295
The Canadian M ineralo gi st
Vol. 32, pp. 295-306 (1994)
FROMKIMBERLITE
MANTLEXENOLITHS
NEARKIRKLANDLAKE,ONTARIO
HENRY O.A. MEYER
Indi.ana
47907,U.S.A.
WestLafayette,
Sciences,
PurdueUniversity,
Department
of Eartha.ndAttnosphsric
MICHAELA. WALDMAN
Drive,#14,Fon Collins,Colorado80523,U.SA.
WaWman
Consubing,636
Cheyenne
BLAINEL.
GARWOOD
Cormvironment,
P.O.Box 111,BearInke, Michigan49614'U.S.A.
AssrRAc"r
tlltramafic mantle xenoliths, predominantlyconsistingof gamet therzolite, occur in the C-14 kimberlite, approximately
25 km east-northeastof Kirkland Lake, Ontario. The xenolitls are coarsegrained and show evidenceof recrystallization
and incipient metasomatism.Conditionsof final equilibrationof the xenoliths,basedon a two-pyroxenegeothermometerand
geobarometer,
rangefrom 917'C and 35.8 kbar to l22l'C and 55.6 kbar. The xenolithsseemto have
A1-in-orthopyroxene
equilibratedat conditionscloseto a 40 mWm2 conductivegeottrerm,andmost within the stability field of diamond.The xenoliths haveprobablybeenderivedfrom a thick cratonicroot-zonebeneaththis portion of Ontario.
Keywords:xenoliths,kimberlites,mantle,diamond,Kirkland Lake, Ontario.
Sovruanr
Nous d6crivons des x6nolithesultramafiques,surtout de lherzolite h grenat, provena[t du manteauet pr6lev6sdans la
kimberlite C-14, sinrdea 25 km i l'est-nord-estde Kirkland Lake, en Ontario.Les x6nolithes,l granulom6triegrossiBre,
pr6coce.Les conditionsd'dquilibragefinal des x6nolithes,telles
montrentdes signesde recristallisationet de mdtasomatose
qu'indiqu6espar la g6othermom6trie
des assemblages
d deux pyroxbneset la g6obaromdtried'aprbsla teneuren Al de
variede 917'C, 35.8 kbm d 1221"C,55.6kbar.Les x6nolithessemblentavoir atteintl'6quilibrei descondil'orthopyroxdne,
tions voisinesd'un profil g6othermiqueselonun modblede conductionde 40 mWm2, et dansle champde stabilit6du
diamant, dansla plupart des cas. Les x6nolithesauraient6t6 d6riv6s de la racine cratoniqueprofonde sous ce$e partie de
I'Ontario.
(Traduit par la R6daction)
Mots-cl6s:x6nolithes,kimberlites,manteau,diamant,Kirkland Lake, Ontario.
from only one kimberlitefield (Mitchell 1977, L978,
1987,Jago& Mitchell 1987,Kjarsgaard& Peterson
Diamond and kimberlite explorationnortheastof 1992).The xenoliths describedby Mitchell (1977)
Kirkland Lake. Ontario.has resultedin a number predominantlyconsistof gamettherzoliteandoccur in
of discoveriesof kimberlite. In at least one instance,a the Elwin Bay kimberliteof SomersetIsland,N.W.T.
drill core in kimberlite has intersecteda numberof The paleogeotherrndefinedby the xenolithsis similar
garnet-bearingultramafic xenoliths of mantle origin. to thosedescribedfor southemAfrica (e.g.,Boyd &
Thesexenolithsprovide information on the hitherto Nixon 1975)and Montana(Hearn & Boyd 19751.
unsarnpledregion of the basallithospherein this part Mitchell (1978) suggestedthat the garnettherzolite
xenoliths in SomersetIsland representthe upperlimb
of the SuperiorProvinceof the CanadianShield.
In spiteof the discoveryof kimberliteselsewherein of an apparentlyinflected geothermand were formed
of deformationandmetasomatismof
Canadamantlexenolithshavebeendescribedin detail as a consequence
INrr.ooucrroN
296
TIIE CANADIAN
MINERALOGIST
the uppermantleby a rising diapir or by thermalinteractionsbetweenkimberlite and xenoliths.Xenoliths
from the Ham kimberlite in SomersetIsland also
outlinean inflectedgeotherm(Jago& Mitchell 1987),
but Mitchell (1987)reassessed
the datausingdifferent
geothermobarometers
than previouslyusedand concluded that tlre geothermsare not hflected. Recently,
Kjarsgaard& Peterson(1992) havere-examinedthe
xenolithsin the kimberlitesfrom SomersetIslandand
concludedthat the apparentpressuresand temperatures of equilibration of the xenoliths define a
44 mW/mzshieldgeotherm.
the GreatLakes(Hobbs1899,Blue 1900,Flint 1957,
Schwarcz1965,Brown et aL.1967).Lee & Lawrence
(1968) describedan occurrenceof kimberlite in the
Upper Canadamine, GauthierTownship,10 km east
of Kirkland Lake, and Arima et a/. (1986a)briefly
notedthe chemicaland isotopicnatureof the Nickila
Lake kimberlite(B-30), Bisley Township(Fig. 1).
Brummeret al. (7992a,b) summarized
the historyand
resultsof kimberliteexplorationin the Kirkland Lake
area,and briefly describedthe Nickila Lake (B-307,
C-14, A-1, A.M47, A^4, MA-2A and B-41 kimberlites.
The kimberlite in the Upper Canadamine occursas
KrvrssRurE:OccunntNcsaNo NanrnB
thin dikes cutting Archeanmetavolcanicrock. It is
describedas a micaceous
kimberlite(Lee& Lawrence
The occurrenceof kimberlite in the Kirkland Lake 1968,Watkinson& Chao 1973,Mitchell 1978),but is
areaof northernOntariohasbeenknown for over four probablya mica-richGroup-I kimberlite (Mitchell
decades(Satterly1948,I*e & Lawrence1968).The 1986).Followingthenomenclature
of Clement(1979),
searchfor suchrock was,in part,initiatedby diamond and Clement & Skimer (1979), the kimberlite in the
finds in glacial outwashmaterialin the region southof Nickila Lake, A-14 and A-l diatremespredominantly
Flc. 1. Generallocation map of kimberlitesin the region of Kirkland Lake, Ontario
(modified after Brummeret al. 1992u b). The kimberliteswere codedby orhersafter
the numberassignedto the geophysicalanomalyin a specifictownship.Thus C-14 is
the fourteenthanomalyin Clifford Township.
MANTLE XENOLITHS FROM KIRKLAN'D I.AKE
consistsof tuffisitic kimberlitic breccia,whereas
AM-47 is predominantly hypabyssalkimberlite
@rummeret aL l992b). Aima et al. (1986b)noted
the occurrenceof upper mantle xenoliths in the
Nickila Lake kimberlite. Chemicaland textural variations in mica from this samekimberlite andthat in the
Upper Canadamine were describedby Aima et al.
(1986b).Fragmentsof hypabyssalkimberlite (phlogopite-rich group-l kimberlite) occur within the Nickila
Lake kimberlite breccia. The kimberlite diatremes
have intruded crustal rocks of the Archean Abitibi
greenstonebelt, Superior Province. Lee & Lawrence
(1968)quoteda K-Ar isotopicageof 151Ma for the
kimberlite in the Upper Canadamine, and Aima et al.
(1986a)determinedan ageof emplacement
of 147Ma
for the Nickila Lake diatreme.Brummeret al. (1992b)
reportedagesbetween147 and 158 Ma for seven
kimberlitesin the Kirkland Lake cluster.
Approximately 9 km eastof the Nickila Lake kimberlite, a small (200 x 100 m) kimberlite, referred to
as C-14, was discoveredin Clifford Township as a
result of a magneticsurvey and subsequentdrilling
(Fig. 1). The kimberlite,examinedin drill core, is a
TABLE I. BIJLK SMPOSITION
C-l4KIMBRLITE
297
tuffisitic breccia, but at least three petrographic
variants are recognized,eachwith a distinct mineralogy. Threexenolithshave actedas nuclei aboutwhich
has crystallized earlier hypabyssalkimberlite. This
material, together with the enclosedxenoliths, are
autoliths in the intenselybrecciatedpresenthost
kimberlite.
Bulk chemicalcompositionsof kimberlite are susceptible to error due to contaminationby crustal and
mantle xenoliths and megacrysts(Clement 1982'
Mitchell 1986). A measureof the proportionsof
crustalor weatheredmaterialin a bulk composition
of a kimberlite is provided by the "Contamination
Index" of Clement(1982) tC.I. = (SiO2+ AlrO, +
NarO)/(MgO+ 2KtO)1.A C.I. closeto unity is conof most uncontaminated
sideredto be representative
kimberlites.The bulk compositionof the leastsample(C.I. -- 7.2)in the C-14 kimbercontaminated
lite is presentedin Table 1, and comparesfavorably
with the range of compositionfor kimberlites
(Mitchell 1986).On the basisof the U/Pb isotopic
system,Brummer et al. (1992b)reportedan age of
156 Ma obtainedfrom an averageof six separate
samplesfrom the sameintrusion.
XENoLnHS
ULTncMAFIc
OP TgE
Description
C-14
SiO2vt% 35.15
TiOt
3.r3
3.03
Al,o.
0.x
6.16
FeO'
3.0
0.15
MnO
6.54
CsO
lvlgO
n.64
MO
aJ4
BaO
0.13
NaeO
0.32
KzO
2,t6
t:n
C1c"
6.96
H'26+
F3&
Total
Worldvide*
Range
25.r-K.4
0.98- 3.39
2.45- 4.45
0.u -025
3.S-6.?8
1.71- 8.68
0.10-0.19
936 -2r.4
17.0-28.6
0.12- 0.48
0.34- r.Ez
4.58- 16.2
3.70- lO.2
9E.34
Cr ppm 64
Nb
140
Rb
105
Sr
145
k
Ia
47
Y
<5
Th5
Ta?s
me&$t
65
L4t
73
E51
1E4
140
17
11
'
Vsldslds r@gesad 0re@ aB t8b ftm
ldihhell{1986).Ilg mcenrorhndtrarsslm€ots,
hctrdbg Ba eas deEdmled dstng X-ray
flsoreso€ore,
€trceptfc Co @d Ni, st€rc eMc
wasM. fte ouo€orad@
ab|6Fd@rye{eosoopy
of ![ drer olemffi eas ds@fud urhg DC
plryna - armig edsst@6pec6oscry.
Severalgarnet-bearingultramafic xenoliths were
intersectedin the drill core, but becauseof the small
diameterof the core (-5 cm), tlre amountand size of
the xenolithic materialare limited. Nevertheless,some
xenoliths are smallerthan the core diameter,but from
the generalcharacterof others,the maximum size for
most xenolithsis probablyless than l0 to 20 cm. A
consequenceof the small size of the xenoliths is that
there is large uncertaintyin the modal proportionsof
the constituentminerals.If lherzolite is defined as
having >l\Vo modal clinopyroxene (Streckeisen
1976),thenall but oneofthe xenolithswouldbe clasharzburgite(t garnet).
sified as clinopylroxene-bearing
In contrast,if nomenclatureis basedonly on the
without recourseto modal permineral assemblage,
centages,then garnet lherzolite would accountfor
60Voof the xenoliths,and lherzolites,407o.Tlrclalter
nomenclatureis conventionallyused in xenolith
studiesandwill be utilized herein.
The xenoliths are relatively coarse grained
(< 6 mm). Two of the ten xenolithsdisplaya porphyro'
clastic texture (Harte 1977), in which strained large
grainsof olivine havebeenin part recrystallizedalong
grain boundaries,to small (0.05 - 0.5 mm) neoblasts
of olivine. One xenolith is characterizedby complete
recrystallizationof olivine to a granuloblastictexture
of small equant crystals (0.3 mm), many having
l20o angularrelationshipswith neighboringgrains
(soap-bubbletexture).The remainingxenolifhsconsist
298
TIIE CANADIAN
MINERALOGIST
TABLE 2. TE)(n'RBr MINBRAI.OCYAND FII{AL EQUILIBRA'IION CONDITIOT.CS
SUMMARZED FORINDTVIDUALXENOLru]S FROT,IC.I4 KIMBBRLTTE
Smple *
Nmber
155
t6t.2
168
r982
r9B.7
308.5
505
ffi2
M.7
506.E
MiMalog/
Secmdarv
Pht Clx-Sp
Texture
Primy
Gt Ol Qx Cpx
P
P
c
G
c
c
(a
(a
(c)
(9
xxx
xxx
xxxx
xxxx
xxlx
xx:r
x(x)x
(x)x
x{x)xx
(x)x
x
xxx
xxx
x
xI
x
xx
x
xrx
x
T("C)
KKbtr)
FB86 BKN Mg,4 I(B
1075
885
1038
rnl
1133
957
11,0
r29l
tw t2to
ws rw
48.8
40.0
51.?
55.6
y.7
6.4
1018
969 rr?3
911
K9
45.4
39.8
j18.3
41.9
54.1
60.9
59.0
12.2
*Samplenumbqsr€present
rslatiw positionanddeprh(ft) in drill cue.
F886,BKN,Mgt4, KB - gecexr
of coarsegranularlherzolite (t garnet),althoughthree
examplesfrom the lowest portion of the drill core
occur in which the olivine is completelyser?entinized
Clable 2). Unfortunately,the samplesof the xenoliths
in the drill core were not sufficiently large to provide
enoughmaterial either for bulk chemical analvsesor
isotopicstudies.
Mineral compositions
The constituentmineralsof the xenolithswere analyzed using automatedelectronmicroprobes(MAC
400 and CAMECA SX50) at PurdueUniversity. The
analyseswere performedwith an acceleratingvoltage
of 15 kV and a beamcurrentof 20 nA. Well-analvzed
mineralsand syntheticglasseswere usedas stand;ds.
Data were reducedand correctedusing Bence-Albee
(MAC 400) and PAP (SX 50) procedures.Resultsof
analysesof the samemineralsusing both microprobes
were found to be similar and within the normal
accuracyfor microprobedata.Concentrationsofmajor
elementsare accurate within *27o of the elemenr
present,whereasconcentrationsof minor elementsare
less accurate,but are reproducibleto t0.08 wt7o.
Resultsof individual analyseslisted in Table 3 are
averagesof a minimum of five analytical points per
mineralgrain.
For the determinationof Ca in olivine, particular
care was taken. At least five analyseswere done for
Ca on each of six different grains per xenolith.
Throughoutthe period of time of the analyses,a speciflc standardolivine (SCKA 1), whoseCa contenr
had beenpreviouslywell determinedas 0.0532wt%o
Ca (G. Brey, pers.comm., 1992),wagmonitored.The
averagevalue for SCKA 1 (20 points) during the
monitoring processwas 0.0544 wtVoCa with a
standarddeviationof 0.0044.
Olivine is predominantin all samples.Grainsare
generally3-6 mm in major dimension,exceptin those
xenoliths that display recrystallization.The olivine is
forsteritic, varying between91 and 92VoFo. TiO2,
Al2O3,Cr2O3and CaO contentsare all less than
0.05 wt%o(Tables3, 4). Orthopyroxeneoccursin
anhedralgrains (< 2 mm) and is unaltered,even
in thosexenoliths in which the olivine has beencompletely replaced.Orthopyroxenealso has a restricted
compositional
rangebefween92-93VoEn (Fig.2). The
concentrationsof the minor oxides TiO2, Al2O3,
CrrOr, CaO and Na2Oare all less than 1.00wt%o,of
which Al2O3is the highest,ranging between0.5 and
0.9 wt%oClable 3). The highestAlrO, contentoccurs
in orthopyroxenein the granuloblasticxenolitl, which
also has the highest apparenttemperatureof final
equilibration.
Emeraldgreen clinopyroxeneoccursas irregular
grains,< 2 mm across,and is characterizedby varying
degreesof a "wormyo' or "spongy" alteration that
proceedsfrom the marginsinward to the core. It is
diopsidic, with minor solid-solutiontoward jadeite.
The proportionsof Ca:Mg:Feare constantand average
(Fig. 2). Chromium contentsvary between
53:.43:.4
0.8 and 2.3 wt%oCr2o.3,whereasAlrO, ranges
between 1.73 and 2.52 wt%o,and Na2Ocontents,
between1.50and 2.51 wtVo(Table3). The altered
"spongy" rim to the pyroxeneis slightly enrichedin
Ca relative to the core, and on averagecontainsmore
Ti and Cr, and lessAl and Na than the core (Table 3).
Similar textural and chemicalfeaturesof alteration
have been noted in diopside in xenoliths from otler
worldwidelocalities(e.9.,EhrenbergL982,Hopset al.
299
MANTLE )GNOLITHS FROM KIRKLAND LAKE
TABLE 3. AVERAGE @I\4FOSITION OF PRIII,IARY MINERALS IN )GNOLITTIS
Ol
Opr
168
cpr
Coro Rim Camst
Si02wt% 4l.t
57.9
54.8
:so
Alzof
CtZOf
FeO
Ii{SO
CaO
r lo
Ni{,
Na2O
KZO
Torals
4tS
543
0.v2 0.09 0.21 0.lE 0.u2
0.02 054 L2r 0.9E l9.r
A.A
8.64
49.6
0.O3
0.06
0.34
0.00
0.00
99,E
2,y
932
LV}
5.39
5.11 2.65 23t
7J0
YS
162
17.1 l9.l
?n2
0J0 lE.6
5.83
0.06 0.41
0.06 o.lt
0.ll
0.03 0,lt
0.{r2
221
lil
024
0.00
0.00
0.m a.v)
0.01
93
99.1 99.0 99.r
Ol
cpr
Cors Rlm Garner
Opx
413 573
0.04 0a
0'q) 0.95
0.05 0.45
7.63 4.61
50.4 vs
o.ut 0.?9
0.rr 0.r2
0.30 0.G
0.00 023
0.00 0.m
99A 99.E
422
'4,3 533
0J2 Q.u
0.s
252 0.E6 183
1.95 \21
5.6?
2.92 255
6.S
17.8 18.6 b.4
r13 19.?
5.r2
0.10 0.06 ai2
0.06 0.@
0,01
Lo' Ln
0.ot
o.ct 0.01 0.01
9.4 99.0 993
cp(
Opr
SlO2wt%40.9 576
Tio
0.00 0.v2
AIZOf
0.00 0.65
0.00 0.23
c{2/4
rc
EJ6 5.18
MsO
49.4 Y.6
CaO
0.8 050
MnO
0.16 o.tl
Nio
o,n 0.08
NaZO
0.00 0.rE
K2O
0.d) 0.00
Totals 9.6 W2
Cors
Rln
Came!
t4S
0.00
t.73
rr0
L52
r73
m.4
0.(r,
0.{n
lJO
0.01
99.3
54.6
4t9
0n
lst
1.45
L6
16.9
m.9
0.D
0.0t
r.l9
0.01
993
0.09
193
4.r2
?.S
r93
5.6E
0.4r
0.m
0.00
0.01
993
506.7
cpx
Opr Co(e llio
SlO2trt%t.8 554 345
Tto
0.0? 0.r5 0.36
At/4
0.61 2,.4 7s7
0.o, 0.(, 0,96
C.pj
rto
1gt 2j'3 L6
Mp
333 r6J 17.0
CeO
O43 20.0 2lA
MoO
0,r2 0.07 Ol2
Nio
0.m 0,05 0.(n
N62O
0.m $2 4.42
K2O
0.00 0.()2 0.Ul
Toulr
96 9,1 992
Ol
Opx
4l.l 37.6
0.04 0.07
0.9, 0.58
0.g7 036
t39 4.92
49.9 yi
0,03 0.,f6
0.t4 0.t2
030 o.il
0.00 0.00
0.m 0.00
r$.0 9a
t(<
308J
Ol
tw:1
1982
Ctarnat
42.1
421
?2.4
2.*
7,78
20.1
4.U
O.4
0,m
0.00
0.O2
1004
Ol
,10.0 s6s
0.00 o.il
0.05 0.E0
0.00 0.38
8.43 5.41
503 359
0.(B 0,6r
0.u 0.11
o.v 0.12
0.00 0.14
0.@ 0.00
993 100I,
---"p"
'tr.2
Qr
Coro RIm
5E.0 552 92
o.ut 0.13 038
0.70 Lr4 rr8
0.31 r.E4 22r
4J8 226 237
343 16.6 l?.1
0J2 193 m.9
0.12 0.09 0.1r
0.05 0.05 0.05
033 1.96 l,&
0.01 0.u2 0.0r
wA 9.6 99J
54.1 t3J
orE
Lst
2gl
L6
16.4
t83
0.r2
0.03
233
0.03
98.8
Rlm
55.6
0.16
Lr6
L31
2S
l6.t
l1J
o.lt
0.04
2sr
0.3r
91s
52:7
0.8r
1.36
L4r
258
t6J
212
0.t4
0.06
0.99
0,00
9E.?
Ofirtet
4r3
0.30
17.3
1.8
7.47
t8J
5.97
0.34
0.m
0.00
0.00
gtA
t672
cpr
C6o RIm
Opx
cpr
Cots
030
tg2
e5l
2J3
r13
zto
0.13
0.05
t.O7
0.m
93
Ol Opx
cpx
Ccc Ritn
395 57.0 543
0.00 0.09 0J8
0.00 05? 0.61
0.02 030 ljo
92r 5.62 423
5lo 363 19.6
0.m 02E 185
0.r3 0.13 0.14
0.35 0.rr 0.(I'
0J0 0.09 0.E2
0.m 0.00 0.01
rm3 1005 1m,4
545
0r5
L4r
L48
L$
r5.7
195
0.09
o.ct
L4S
0.@
9:l
505
5064
Ops Cq€ Rttrl
opr
Gaoet
5?5 539 5L9
0.09 0.t, 0.60
LzE
o.72 2A
0.30 l&4 r.E3
5.X 23' 2.56
35.r l6J t7.4
0.38 203 ZLL
0.r3 0r0 0.09
0.08 0.04 0.08
0.m 196 0.99
0.00 0.Q 0.4
995 99J 99,7
585
0.05
0.n
0,49
43r
y4
0.m
0.16
0.01
0.m
4tJ
02tt
n9
7.6
613
20,4
5.,1E
0-D
0.03
0.6
99.6
990
0.m orn
All ko,nmportsdasFeO.
that the alteration
1989);thoseauthorshavesuggested
of the diopside margins may representan attempt to
chemicallyre-equilibratein responseto rapid decompression.
Pale phk to pale purple garnet,less than 3 mm in
size, is invariably surroundedby a kelyphitic rim of
varying thickness.The garnet is Mg-rich (pyrope)
(Fig. 2, Tabte3), with TiO2contentsbetween0.02and
0.88 wt7a. Na2O contentsare below the limit of
detection(0.01wtvo).Chromiumrangesfrom 238 to
7.23 wt%oCr2O3,and calcium, between4.8 and
6.0 v,tToCaO (Fig. 3). Includedin Figure3 for comparison are data for a number of garnet macrocrysts
from the kimberlite. Most of the garnetcompositions,
including thoseof the macrocrysticgamet,fall within
the field defined as G-9 (Dawson& Stephens1975)
300
TI{ECANADTAN
MrNERALocIsr
TABLE4. CI)NCENTTATION
OFCALqUMIN OLMNE
Sample
Nrob€r
itro&of
Points
Ca
FI%
Sd-dsv.
CaO
wt%
n
0.0212
0.0129
0.0186
0.0371
o.o229
0.0lyt
0.0058
0.wr3
0llc42
o.m52
0.@47
o.(n47
0.0296
0.0180
0.0260
0.05a
0.0321
0.uzr6
155
t6t2
168
L9t.2
r9B.7
gB.5
A
t5
A
25
25
and have CrrO3 contentsbetween2.2 and 10.5 WVo,
CaO from 1.98 to 7.57 wt%o,and TiO, less than
0.70wtVo.
Associatedwith the gamet and developedbetween
the kelyphitic rim and the neighboringminerals(mostly olivine) arenanow small irregular platesof phlogopite, tiny grains of chromite, some enclosedin
phlogopite,and occasionalAl-rich diopside(Table 5).
The spinelassociatedwith the kelyphitic rim is Al-rich FIc. 2. Compositionsof coexistingorthopyroxene,clinoandTipoor; in contrast,the spinelincludedin phlogoplroxene and gamet from ten xenoliths in C-14 kimberpite is richer in Cr and Ti (Table5). The mic4 Al-rich
lite, plottedin termsof the major elementsCa-Mg-Fe.
pyroxene and spinel representthe result of incipient
modal metasomatism.Similar featureshave been
Concentrote gornet
Xenolith gornet
herzolit
Horzburgite
ond
3
DuniteField
-{
2345
9
Cr
1 0 ' l1
12 13 14 15
rO 3Wt%
Flc. 3. CaO versus Cr2O3relationshipsfor gamet from xenoliths and kimberlite, C-14
diatreme,Ontario. Fields outlined for garnetfrom lherzolite and haxzburgite-dunite
from Sobolev(1974).T\e dotted line representsthe boundarybelow which plot
857oof ultramafic-suitegarnetinclusionsin diamond(after Gumey 1984).
301
MANTLE )GNOLITHS FROM KIRKLAND I.J'KE
TABI";E 5. AVERAGE @MPOSITIONS
SiClrwt%
Ttd
Alr6a
g6q
Mgo
CsO
MnO
ldo
Na,rO
KzO
Totals
.
Ar
168
llfica
$.9
o23
6.52
0.?5
3J.2
16.5
2no
o.n
0.m
0.6E
0.01
99,4
4t,5
0.9E
tL9
l.(ts
3.40
4.4
0.00
0.08
o24
0.u3
105
95.2
Clx
0.25
o.?3
4L2
23.3
13.7
19.r
0.04
0.36
52.8
o.?5
42t
1.05
4.9
m.a
14.6
0.30
0.05
o.41
0.01
98.9
utz
0.m
o.00
99.3
,,s67
SiOrwt%
fid
Al,or
F3F
Mgo
CaO
Irtno
Nio
NarO
KrO
TAnk
. .198.2
Mica SPin€[
SpineF
Cpx
Mica Spinel*
47.t
0.86
11.8
0.90
429
13.6
19.6
o.L7
0.m
I.ffi
0.01
99J
n.6
2.E9
r7,l
2,m
4.61
m2
0.m
0r5
0.(B
0J7
9.&
94.9
a.D
0.16
g2
125
132
8.L
0.14
0.31
0.m
0.m
0.01
101.0
38.7
4.A
t43
,.43
3.89
nJ
0.0
0.m
0.(r'
0.26
9.30
945
IN XENOT.TrI{T;
MINERAIJ
OF SE@}DARY
0.71
3.33
4.6
&.4
16.4
rc.6
0.18
432
0.m
0.m
0.gz
98.4
Qx
54.5 39.3
0.23 3.X
3.2r r3.9
o.94 1.t6
5.95 5.14
22,4 20.9
ll.t
0.m
a.42 0.{n
0.m 0.l5
o23 0.04
0.02 929
93 y33
rs7.1
jmj
505-
lvfca
Mioa
Spiael*
4r.6
a2A
t?-4
0.57
t.?2
a.6
0.00
0.04
o8
0.00
10.0
q!.0
0.50
0.55
39.8
26,2
rr.7
l9.l
0.r3
0.19
0.11
02r
0.m
98.5
4t).6
2.12
13.2
l.t3
4n
23.1
0.0
0.ot
0.18
0.crt
10.3
95.1
1q9.7
.,
!fics
Spinel
A.U
4.57
11.3
6.4
U.l
132
0.02
4.42
0.m
0.m
0.@
1m.4
Total Fe reportedastr'eo.
*Splnelassoctated
with kelyphite.In samples198.2and 199.7the splneloccursas
lncluslonsin phlogopite.
recognizedin the SomersetIsland xenoliths (Mitchell
1977) and worldwide (e.g., Boyd & Nixon 1975,
Dawson1987"Erlanket al. 1987''t.
The compositionsof the major mineralsin the
xenoliths are all generally comparablewith those in
lherzolite (* garnet) xenoliths from kimberlites
in Canada(Mitchell 1978, 1987) and the United
States, e.g., Montana (Hearn & Boyd L975),
Colorado-Wyoming(McCallumet al. 1975,Eggleret
al. 1987)and Kentucky (Schulze1984).Equivalent
minerals also occur in xenolithsfrom southernAfrica
(seeNixon 1987,for detailedreferences).
Temperaturesandpressuresof equilibration
Temperaturesand pressuresof final equilibration
have been estimatedfor the xenoliths on the basis of
mineral composition.For comparison,two different
pairs of geothermometer- barometerhave beenused
(Table 2); the resultsare shownin Figures4a arLd4b.
Onepair consistsof the fwo-pyroxenethermometeras
modified by Finnerty & Boyd (1987) [FB86], with
pressureestimatedfrom the Al content of the orthopyroxenecoexistingwith garnet (MacGregor1974)
MC74l @g. 4a). The secondpair chosenis a combi-
nation of the Brey & Ktihler (1990) geothermomeler
lTsKNl, basedon coexisting pyroxenes,q9 ry"ttq"
dei6rminedfrom the Ca contentof olivine (Iable 4) in
the presenceof clinopyroxene(Kdhler & Brey 1990)
tPKsl (Fig. 4b). Unfortunately,becauseof the comp16teserpentinizationof olivine, pressurecould not be
determinedin threexenoliths.
result in a simiBoth pairs of geothermobarometers
lar relative sequenceof pressuresand temperaturesfor
the individual xenoliths.However,temperaturescalculated using Tsps ver,rasPKs are consistently higher
than those determinedfrom the FB86-MC74 pair'
This is illustrated in Figure 4b, in which the xenoliths
plot above the reference40 mWm2 conductive
seothermof Pollack& Chapman(L977)nd arecloser
lo a q4 mWm2 geotherm.In contrast,use of FB86
versusMC74 resultsin temperaturesand pressuresof
equilibration for the xenoliths close to the reference
geotherm(Fig. 4a). The xenolith having a granulo6lastic texture falls off the generaltrend at a higher
temperaturethan the others. Irrespectiveof which
is used,the majority of the xenogeothermobarometer
liths seemto have equilibratedwithin the stability
field of diamond.Temperaturesand pressureswere
also determinedfor five xenoliths ssing T"oq versas
302
TIIE CANADIAN
r 400
1300
o
@
!o
o
p
120A
11 0 0
1000
900
F
800
40
50
60
Fressure kbor (r\rc74)
z
t
Y
m
1200
8 rtoo
E
t
b
ts
1000
soo
40
OU
Pressure kbor \
KB/
MINERALOGIST
rocks in the United States,for example,the Williams
diatreme,Montaaa(Hearn& Boyd 1975),StateLine
kimberlites(Eggler:seeFinnerty& Boyd 1987),The
Thumb minefte,New Mexico (EhrenbergL978, 1982)
and Hamilton Branch kimberlite, Kentucky (Schulze:
see Finnerty & Boyd 1987) (see also Eggler et al.
1987for other localities). Temperaturesand pressures
of equilibrationfor xenolithsfrom the abovelocalities
are shownin Figure 5, togetherwith datafor xenoliths
from the C-14 kimberlite nearKirkland Lake.
Comparisonof the temperature- pressuredistribution for xenolithsfrom the localitiesmentionedabove
in North America suggestsdifferencesand similarities. For exampleothe generaldistributionof T-P
determinationsfor SomersetIsland. Northwest
Teritories, and The Thumb, New Mexico, are somewhat similar; whereasthe SomersetIsland data lie
along the graphite-diamondequilibrium curve
(Kennedy & Kennedy 1976), datafor The Thumb
define a lower-pressuretrend lying entirely within the
stability field of graphite (Fig. 5). Xenoliths from
the Williams diatreme,Montana, overall have higher
T-P of equilibration than those from other localities,
and are, on average,about 100'C higher than the
closest data for xenolitfis from the Hamilton Branch
kimberlite, Kentucky.Xenoliths from the C-14
kimberlite near Kirkland Lake havea T-P distribution
somewhatbetweenthosefrom Hamilton Branch and
the StateLine (Colorado-Wyoming)kimberlites.
Flc. 4. Estimatesof temperatureandpressureof equilibration
for xenoliths from the C-14 kimberlite. Ontario.
a) T determinedfrom Finnerty& Boyd (1987) fFB86l,
andP derivedfrom MacGregor(1974)U{C741.b) T estimatedfrom Brey & Kdhler (1990) [TBKN]and pressure
calculated using Kiihler & Brey (1990) tpKBl.
Diamond-graphiteequilibrium curve after Kennedy&
Kennedy(1976), and 40 mWm2 conductiveseorherm
fromPollack& Chapman
(1977).
I 400
1J00
ro 12OO
g 1100
E
; 1000
P"* (Brey & Kdhler 1990)and resultedin the esti- o
mated conditions of equilibration for the xenoliths o " ' "
lying closer to the 40 mWm2 geothermthan the
MmW/r*. Useof Tr^ andPBKN
requiresknowledge
of the compositionsof four phases(garnet+ olivine r
orthopyroxene+ clinopyroxene).Consequently,
becausegarnet and olivine are absentin some xeno- '
liths, T"* and P"K,r could not be deterrninedfor all
7
samples.
:
Drscussrou
BesidesSomersetIsland(Mitchell 19g7,Jago&
Mitchell 1987, Kjarsgaard& peterson 1992), dataon
xenoliths also are availablefor kimberlites and allied
ffllll]
40
50
.Pressure kbor
1.r'.,
county, K€nluciy
willlohs, Montono
t* rhurb,NesMexia
l-_-l
60
(Mc74)
so-u^"t
o"*
N\\\
o
7A
B0
t6tohd, Nwr
Line, Cororodo,/Ytyoming
c-r4, onto,to
Ftc. 5. Temperaturesand pressuresof equilibration for
selectedsuitesof xenolithsfrom localities in North
America (seetext for references).
MANTLE )GNOLITHS FROM KIRKLAND LAKE
However,comparisonsof conditionsof equilibration of the xenoliths needto be treatedwith caution:
other factorsosuch as the age of intrusion of the host
kimberlite, must be considered.It is also,important
that comparisonsonly be done on xenolith suitesfor
which estimatesof temperaturesand pressuresof
equilibration have been determinedusing the same
ggothefinometer- barometerpair. For example,in
contrastto the reconstructionof the xenolith suite
from Somerset Island, which lies along the
graphite-diamond
boundaryGig. 5), Mitchell (1987)
concludedthat the samesuite equilibratedin the
of his
stability field of graphiteas a consequence
adoptionof the geothermometerof Wells (1977) and
the geobarometerof Wood (1974).Although the original formation of the ultramafic rocks from which the
xenoliths were derived is probably Archean or
Proterozoic in age, subsequentthermal and metasomaticevents,including effects of entrainmentin
the host kimberlitic magma,have probably modified
both the original chemistry and fabric. Furthermore,
thermal events,such as the heat sourcetlat provided
the impetusfor kimberlite eruption, also may have
locally perturbedan otherwisepossibly stablecratonic
geotherm.Nevertheless,it has been noted (e.g.,
Finnerty & Boyd 1987) that most low-temperature
xenolithsassociatedwith cratonicregionshave equilibrated close to the 40 mW/m2 geotherm, if the
FB86-MC74 combinationis utilized, irrespectiveof
age of eruption. As noted earlier, xenoliths from the
C-14 kimberlite in Ontario follow this worldwide
trend.
Xenolithsanl diamond
Diamond has been found in severalultramafic
xenoliths from southernAfrica (Dawson & Smith
1975,Sheeet al.1982),Siberia(Sobolevet al.1969a,
Pokhilenko et al. 1977), the State Line region of
Colorado and Wyoming (McCallum & Eggler 1976)
and Australia (Jaqueset al. 1990). In spite of these
discoveries,the occurencesof diamond-bearingultramafic xenoliths are relatively rare, and most are
harzburgitic,althoughexceptionaldunitic xenoliths
containdiamondin Siberia(Pokhilenkoet al. 1977).
Early isotopic evidence (Kramers L979) and
suggestions(Meyer 1979)that diamondis a xenocryst
in kimberlite were proven correct by the radiometric
age determinationsof syngeneticultramafic garnet
inclusions in diamond (Richardsonet al. 1984). T\e
unusual chemical composition of these garnet inclusions,first notedby Meyer (1968),was also notedir
rare garnet xenocrystsin kimberlite (Nixon &
Hornung1968,Sobolevet aL.1969b,1973,Gurney
& Switzer 1973).Garnet,as well as magnesian
ilnenite, chromite, and chromiandiopside,have long
been used as indicator minerals in kimberlite explorationofor example,in the discovery of the diamond-
303
bearing Orapakimberlite, Botswana,by geologistsof
the DeBeerscompanyprior to 1967, andearlier in the
discoveryof the Mir and Udachnayakimberlites,
Siberia(Sobolev1959).Sincethe early 1970s,on the
basisof chemistryof garnetinclusionsand work by
Sobolev(1971,1974)and coworkers,the calciumchromiumrelationshipof kimberlitic gametshasbeen
used to assistin kimberlite and diamond prospecting
(e.9.,Gurney1984).
Currently, the generalconsensusis that diamond
formed in ultramafic and eclogitic rocks in the preexisting mantle, which were subsequentlydisrupted,
possibly by eventsthat eventually producedkimberlitic magma.Fragmentsof these disrupted rocks are
the xenoliths that occur in kimberlite. The disruption
and disintegrationof the mantle rocks may havebeen
a consequenceof incorporationinto a raising thermal
diapir associatedwith kimberlitic magmatism.This
disintegrationalso resulted in the incorporation of
mineralconstituents,including diamond,asxenocrysts
into the ascendingkimberlite. If the suggestionis
correct that diamond and xenoliths in general are
related,thenthe questionarisesasto what information
can be provided from xenoliths concerningdiamond
and, more specifically,the prospectsof diamond
occurringin a particularxenolith-bearingkimberlite.
Evidenceto date has demonstratedthat kimberlites
whose suite of xenolithsprovidesvaluesof P-T of
equilibration generally within the stability field
of graphiteare for the most part barrenor uneconomic
in terms of diamond content (e.g. Louwrensia and
Hanauskimberlites. Namibia: Mitchell 1984; East
Griqualandkimberlites: Boyd & Nixon, in Finnerty &
Boyd 1987; Belmontekimberlite, Brazil: Boyd
& Nixon, in Meyer & Svisero1987).The absenceof
diamondand the low physicalconditionsof equilibration for the xenolithsin the African exampleshave
been interpretedas due to thinner lithosphereoccurring close to the margin of the craton than in the
central cratonic region (MacGregor 7975, Boyd &
Gurney1986).
In the case of the North American localities of
xenoliths, and using the southernAfrican model, The
Thumb minette, New Mexico (Ehrenberg 1982)
should be devoid of diamond;none has yet been
recorded.In contrast,all other localities, including the
SomersetIsland anay of xenoliths, which are closest
to the diamond-graphiteequilibrium boundary,have
diamond potential. Nevertheless,diamond has not
been recorded either from the Williams diatreme,
Montana(Hearn& Boyd 1975)or from the Kentucky
kimberlite, in spite of periodic explorationsincethe
turn of the century in the caseof the latter location.
Although diamond does occur in the StateLine kimberlites, the gradeis generallytoo 1ow, other than in
the GeorgeCreekkimberlite, to sustaineconomicproduction. Equally well, the presenceof diamond in
subeconomicquantitiesis known in the kimberlitesof
304
TIIE CANADIAN MINERALOGIST
SomersetIsland (Brummer 1978).Diamond does
occur in the C-14 kimberlite, Ontario, and its occurrence is the subject of further prospection;currently,
theindicationsaretlat it is subeconomic.
Thus,at present,P-T anaysof conditionsof equilibration basedon xenolithsin kimberlite can be usedro
indicate the likely absenceof diamondin a particular
kimberlite, but cannotbe used to unequivocallyindicate the presenceof diamondin economicquantities.
In caseswherethe P-T array indicatedby xenoliths
suggeststhe absenceof diamond,it is possiblethat
diamond was once presentin the kimberlitic magma
that sampledand carried the xenoliths to the surface.
However, temperatureand redox conditions in the
magmamay have been sufftcient to causeresorption
of diamond.Information on suchredox conditionscan
be estimated,in part, from spinel and ilmenite occurring in the kimberlite. Nevertheless,kimberlites
generally occur in clusters,and variation in diamond
contentfrom pipe to pipe is the norm. Obviously, the
samplingof all kimberlitesin a clusterfor xenoliths
may producea more significantresult.
Mantle structure
The xenoliths describedherein from the C-14
kimberlite in Ontario are similar in type, mineral
chemistry,texture and possiblemetasomaticeffectsto
xenolithsfrom kimberlites worldwide, including those
from other North Americanlocalities.It would thus
appearthat the petrology of the cratonicroot-zones
sampledby kimberlitesfrom the Proterozoicto late
Cenozoicis broadlysimilar'.
Basedon the interpretationof xenolith data,
MacGregor(1975)and Boyd & Gumey (1986)have
suggested
a thick cool root-zoneap to 220 km thick
beneaththe Kaapvaalcraton.At the marginsof the
craton, this thicknessdecreasesand, correspondingly,
the lithosphere- asthenosphereboundary lies closer
to the Earth's surfacetoward tJtesouthernAtlantic
and Indian oceans.Extensionof this model to other
cratonic areasoespeciallythose with diamond-bearing
kimberlites, has been suggested,but insufficient evidence,comparedto the abundantdata for southern
Africa, has precludedsignificant discussion.Highresolutionseismictomographyis a relatively new
methodof imaging the Earth's interior. Anderson
et al. (1992)suggested
the presence
of a high-velocity
zone or stablecratonbeneaththe areaof interestin
Ontario,to depthsgreaterthan 220km. Similar results
of tomography are apparentbeneaththe Kaapvaal
craton.
Finnerty & Boyd (1987) consideredthe xenolith
datafrom the few localities in North America to indicate a similar upward shelving of the lithosphereasthenosphere
toward the Arctic (Somenet Island) in
the north and New Mexico (The Thumb) in the south,
comparableto that which occursin the Kaapvaal
cratonin soutlern Africa. Basedon the overall pattern
of P-T conditionsof equilibrationfor the xenoliths
from tlte C-14 kimberlite, we suggestthat the locality
has a non-marginalposition within the North
Americancraton.
AcKNowLEDGENEI{TS
HOAM and BLG gratefully acknowledgepermission from Lac Minerals (USA) Inc. to report on
samplesand information madeavailableto them since
1987. HOAM also acknowledgessupport from
National ScienceFoundationfor the purchaseof an
electron microprobe (EAR-8720584). Bruce
Kjarsgaardand an anonymousreviewer provided
constructivecriticism and help, which the authors
appreciate.
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