis Duplicate runs on the same' sample showed thai: i:hesr86/sz:88 ratio c~ vaxy .by -2S0cro> as much as 0.0021 (1.8%) between successive the .samesample (Schnetzler ,T3990) due to isotop1c fractionation 0 analyses of 'lhis variation is in 'the mass spactrometer. ~e sr84/sr88 zoatios are generally based on no JQOre~han 12 to 18 scanso ord~r oft .,.st~dard ~e precision 0.0002 or 3.0% (~able 9.1) deviation error is of the expressed as ~e for a ~1ngJ..e.analysis •. This ratio was found to.vary.by as much as 0.0001 .(1%) between successive analyses of the same sample (Faure R1292, ~chnetzler T3986). Average values of 'the sr86/sr88 and the sr84/sx8B raUos were.calculaUKJ for each of the three types of material as well as for all analyses combined (Table 9.1). Table 9.1 AVerage.sr86/s:8B ,and sr84/sr8B sr86(sx88 Ha'terial sr~3 0.1195 a: :t: 0.00018 <r '1:. 0.0005 O.la192 .. ~kt.1t:e8 :t 0.00015 ::t 0.00077 TeZ'%8strlal Rocks. C!? cr Comb1rtedAverag9 0: .(j 0.1190 sr8~/sr88 0.0066(7) 2S .' 0.0066(55) 63 .:t. 0.00055 :0.0002' :t 0.0006 8 . 0.0068(18) % 0.00006 1:0.00023 .:to.00003 0•.1191 No.of Analyses. :t 0.0001 :t0.00027 :to 0.00007 Or 0.00006 hUos. 0.0066(80) :0.00003 ~0.0002 96 -251- No. I 2 .1180 .1182 .1184 .1186 .1188 .1190 .1192 No.2 2 SrC03 (8 ANALYSES) .1194 .1196 .1198 .1200 .1202 .1204 86 88 Sr/Sr TEKTITES (25 ANALYSES) II .1175 .1180 .1182 .1184 .1186 .1188 .1190 .1192 .1194 .1196 .1198 .1200 .1202 .1204 10 S8r~B~ No.3 IGNEOUS AND 3 SEDIMENTARY ROCKS. 8 )- (63 °6 z w ANALYSES) =>4 0 w 0::2 lL .1180 .1182 .1184 .1186 .1188 .1190 .1192 .1194 .1196 .1198 .1200 .1202 .1204 86 88 Sr/Sr No.4 IGNEOUS a SEDIMENTARY (3) ROCKS. TEKTITES, AND SrC03. 12 10 (96 ANALYSES) )- °8 z l&I • =>6 o l&I .1208 0::4 lL 2 .1180 .1182 .1184 FIGURE 9.1 .1186 .1188 .1190 .1192 DISTRIBUTION OF .1194 86 Sr/Sr .1196 .1198 88 .1200 .1202 RATIOS. .1204 86 88 Sr/Sr Discussion of Raeults~ In o~der to s~udy the distribution of values of ~le sr86/Sg88 ratio in i:be th:8:'eetYPsa ~f ma'ter1al, a ,aezies of histograms was construc~ed (Pigure 9.1). of analyses The numbe~ of the srC03 and the 'tektites is not tBuff!1- ciant to show a distinct distribution patterno .analyses of the igneous dency toward a nomal r001(3, The however, do show a i:an- distribution. The high frequency of values in the range from .1184 to 01187 is striking but is not considered ~o be significant. An examina- tion of the da~a shows t.ha;t no one rock type favours this particular this range. is an instrumental It is t:herefore suggas'ted that. effect and is due to the fact that a large number of a.nalyses \~as made during a pexiod of time when fractionation processes spectrometer favoured this range of values. in the mass The fluct- uations in the measuremen1: of the sr86/Sr88 ratios are well displayed by sample R1292 (Chapter 5, 'table 5.1) and the sreo] sUlldard (Chapter 5, Table 5.2). All 96 analyses are combined in h1stogxam number 4, Pigure 9.1. A normal distribution same mean and standard deviation curve having the was fi ~tad to the h1stogxamo The goodness o~ fit was tested by replo~tin9 the dat:a on normal probability performing paper (Figure 9.2) a c:hl'square test. The plot on the probability graph allows some ev1- Qence of the presence of ~wo superimposed no~al bu1:1ons. and by However, as mentioned before, to instrumental isotope fractionation this d1stri~ is ascribed effects. More data taken over a period of time would be needed to confirm the existence 'of two normal populat1ons. A single line can be fitted to the pOints indicating overall. distribution A chi square is approximately one. to detexm1nC! the gOodness of fit of the data to a nomal I that the a no~al tes't was performed straight dist.ribution . (Dixon and Massey, 1951 ~ p.221). Thestatistic used is defined as where k II number of ca1:egories £1 = observed fre~ency p1 = for each ca~egory ~eore't:l.cal. frequency for each category assuming no~al distribution • .Values of Pi are obtained by calculating 'the normal distribution the areas under curve between the l1m1'1:sfor each -254- CUM. 0/0 FIGURE 9.2 PLOT OF CUMULATIVE DISTRIBUTION OF THE S~'S~8 RATIOS 98 ON NORMAL - PROBABI LITY 95 PAPER. 90 80 70 30 7 20 o 0/ / 10 / / / 0//0 5 / / / 0/ 2 .1182 .1184 .1186 .1188 .1190 86 88 SrlSt .1192 .1194 .1196 .1198 -255- Of,it P-are 'rhea1str1butiom categozoyo ant; degrees of freedomo ~e tabulated fox: differ- number of da9xoses of freedom is determined by d~ = (lc-l) - number of assumpt1ons:o Since the meoan and ~he standardodev!a~lonfor of .data mus't be knOWn before dof = Pi °can°be calculated, k-3. It. is" cusi:omaEY to accept: the hypothesis 'tribution a given set " is n~mal when 'the calculated less titan l.tsopredlct.ed that a g1ven die- value of value at t.he 9~ X 3- 1s confidence limit. ~ The value of ~ . " for the distri~t1on of the sr86/sr88 ratios 1s 15.50 ("able 9.2). Since ~b1s 1s less than 18.31 t.he hypothesis for 10 deg1t'8ssof freedom, tba~ th~ d1s'trl- but:1on1s nomal is accepted at the 9!?"confidence lim1t (D1xonand Massey, 1957, Table A-62°, " '---v- p.385). "The average"oyalue of 'the sx86/Sz88 ratio for the entire" assemblage of" 96 analyses ls0.1191,cr-~ 0.0001," q-. % .. 0.0006. "The spread of °tile dlst:w:1but1on o 1s suell tha~ 0 ~11 previously reported averages .(Chap~~ 3, Table 3.1) as ~ ,,,"" .. o " well as" tbe averages for "tbe'tbree ponedhexe fall 0 types of ma1:erlal re- into t:he band O.1191:t 0.0006 «T) at the 68% confidence l1m1t. 'lite difference between the previous- lYoaccep~edvalue" for t~e sr86/sr88 ofoO~1194 and the -256- Table 902 Chi SquaX'e Teat Fi Inte~al (Ft-fi)2 £1 .!!i--fi) 2 Pi .1180 3.2 1 4.84 1.51 .1180 - .1182 .1182 - .1184 3 ..2 3.24 1.01 5.2 5 ~:i7 3.24 0062 .1184 - .1186 .1186 - •118a .118a ...•1190 7G8 14 38.44 4.93 10.2 12.0 12.7 15 9 23.04 900 11 2.89 12.0 6 36.00 2.26 0.75 0.23 3.00 10.2 8 4.84 0047 7.8 5.2 3.2 ., 7 0.64 0.08 0862 3 3.24 0.04 3.2 3 0.04 95.9 96 .1190 - .1192' .1192 - .1194 .1194 .- .1196 .1196 - .1198 01198 - .1200 .1200 - .1202 .1202 Totals 15.50 overall aveX-age obta1n~dhere i.a statistically flcant. 0.01 0.01 not 81.;01.• This conclusion is conflxmedby c.ompar1ng mean reported by Ald:r;1c:hat ale (1953) the to the average re,ported here • .1l1dr1ch et: al '!'his work _ D 0.0004.0i> = '(1953) CII I sr86/sr88 ar86/sr88 % 0.00032 •. Since = O.119S:t 0410003(0=) a O.1191:tO.0001(~) D < 2. <rbJ 'the 'tWomeans are ident.1cal at. the 95" confidence limi~. Because the difference between 0.1194 and 0.1191 is not statistically significant, or 'modify 'the fo~!: value. values of the sr$1/srSS ratio The distr1butionof is shownin Figure 9.3. standard deviation ,The means is 0.0066 with Cl of 0.0002 (cr). fitted to 'the histogram fit was obtained. b111ty paper there is no reason to %eject normal curve was A (Figure 9.3) &nd a satisfactory The data were also plotted on proba- (Figure 9.4). approXimate a stra19h~ , ~at: i:he distribution The experimental line quite well thUB points confixm1ng is normal. Conclusions. 1..t... The distribu1:ion of the measurements of the sz:S6/srS8 is no:r:malabout: ~e mean at the 95%confidence limit. The average for the total of 96 analyses is 0.1191, - Because of + ' 0-.:= - 0.0001,<:1- Z 0.0006. the da~, indicated the large spread of by a s$;andard dev1at.1on of :t.O.OOQ6«01l. this value is not significan~ly different. from tite prev10usly accepted value of 0.1194 (Nier. 1938) or 0.1195 (Aldrich et al., 1953). The meaBu~nts nomally of ~e sr84/sr88 xatlos are likewise dlstr1bu<ted abou't the mean of 0.0066,0:. :! 0.00003, -258- 14 12 >- 10 (.) Z8 I&J :)6 a 1&J4 a: u.. 2 .0060 .0062 .0064.0066 .0068 .0070 .0072 84 88 SrlSr FIGURE 9.3 DISTRIBUTION OF S8r1S~8 RATIOS IN ROCKS TEKTITES AND StC03 (80 ANALYSES) -259- CUM. °/. 99 FIGURE 9.4 98 PLOT OF CUMULATIVE DISTRIBUTION OF S~iS~8 RATIOS 95 90 80 70 60 50 40 30 20 10 5 2 .0060 .0062 .0064 .0066 84/ 88 5 .. Sr .0068 .0070 .0072 -260- O-='~ 000002, which 1s ,identical to the values previously reported by Nier (1938) and Aldrich at al. (1953). Ev~n though the distribution ofthemeasuremants 1s appZ'oximatelynormal, the wide spread of 'the .d~ta require's a large number of 1nd1v~dua~ mea~urements before a sta~!st1cally reliable mean is obtained. -261- DESCRIPTXONOP Oceanic S~~LES. Basalt:s R1292 . Olivine basalt Aphanitic: 'to glassy, highly ves1cular,almost .a pumice. Scattered phenOcrysts of olivine. Erupted in 1894, Kilauea, Hawaii. Collected by N.E.~. Hinds near Postal Cavern. R1282 Basalt glass Glassy, highly vesicular with bubbly surface indicating top of flow. probably erupted in 1921, K11aue~, Hawaii. R2001 Olivine basalt. Glassy, higbly vesicular, containing sca~tered phenocryst.s of, olivine. Recent: flow on floor of Kilauea sink (Birch #20). Rl993 . Basalt Apbanit:l.c•. ves1cular, no olivine observed. R2002 Mauna Kea, Hawaii phenocrys~s (Bircb #H63). Basalt:. Pine grained, composed of plagioclase and pyroxene. No 'olivine observed. Maui Island, Hawaii (Birch *858). ' ~2029 Oliv1nebaeal~ Aphanitic, ..vesicular w1~h sca~~red phenocryst.s of olivine ..Tu.t.uila, Samoa. (Birch #2594). RiggS Olivine gabbro .Medium. grained, containing euhedral phenocrysts of olivine. Volcanic neck on .Ofu Island, Samoa (Birch 12661). -262."...' R2030 Basalt Aphanitic, ves1cul~r wi~hout obsegv~le olivine phenocrysts. Ofu Island, Samoa (Birch ~2651). R2031 Basalt Aphan:l.t1c, vesicular clase. R4125 Ascellsion with phenocrysts' of plagio- Island' (Blz.ch *2751). Basalt Aphanitic and slightly VG$iculax. Mid-1\tlantic Ridge Lat. '31~18.'8N, LO~9. 40Q 54.'3W. Collected "rift- f~ sloping bot~om.at depth in of about. 1250 meters." Bcho' sounding traces ava11~le from Capt:. Cous'te'au'. Gift: of Lloyd Breslau, 1960. R4156 Ollvinebasalt Horta Payal, Azores. M.I.T. Pe~rology Collection «"6412. R41S8 Obsidian San Miguel, Azores. #6410 Continental R4161 Petrology collection' H.I.T. Volcanic Rocks Hypersthene, augite andesite (bomb) Pine grained, composed of plagioclase and pyxoxene. Erupted in 1909 at: Sakurajima, Kyosho, Japan. M.I.T. Petrology .collec~:Lon. #2081 •. Collected by Sidney Powers, R4221 1915.' . Andesite v1'trophyn Contains hypersthene and augate •...... ~rupU!a.1n 1914 at Sakurajima', Kyosho., .Ja~an.."q . M~I~'1' •. Petrology Collection #2082 •. COllectetl by Sidney.powers, 1915. R4244 Dacite Light. grey, fine grained, pyroxene .and quartz. cOmposedof plagioclase, El'Upted in 1888.81: aandai San, Japan. H.I.'1'. Petrology Collection Collected by Sidney Powers, 1915•. #2079.' .. R1864 Basalt samples from bore holes for KCgna ~1ver Project, Bombay State •. Mapsheet No.47, Gill and 15, between Pophal1 (17026 N, 73046°E). El.2000(M.S.L.)60/922. t R186S Basalt .Same as above. B.H.TS, El.978°(M.S.L.)60/937. R1868 Basalt Same as above. B.H.T4, Elo1811'(M.S.L.)60/949. R1428 Basalt Aphanitic, slightly vesicular. P.M. Hurley and R.lj. Trail ate. u.s. Fairbairn B.H.T4B, Collected by .along Wahkeenah 30 jusi: east of Portland, Ore90~.. Samples from flow cen'ters only, starting at t.op,.~. of stratigraphic section of about 1000 fee't. 1U435 Sasalt Aphanitic, slightly vesicular. No minerals identifiable.' Same as fo: R1428. R1443 Basal~ Aphani~ic:, sllghtly. vesicular. Same as for R142Bo Bottom of s~ra~lgraph1c sac~lon. R4209 Diabase Pine grained, composedof plagioclase and pyroxene. cliffe R4210 Palisades Inb!rs~ate Park, 'top of Diaba~~ pa~1sades Interst~te. '1'ouriB~ Camp, 100. f~. Hudson River. R4211 Diabase. Sleeping Giant Quarzy, .Mt. Camel, Haven, Connec:Ucut. R4159 above north of New Olivine basalt Vesicular, apban1't1c: wi'th olivine phenocrys1;s. Post-Pleistocene ~n.age, Iceland, from M.Z.T. petrology Collec~1on. R4243 R1574 R4245 Taehy11~e (basalt glass) Glassy, scorlaceous flow surface. I~aly, M.I.T. Petrology Collection Mt. Vesuvius, i459. Basalt Aphanitic, ves.icular, Yellowstone Park. Basalt Tert1ary age, SquawCreek, Montana. M.I.T. Petrology Co~lection #4805. Precambrian Basic Xn~rusives R4193 Anorthosite, with shaxpbands of ensi:at1te Wa~erkop 602, Bushveld complex, Shaler Memorial Expedi1:ion. Harvard #13444x. Donated by Dr. J.B. ~ompson. R4194 Norit:e Mosego Mountain, Buehveld comp~ex, Hackney 700, Shaler Mem. Expedition. Rarvazd #13434x. Donate~ by Dr. J.B. Thompson. R4195 Pyroxenit.e - overlying cbz:onl'teband Mooihoek 147, Bushveld Complex, Donat.ed by Dr. R1231 J.B. Harvard #13452x. Thompson. Gabbro (Duluth Gabbro Complex) Eight dnu.les south of bridge at Zsland - orchard Lake, west: siae of Rte. 4, Minnesota. Specimen 'taken 6- below glacialsuxface. R4l86 Anortho81~e Stillwater complex, Montana. MontereQ1an 81119 R4i79 Yam~s1dte Mt:. Yamaska, .Quebec. 1960. Collected by G.R. Beall, -265- R4180 Essex1te f4t. Johnson, OUebec •. Collected by G.B. Beall, 1960• .R4181 Nordmarkite Shefford, Quebec. Collected by G.R. Beall, 1960. R3069 T1ngua1te (1) . St. R3253 Joseph's. Boulevard, Montreal Eas't, Quebec. Carbonatite. Ore grade Nb. concentration. Collected by J.A. Gower. C3253 R3111 Oka, OUsbec. Calcite concentrate prepared from R3253 by means of Frantz Isodynamic Separator. Bet~er . ~ha~95" pure calcite. Eclogite composed of pyroxene and gaxne~. Entire sample ground up, no. hand specimen available. Inclusion in kimberlite pipe, Robert Victor Mine, S. Africa. Given by F. B1J:ch. P3111 Pyxoxene concentrated G3111 Garnet concentrated Keew&t1ft-.Volcan1cs from R3357A. Chlorite'schist' framR3111. from R3111. M1nnesot:a (Bly gre8ns~one) . Dark' Green•. Bntire specimen w~s cxushed. COllected by P.M. Hu'rley, 1956, at SOudan Mine, Vermilion Range, Minnesota •. Geological SOc:Le~y of America Pield Trip Guldebook~ Precambrian of Northeastern Minnesota, p.100. -266- R3358B Qsaenatone (Ely greenstone) Pl.llegreen, not schistose. Contains calcite occurring in t1:lin bands between .pillo\nJs. Collected by P.M. Hurley, 1956, at City limits of Sly on B1g1tway 21 enroute ''to Babbi1:t. GoS.A. Guidebook 1956, p.104. C3358B C~cite leached out of R3358B by means of O.SoN. vycor-dis~illtad hydrochloric acid. R3354 Shale or fine tuffaceous sed1men~ Soudan foxmatlon. Collttcted at Soudan Mine, Venti110n Range, M1nnesota by P.M. Hurley, G.S.A. Guldebook 1956, p.9S. 1956. ,Shale composites R4184 composl~e of App~lac~an shales of Paleozo1c age. S~le~ WSES col1ec~ed from exposures of, shale of pennsylvania '.tUrnpikegoing east from Harrisburg_ Thickness of individual shale beds used 'to weigbt proportions of samples in the composite. by prepared Jul1anPe1ss. R4l85 composite of Paleozoic shales from the west coast. Prepared by Pe1s8 in same way as described above. 'R4132 Gneiss Labrador trough,' eas'tem gneiss, zone B. Unconformably underlying tueUmoJ:phozed trough sediments. Collected byPierxe Sauveo (8-42-57) fxom 58°15°5, 69014°W. , .,,267- BIBLIOORAPh.~ Aldrich, L.T •• L.F. Herzog, W.K. Holyk, F~B. Whiting and L.F. Ahrens, 1953, variations in isotopic abundances of strontium" Physo Rev., 89, 631-632. Aldx:1cb, L.T., LoP. HeJ:zf?9, J.B'o Doak and G.L. Davia" 1953, Va:iations 1n stgont1umlso~ope abundances in ~nerals, p~rt I, mass spectrometric analysis of mineral sources of ,stront.ium, Trans. Am. Geophys. 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Mit:chell, 1953, Trace elements in a suite of Hawaiian lavas, Geocb1m. Coamocb1m. Ac1:a, et 3, 217-233. Webs1:er, R.K., J.li. Morgan and A.A. Smales, 1957, Soma Recent Harwell analytical work on geochronology, Trans. Am. Geopbys. Union, 38, 543-545. 'iiclaoan, P.E., 1948, Isotope ratios.: a clue to t;he age of certain marine sediments, J. Geol•., 56, p.61. Wickman, If .B., 1954, The "totalamount: of sediments and the compos:LUon. of the average igneous rock, Geoch1mo e't C:osmochlm. Acta,S, p. 97 • Williams, A.P., 1932, Genesis of the diamond, 2 vola., B. Benn,' London. Wilson, J.'t., R.D. Russel and R.M. Farquhar, 1956, Rad1oactivi1:y and age of minerals, Handbuch der Physik, 47,' p.2Sa. -277BIOGRAPHY. The author was born on May 11, 1934, in Tallin, Estonia. As a result of the outbreak of the.second World War the. family was forced to move to ~astez:n Germany in 1939 and to wes~ern Germany 1n 1945. .family immigrated ~o Canada1n London, The 1952 and settled in Ontario. '!'he author c:omple~edhis high schOOl education at .the' LOndon Central Collegiate ~n 1953 and 1n 1957 received a B~SC. degree in Geology from the University of Wesf:ernOntario. student He was enxolled as a graduate in the Departmen~ of Geology and Geophysics in 1957. ~ile at -M.I.T. he. was a balf-time res~arc:h assis- tant for two years from 1957 to.1959 and.a part-time instructor for Sclen~lfic German in the fall semesters "\ ~f 1959 and 1960. in geolog1cal Foothills field 'the aut:hor has also spent six Bummers wor~ on t.he Canadian ~leld and ~e of 'theRoc'ky .Mo~n1:a1n.B•.. As an undergraduate and as a graduate student at: M~I.T. 'the author .was awarded -the Board of Goveznors' ~m1ssion ScholushipII, the .J.P. Bickell PounClat1on -278Scholarship, the Board of Governors' Scholarsh~p, .California Standard Company Scholarship. Gold Medal, the 'Imperial and a scholarship the ~he University Oil Graduate Research scholarship from the 14.1.'1'. Ca!2adlan T~st Pund. The author is a member of the hner1c:an ~ophyslc:al Union,' the Geochemical Socie1:y and ~be SOciety of Sigma Xi. In september 1959 the author was married to Miss BarbaraL.L. Goodell of East Dennis. Massachusetts.