JOURNAL
OFGEOPHYSICAL
RESEARCH,
VOL.95,NO.Bll, PAGES
17,661-17,675,
OCTOBER
10,1990
Small-ScaleDisequilibriumin a MagmaticInclusionand
its More
Silicic Host
JON P. DAVIDSON1
Department
of Geological
Sciences,
University
of Michigan,AnnArbor
SHANAKA L. DE SILVA
Lunar and PlanetaryInstitute,Houston,Texas
PETER HOLDEN1 AND ALEX N. HALLIDAY
Department
of Geological
Sciences,
University
of Michigan,AnnArbor
Basaltic
andesitc
inclusions
andtheirhostdacitefromthePurico-Chascon
complex
in northern
Chileare
isotopically
distinct.Texturalcharacteristics
of the inclusions
are typicalof thoseresulting
frommagma
mingling. Serialsectioning
acrossthe interfaceof an inclusionand its hostdacite,complemented
by
microdrillsampling
anddetailed
microprobe
work,hasenabled
usto examine
thescaleof mixingandchemical
(isotopicandtraceelement)disequilibrium.
The resultsof this workshowthat (1) the composition
of the
inclusionis relativelyhomogeneous;
(2) the dacitehostis generallyhigherin 87Sr/86Srand lower in
143Nd/144Nd
thantheenclave,
butit is heterogeneous
ona smallscale,
andprobably
a "hybrid";
(3) the
isotopiccomposition
in themarginalzone,apparently
on bothhostandinclusion
sidesof the weaklychilled
interface,
actually
shows
thehighest
87Sr/86Sr
andlowest
143Nd/144Nd;
(4) largeplagioclase
crystals
in the
inclusions
andhostarexenocrystic,
withhigher87Sr/86Sr
thananyof theothersamples.Theseobservations
are reconciledwith a modelof magmaevolutionin a crustalmagmachamber.In sucha scenariothe mafic
magmais overlainby a cap of rhyolite - a partial melt of the daciticignimbriteswhichnow underliethe
Purico-Chascon
complex. Overturnof sucha magmasystemgivesriseto a hybriddacitecontaining
discrete
mafie
inclusions.
1. LNTRODUCTION
magma interactioninclude temperature,latent heat of fusion,
Magmatic inclusions more mafic than their host are a
commonfeature of silicic volcanic and plutohie rocks.
While someare demonstrablyof cumulusorigin [Bacon and
Druitt, 1988; Tait, 1988; de Silva, 1989a], and others are
interpretedas 'resrite' [Wyborn and Chappell, 1986], the
inclusionsdiscussed in this paper belong to the most
commongroup: those thought to be the result of magma
mingling[Walker and Skelhorn,1966; Eichelberger, 1975;
Bacon and Metz, 1984; Bacon, 1986]. The inclusionsare
typically spheroidal, are vesicular, have 'cauliform' or
crenulatechilled marginswhich are convex towardsthe more
composition, water content, and the relative mounts and
total volumes of each endmember, as well as the time and
length scales of mixing. However, the interaction of marie
and silicic magmas at small length scales (thin section to
micron scale) is less well documented and understood.
Resultsof the few detailedexperimental[e.g., Lesher, 1988]
and petrologic studies [Koyaguchi, 1986; Clynne, 1989]
indicate complex chemical and physical interactions which
are lost in studiesutilizing bulk samples.
Here, we presentan investigationof small-scaleisotopic,
compositional, and mineralogical variation, by using
microsamplingtechniques,acrossthe interface of a basaltic-
silicic host, and show crystal morphologies (quench andesitc inclusion and its dacitic host from Cerro Chascon, a
textures)consistentwith incorporationin a liquid, or at Holocenedome in northernChile (Figure 1). Earlier work on
leastpartially molten state.
bulk whole rock samples[Hawkesworthet aI., 1982] showed
In general, studies of such inclusions and their hosts
that the inclusionsand hosts are isotopically distinct. We
havebeen directedtowardsunravelingtheir petrogeneticcontendthat our observationslend further insight into the
evolutionusing bulk samples [e.g., Eichelberger, 1975; nature and time and length scales of mafic-silicic magma
Grove and DonnelIy-Nolan,1986;Holden et al., 1987] or interactions.
modelingthe dynamics of the interaction of marie and
silicicmagmas[e.g.,Sparksand Marshall,1986;Frost and
2. GEOLOGICALAND GEOCHEMICALBACKGROUND
Mahood,1987]. The physicalandchemicalinteractions
of
the hostand intrusivemagmaon a medial to large scale The inclusion discussedin this study, P8815 (Figure 2;
(handspecimen
to magmachamber)
arenowreasonably
well from Cerro Chascon), is one of a suite collected from two
understood.Variables that control the style and amount of domes, Cerro Chascon and Cerro Aspero, located in the
Purico complex (67ø45'W, 22ø57'S) in the Central Volcanic
Zoneof the Andes(Figure1). The Puricocomplexis a broad
a summit complex of cones and
domes(Figure la). Earliest activity in the region (from 1.3
1Now
atDepartment
ofEarth
andSpace
Sciences,
Universityignimbrite shield with
ofCalifornia,
LosAngeles.
to 1.0 Ma) produced
the ~80 km3, daciticPuricoignimbrite
member [de Silva, 1989b]. Following thesemajor ignimbrite
Copyright
1990by the American
Geophysical
Union.
Papernumber90JB00427.
0148-0227/90/90JB_00427
$05.00
eruptions, the andesitc to dacite cones and domes of the
summit complex formed. Two groupscan be defined on the
basisof age and chemistry;the post-ignimbritebut glaciated
andesitic Purico group (Cerros Purico sensu stricto, Toco,
17,661
17,662
DAVIDSON ET AL.: SMALL-SCALEHOST-INCLUSIONDISEQUILIBRIUM
N
ß
Fig. la. LandsatThematicMapper image of the Puricocomplex(-45 x 45 km). The complexconsistsof the Purico
ignimbritemember,a large dissectedignimbriteshield (light tone) and endogenous
dome (D), on which the younger
Puricolava memberis built. The latter consistsof the Puricogroup;CerroPuricosensustricto (P), CerroNegro (N), and
CerroPutas(PT); and the Chascongroup;CerroChascon(CH), CerroCerrillo(C), and CerroAspero(A). Also shownare
the northem extensionof the Miscanti lineament (ML) on which Cerros Chasconand Asperolie and terminal moraines
(M) relatedto the Holoceneglaciation(10,000 years B.P.). Image processed
and producedat the Lunar and Planetary
Institute (Landsat scene 5050614004).
Negro, and Putas, none of which is known to contain
inclusions)and the post-glacial (Holocene) dacitic Chascon
group (Cerros Cerrillo, Chascon, and Aspero), from which
inclusions have been collected. The morphology of the
constructsof the two groupsalso appearsto be different; the
found, and someinclusionsshowa vesicularcore region.In
handspecimen,
inclusions
are aphaniticto microporphyritic
in texture, with a rough correlation between groundmass
grainsizeand size of inclusion. The inclusionscomposeup
to an estimated20% by volumeof someof the Cerro Aspero
outcrops.AlthoughHawkesworth
et al.
Purico group are small andesiticpolygeneticcones,whereas andCerroChascon
only one typeof inclusion(theyreferto
the Chascongroup are dacitic flow-dome complexes;Cerro [1982] recognized
Aspero could be classified as a Pelfan dome. The rocks of them as "xenoliths"),Francis et al. [1984] recognizedtwo
with our observations.
One
the complex are predominantlyhigh-K dacites, with minor types, which is consistent
xenoliths"
of Franciset al. [1984],has
volumesof andesireand rhyolite. Domes and lavas range variety,the "basaltic
felty groundmass
dominated
by skeletal
from 56 to 70% SiO2, while the inclusionsanalyzed by a fine-grained,
(resorbed
or reactedrimsbut freshcores),and
Hawkesworthet al. [1982] are basalticandesiresin the range hornblende
plagioclase
with subordinate
smallphenocrysts
of olivine
52 to 56% SiO2.
and augire.The other variety, the "plutonicxenoliths"of
3. PETROGRAPHYAND MINERAL CHEMISTRY
Francis et al. [1984], has a fine-grained porphyritic texture
but contains no skeletal crystals and does not contain
The inclusionstypically occuras 4 to 25 cm ovoid, sub- hornblende in the groundmass. Preliminary analyses
roundedto well-roundedblobswith crenulatemarginsconvex demonstratethat the olivines and augitesin both inclusion
towards the host. Well-preservedchilled margins are often types, while different in size, are very similar in
DAVIDSON
ETAL.'SMAI•-SCALE
HOST-INCLUSION
DISEQUILIB•
17,663
70øW
PERU
ß
BOLIVIA
AREQU!;A
20øS
PACIFIC
OCEAN
o POTOS!
20øS
C^L^M^
•,•,,Figure
la.
25øS
.....
A•
'TOFAGASTA
•
-...
25øS
Antarctic Phtc
,.....
,
X.[ ....
70øW
Fig. lb. Map showingthe distributionof activevolcanoesin the Central VolcanicZone of the Andes [from Francis and
de Silva, 1989]. The locationof Figure la is shown.
composition.Inclusion P8815, the subject of this paper,
belongsto the secondgroup (i.e., "pintonicxenoliths"of
Franciset al. [1984]).Representative
mineralanalyses
from
thisinclusionandits host are givenin Table 1.
Basaltic Andesitc Inclusion
compositionalevidenceof being xenocrystic.For instance,
quartz is highly resorbed and rimmed with clinopyroxene
(Figure 3b), while biotites are rimmed with opaques.The
core of a large hornblendexenocrystin the analyzedsection
has a lower Mg# composition very different from those
typical of the host and margin (Figure 4c).
The inclusioncontainscrystalsof plagioclase,olivine Selvage
(0.5- 8 ram)andaugitc(oftenin clots)in a microcrystalline
groundmassof plagioclase microlites and oxides (Timagnetite)
up to 0.05 mm with high-Si rhyolite glass.The
plagioclases
haveresorbed,
normallyzonedcores(Figure3a)
rangingfrom An47 to An43, anda rim of An63 (Figure4a).
The selvage of the basaltic andesitcinclusion is a relatively
coarselycrystallinezone with up to 70% crystalsdominated
by a groundmassof 0.1 - 0.2 mm plagioclase(65%), the
majority of which have compositionswithin the range An74
Another population is represented by plagioclase to An64 (Figure4a), hornblende(30%), and smalleropaques
phenocrysts
whicharenormallyzonedfromAn78to An57. (5%), defining a hypidiomorphic granular texture set in a
Groundmass
microlitesare dominantly
An72to An65,but matrix of fresh high-Si rhyolite glass (Figure 3c). Larger
rangedownto An49 (Figure4a). Olivine (Fo81-82)is fresh crystals set in this groundmass are olivine, augitc,
but showsminor resorptionand containsCr-spinel plagioclase,and quartz.The olivines (Fo81-82) are typicalof
inclusions.
Augitc(Mg# 73-82)rangesfromWo45En40
to those in the inclusion but are rimmed with augitc
Wo41En44
(Figure
4b) andhasvariable
A1contents
(Table (Wo45En44) and sometimeshornblende(Figure 3c). Large
1).Quartz,hornblende,
biotite,andsphene
(up to 0.2 mm) plagioclasecrystals,up to 0.5 mm, are typically resorbed
are also present and show either textural and/or with normallyzonedcores(An59to An56) andmoreAn-rich
17,664
DAVIDSON
ETAL' SMALL-SCALE
HOST-INCLUSION
DISEQUILIBRIUM
Fig.2. Photograph
of slabbed
sample
P8815afterslicing
samples
(chips
labeled
A through
G, shown
in Figure5 and
Table2) in a traverse
perpendicular
to interface.Thecontact
between
theinclusion
andhostis marked
by a narrow
reaction
zone/chill,
referred
to in thetextas the "selvage".
Notethelargeplagioclase
xenocryst
(labeled"p" and
truncated
by slicingsubsequent
to microdrillsampling(cf. Figure6a) incorporated
withintheenclave.Scalebaris 1
cm.
rims of An73. The other plagioclases
are typicallynormally
zoned and define two groups with compositionsranging
from An65 to An45 and from An42 to An38 (Figure4a).
Resorbedquartz and sphene(titanire)are also presentand
show signs of disequilibrium,althoughthe quartz has no
reactionrim. Rarehornblendes
with low Mg# coresare found
(Figure 4c).
textural relations and compositionsof augite and olivine
indicate that they were originally phenocrystsfrom the
inclusion magma, now present as xenocrystsin the more
silicic host. Rare low Mg# hornblendes are also found
(Figure 4c) and are interpretedas xenocrystsfrom another
silicic magma. This mixed crystal population and the
presence of small fragments of inclusion indicates that the
host dacite is a "hybrid".
Disequilibrium
mineralassemblages
andtexturesoccurin
both inclusionand host, indicatingthat phenocrysts
from
The host dacite containsphenocrystsof plagioclasewith both magmatypeshave beenexchanged,
at leaston a local
complex zoning, rounded quartz, fresh hornblende and scale.For instance,
the association
of Mg-richolivinewith
biotite, abundant euhedral sphene (both as inclusions in quartz,reactionrimson quartz(Figure3b), augireandolivine
biotite or discretemicrophenocrysts)
set in a fine-grained rimmed by hornblende,and the complexcompositional
groundmassof plagioclase microlites, opaques,and fresh zonationsin the plagioclases(see below) are all common
high-Si rhyolite glass (Figure 3d). The plagioclase indicators
of boththermalandchemical
disequilibrium
[e.g.,
phenocrystsdefine two distinct populations; the first are Baconand Metz, 1984].Texturalevidence
suggests
thatthe
normally zoned (Normal, type N) with compositionsin the mafic magmawas at (or below) the liquidus,crystallizing
range An78 to An67 (core to rim), while the second are olivine, augire and plagioclaseprior to being included.
reverselyzoned (Reverse, type R) over a small range from Interactionbetweenthe two magmascan be examinedat a
An41 to An45. The plagioclasemicrolites generally have higher resolution by considering the plagioclase
compositions
rangingfrom An41 to An49 similarto the type compositionsin more detail. Two broad populationsof
R plagioclases (Figure 4a). Hornblende and biotite plagioclasecan be recognized;those with compositions
plagioclases
of boththe
phenocrystsare unzoned and have restricted compositions. An>60 definedby the groundmass
Quartz is sub-rounded.
Augiresand rare olivinesshowing inclusionand its selvage,and thosewith An<60 typicalof
Dacite
Host
resorptionor mantling by hornblendeare also common.The groundmassplagioclases(microlites)in the host. Thesetwo
DAVIDSON
ETAL.:SMALL-SCALE
HOST-INCLUSION
DISEQUILIBRIUM
17,665
••
'o•o,-¬•o•d
ß
.
c•
•.
o
o', o
c:',o,--, •
o
•
or--.
o •
o-, •
•
o
o
o o •
o
o
o
•..•, o
,--, o
•
o
•D
c• •
0
0
0
00
•
o
o
,•.
•
o •
0
•
o,-4•c•c5
•,-•o•o•
o•o
,.--'• Ox
DAVIDSON
ETAL.'SMALL-SCALE
HOST-INCLUSION
DISEQUIZ!B•
17,667
..TABLE
lc. Representative
Microprobe
Analyses
ofMi.nerals:
Host
Plag'i'•clase
Biotite'
Hornblende
C
R
Augitc
Glass
Groundma
ss
--SiO2
TiO2
A1203
FeO
•0
51.13
48.7'0 •"•.44
56.88
55.76
56.71
41.87
43.13
0.07
30.64
0.02
32.41
0.00
26.64
0.00
27.39
0.02
27.02
4.23
14.35
2.27
12.58
1.39
9.39
0.60
0.00
0.53
0.0 •
0.18
0.00
0.22
0.00
0.00
0.07
16.03
0.20
11.75
o. • 4
16.68
0.48
8.02
1.03
0.29
0.02
50.19 ....
78./)1
0.37
0.29
2.65
12.50
MgO
0.09
0.07
0.01
0.01
10.08
14.22
14.08
11.28
16.60
0.31
CaO
14.69
15.46
8.31
9.22
7.40
0.01
11.53
11.75
19.40
0.81
Na20
K20
•2os
1.95
0.53
0.05
2.57
0.15
0.00
6.16
0.56
0.0 •
5.86
0.49
o.00
4.78
0.54
0.05
0.63
9.33
0.00
1.92
0.67
0.04
1.37
1.18
0.02
0.18
1.55
0.01
2.65
0.01
0.03
SrO
0.11
0.17
0.15
0.10
0.00
0.02
0.07
0.00
0.08
0.00
Cr203
0.00
0.00
0.00
0.00
0.00
0.02
0.01
0.04
0.02
0.00
98.52
97.91
Total
99.87
100.07
99.46
100.17
99.07
95.75
96.92
96.71
,,
Formula
Units
perindicated
Number
ofOx7•ens
Plagi0clase
(8O's)
C
Bi (22 O's)
R
,
....
'Hb(23
Q's) ..
Aug 6 O's)
Groundma
ss
.....
2.'•356..... 2.2337
si
0.0023
Ti
0.000s
2.5905 .....
2.5527
2.4757
5.5125
6.2144
6.5810
1.9026
0.0001
0.0007
0.4778
0.2530
0.1596
0.0105
0.0000
A1
1.6496
1.7519
1.4164
1.4488
1.4141
2.5400
2.2005
1.6864
0.1185
Fe
0.0230
0.0204
0.0068
0.0082
0.0000
2.0128
1.4580
2.1270
0.2543
Mn
0.0001
0.0002
0.0000
0.0000
0.0026
0.0258
0.0174
0.0616
0.0093
Mg
0.0062
0.0045
0.0003
0.0004
0.6672
3.1828
3.1153
2.5643
0.9378
Ca
0.7189
0.7597
0.4014
0.4436
0.3552
0.0018
1.8334
1.9193
0.7878
Na
0.1723
0.2286
0.5388
0.5099
0.4115
0.1843
0.5538
0.4050
0.0132
K
0.0311
0.0089
0.0322
0.0283
0.0306
1.7876
0.1273
0.2291
0.0003
55.62
Mg#
An
77.95
76.18
41.28
45.18
Ab
18.69
22.93
55.41
51.94
Or
3.37
0.89
3.31
2.88
58.60
60.33
88.78
44.55
51.61
3.84
,
,
,
C,core;
I, intermediate
andR,rim'.'
groupsare believed to reflect crystallization from mafic further later, was most likely an ignimbriteprecursorto the
(An>60)and silicic (An<60) endmembersand on this basis presentdacitehost. Most of the crystalsin the hostare true
thefollowinginterpretations
canbe made:
phenocrysts;
i.e., they crystallizedfrom the hybrid dacite,
1. Microlite compositions
in host and inclusionare but they crystallized
after interaction
andmixingwith a
distinctand reflect crystallization
within eachrespectivemafic component
(inclusionprecursor).This is required
system
(hostandinclusion)
aftertheyhadformed.
because
the hornblende
crystallized
on rims of xenocrystic
2. Thecoresof thetypeN andR plagioclases
of thehost augitcin the host has the samecomposition
as the
represent
phenocrysts
whichwerein equilibrium
withmafic phenocrysts
in thedacite(Figure4c): thiswouldnotbe the
andsilicicendmember
magmas,
respectively.The rim caseif thehornblende
phenocrysts
hadbeenpresent
in the
compostions
reflectre-equilibration
with a new "hybrid"dacite
before
interaction
withthemaficmagma.
meltenvironmentresultingfrom interactionwith the mafic
magma.Notethatthe (hypothetical)
maficandsilicic
endmember
componentsare not strictly equivalentto the
inclusion
and host, as someinteraction
has takenplace
between
themduringinclusion
formation.
3. The type N plagioclaseof the host indicatethat
phenocrysts
crystallizingin the mafic magmaweremixed
intothe silicic magma;which is also borneout by the
4.SMALL-SCALE
ISOTOPIC
RESULTS
SampleP8815wasslabbed
(1 cm thick),anda detailed
traverse
wasmade(Figure2) by cuttingsevenchipsovera
distanceof 5-7 cm perpendicularto the inclusion-host
interface(chipslabeledA to G in the diagrams). The
inclusion,not recoveredwhole, is estimatedto have a
presence
of ubiquitousdiscreteolivine and augitcxenocrysts diameterof about5 to 10 cm. The chipswere dissolvedand
in thehost.
analyzed
for Sr, Nd, andPb isotopic
composition
(Table2
4. The normally zoned resorbed cores of large andFigure58). Pb isotopevariationswerenot very large,
plagioclases
in the inclusionare capturedplagioclasebut overall206Pb/204pbmirrors87Sr/86Sr (Table 2).
phenoprysts
from the silicic endmember
whichwere then AlthoughPb isotopes
will forman importantpart of future
resorbed
andovergrown
by a moreAn-richrim in equilibriumwork,we will not discuss
thePb datafurtherhere. Aliquots
withtheinclusion
magma.
were also spiked for determination
of Sr, Nd and Pb
A groupof xenocrysts
foreignto bothinclusionandhost elemental
concentrations
(Table2 andFigure5b).
arepresent
- a few largehornblendes
havelowerMg#'sthan
Before sectioning,eight sites were sampledwith a
hornblendes
in thepresent
hostandmargin(Figure
4c). This microdrill
to determine
Sr isotopic
variationon thesmallest
suggests
that thesexenocrystic
hornblendes,
andprobably scale(Figure5c). The microdrill
siteson sample
P8815and
some
of thequartzandplagioclase,
arederivedfromanotheran additional
host-inclusion
sample,
P8823,areillustrated
in
(lowertemperature)
silicicmagma,which,as we developFigure6. Somesimscomprised
multipledrillholes(threeto
17,668
DAVIDSON ET AL.: SMALL-SCALE HOST-LNCLUSIONDISEQUILIBRIUM
o
,
/
& *,
ß Core
AI
o Intermediate
I Zoned
Phenocrysts
o Rim
x GroundMass(unzoned)
Inclusion
An
Ab
Margin
Host
El
Wo
ß Inclusion
ß Margin
ß Host
Opensymbols
indicate
rim
analyses
of zoned
crystals
Rim of an Olivine
Cl
3.40
3.20
•::ß_•x.
•..-'.-'i'•:...:;•':...
?.-:•¾....
'•l•i¾.?.:.-.::'!.•.'.-'::•i•;•-'.:
3.00
2.80
ß•....
................
..............•............•••••......:¾...
Mg (0=-23)
2.60
.....Cor6':.'•..•".-_:.i.
j-".':.
'j.;-'..':'.;::•:...
2.40
2.20
2.00
,
1.00
1.20
i
t
!
1.40
1.60
1.80
,
I
I
I
2.00
2.20
2.40
2.60
Fe (O = 23)
Fig. 4. Variationdiagramscomparing
mineralchemistry
from inclusion,marginand hostof sampleP8815 (datafrom
Table1). (a)Plagioclase,(b)augite compositions,
and (c) hornblende
compositions
(inclusion,solidsquares;
margin,
solidcircles;host,opencircles;and rims to augirexenocrysts
in host,crosses.)Stippledfields in Figures4a and4c
representthe rangeof compositions
in daciticignimbritesin the region(datafrom de Silva [1987]).
17,670
DAVIDSONETAL.' SMALL-SCALE
HOST-INCLUSION
DISEQUII3BRIUM
TABLE2. Elemental
andIsotopic
Compositions
ofChipsCutinTraverse
(Figure
2)
Section
,
A
B
C
D
E
F
G
Sr
Nd
Pb
(ppm)
(ppm)
(ppm)
470.7
523.8
500.2
495.0
755.1
214.5
418.1
15.3
16.2
16.7
17.3
33.3
9.1
17.0
8.76
9.33
9.36
9.36
14.26
14.05
13.71
87Sr
143Nd
86Sr
144N
d
0.706113 +
0.706209 +
0.706110 +
0.706129 +
0.706735 +
0.706650 +
0.706432 +
15
14
12
16
13
13
15
0.7068
0.512430 + 8
0.512428 + 9
0.512436+7
0.512441 + 8
0.512359 + 9
0.512373 + 10
0.512395 + 15
206pb
ENd 204p
b
-4.1
-4.1
-3.9
-3.8
-5.4
-5.2
-4.7
18.86
18.85
18.84
18.85
18.88
18.86
18.89
207pb
204pb
208pb
204pb
15.70
15.68
15.67
15.68
15.70
I5.68
15.70
38.91
38.86
38.16
38.84
38.97
38.89
38.96
six) but a set of single holes, some of which can be seenon
Host
Inclusion
the photographs
(Figures2 and 6), were drilledin thehost,
inclusion, selvage, and one of the large plagioclase
6St
0.7066
xenocrysts.Massestimates
basedon the volumeof sample
recoveredfor a drill diameterof 0.5 mm and comparable
hole
depth suggestthat about 1 mg of samplewas obtained.The
EN
d
0.7064-
!•Nd smallsamplesizeandthe difficultiesinherentin determining
precise sampleweights precludeddeterminationof elemental
concentrations through isotope dilution, and causedSr
isotopic ratios to be less precisely measuredthan wouldbe
0.7062-
possibleon large samples. Nevertheless,the total rangein
A,
0.7060 •
-4
i
B
E
CD
F
G
-5.0
4
80
bx5
60-
40-
Nd
20'
Sr isotope ratios is far greater than analytical error, and
these data can be used to place important constraintson the
evolution of the magma system. Analysis of Nd isotopic
ratios on the microsamples was largely unsuccessfulbut
shouldbe possiblewith improvementof mass spectrometric
technique.
Isotopic analyses were made on a VG Sector mass
spectrometer at the University of Michigan, using a
multidynamic (peak-hopping)collection routine. Analysis
of the small Sr sampleswas facilitated by loading in a Ta
oxide slurry to improve the Sr signal. A total processSr
blank of 70 pg was measured(amounting•o lessthan0.02%
of the Sr analyzedin t_hemicrosamples).The micro&illbit
(after total dissolutionof the bit) was found to contain400
pg (St) and 130 pg (Nd). However, no significant
degradationof the bit was observedafter each hole was
drilled.
0.7068
The main points of note from Figures 5 and 6 are as
'
follows:
0.7066-
1. Nd and Sr isotopedata on the finest scale(micro&ill
samples)exhibit more variability in the host than in the
0.7064 -
inclusion-host
interface(chipE; Figure2). 87Sr/86Sr
is
inclusion.
2. Trace element concentrations reach a maximum at the
I
Fragmentof
inclusion
0.7062 -
highest,and143Nd/144Nd
lowest,alsoat the interface,
both
on the hostside (chip E) and,apparently,
on the inclusion
sideof the weakchill (seethesetof threemicrodrillsites
in
the selvageanalyzedat 87Sr/86Sr= 0.706671+ 54 Figure
6a).
0.7060
'
-4
'
,
,
"
Distance from interface
(cm)
0
3. The presence
of smallchilledfragments
of inclusion
within the host with disequilibrated
isotopiccompositions
(Figure 5c) suggeststhat solid-statechemicaland tracer
diffusion was insignificant.
The microdrilledsamplesfrom the coresof the largest
(>2 mm) plagioclase
crystalsin bothinclusions
and host
Figure
2; datain Table2). (a) $7Sr/86Sr
ratios
andend determined (for two samples:P8815 and P8823 (Figure 6))are
onchips,(b) elemental
concentrations
of St, Nd, andPb determined
by consistently
higher 87Sr/86Srratios
on chips,and(c)8?Sr/$6Srresultsfor finestscale(microdrill) characterized
with crystallization
sampling (sites shown in Figure 6a). Drilling locations are (>0.708). This cannotbe reconciled
-- 0.7068)andsuggests
that
expressedin terms of distance normal to the inclusion-host fromthehostdacite(87Sr/86Sr
froma
interface. Some of the drillholes can be seen by closeinspection theselarge crystalswere originallyphenocrysts
magmawith a greater crustalSr component,
and are
of Figures2 or 6a.
Fig. 5. Resultsof traversefrom inclusionto host (slicesas in
DAVIDSON
ETAL: SMALL-SCAI•HOST-INCLUSION
DISEQLIIZBRIUM
17,671
P8823b
i
HOST
.o
[ •lagxenocryst
[0.708224
+ 75
..
..o
Fig.6. (a)Microdrill
sites
and87Sr/86Sr
results
fromrecovered
material
onsample
P8815.Thesingle
holedrillsitein
thehost
actually
sampled
a tinychilled
fragment
ofmafic
inclusion
disseminated
inthehost
(thisisvisable
onhigh
resolution
large
photographs
oftheslab),
andthus
gives
alow87Sr/86Sr,
relative
tothemultiple
drillsite(upper
left)
andthechips
ofthehost.Photograph
taken
prior
tocutting
chips
(cf.Figure
2). (b)and(c)Microdrill
sites
andresults
' fortwoslabs
fromsample
P8823.Notethehigh
87Sr/86Sr
forthelarge
plagioclase
crystals
inbothinclusion
(Figure
6b) andhost(Figure6c), indicative
of a xenocrystic
origin.
17,672
DAVIDSONET AL.' SMALL-SCALEHOST-INCLUSIONDISEQUII•BRIUM
accordinglynow most appropriatelydescribedas xenocrysts. compositions
of the xenocrystic
plagioclase
andhornblende
The interface of the inclusion also has generally higher cores(Table 1) are alsotypicalof thosein the ignimbrites
87Sr/86Srthanthe"bulk"hostdacite.Theinterface
appears(Figures4a and4c). We thereforesuggest
thata reasonable
to havea 87Sr/86Srratio of about0.7067 (chipE on the candidatefor the silicicendmemberis a typicalignirnbrite
host side of margingives 87Sr/86Sr - 0.706735ñ 13: (or subvolcanic
plutonicequivalents)
from this region
microdrill sites on the inclusion side of the margin shown in (Figure7). This couldeitherbe residualmagmaleft over
Figure 6a are87Sr/86Sr- 0.706671+ 54 and0.706450ñ from an ignimbrite
eruption
or a melt of pre-existing
24), whilethehostdaciteis characterized
by a 87Sr/86Sr• ignimbrite. For two main reasonswe prefer the latter
0.7064 (chip G, furthest into the host dacite from the scenario;
first,we consider
it unlikelythatresidual
magma
interfacegives87Sr/86Sr= 0.706432+ 15, consistent
with from the last ignimbrite eruption (-1 Ms) would have
the multi-site microdrill result of 87Sr/86Sr - 0.706437 + 14
shown in Figure 6a), so binary mixing between the
inclusion and the dacite host cannot explain the isotopic or
concentrationdata at the inclusion-hostinterface. The high
surviveduntil the Holocene(but see data of Hallidayet al.
[1989] for the still active2.1 Ma Long Valley system).
Second,to producethe characteristics
of the "hybrid"host
and the marginal zone, the mafic magma had to interact
Srppmand87Sr/86Sr
ratiosobserved
in chipE mightbe due with a high-silica melt enriched in incompatibleelements
to incorporation of a larger amount of xenocrystic (i.e., Nd). The most likely composition for a residual
plagioclase; however, this is not consistent with the ignimbrite
magmain thisregionwouldbe a dacitewithSiO
2
observation that this slice also has the highest Nd -64 - 68%, which is too low for the silica-rich endmember
concentration
andthelowest143Nd/144Nd
ratio (plagioclaserequiredto producea mixedhybridwith up to 70% SiO2 (see
typically containsvery little Nd, and would not control the S iO 2 data of Hawkesworth et al. [1982]), but more
Nd isotopiccompositionof the sample).
important, such a magma would not have the enrichment
in
Nd that is required. However, a partial melt of suchan
5. DISCUSSION
ignimbrite, would not only have a cutecftc composition,but
also would be enrichedin incompatibleelementssuchasNch
Our observations indicate that inclusion P8815 resulted
from the mingling of a mafic magma with a silicic magma.
However, the microsampling techniques used here have
unraveled
a set of
characteristics
of
host
and
inclusion
and therefore would be a more suitable endmember.
0.5125
P8815 which require a complex interaction to account for
ß
ChipsA to G (Fig 3)
them. For instance,
the fact thatthe highest87Sr/86Srand
lowest143Nd/144Nd
ratiosareobserved
at themargincannot
A
Purlco Chascon Dacites
[]
Puricoignimbrite
(basement)
be reconciledwith the simple injection of marie magma into
an existing daeitic chamber.Further, if the marginal zone of
the inclusion (either side of the weak chill defined by the
selvage to the inclusion indicated on Figure 6a and a few
mil!imetersinto the host, representedby chip E in Figure 2)
formedduringinteractionof the mafic and silicic magmaand
its isotopic compositionreflects mixing between these two
isotopicallydistinctreservoirs,then a third, now completely
ß
0.5124
A
Bulk
mixing
0.5123
A
irradicated
silica-rich,
high 87Sr/86Sr, endmember
hasto be
postulated.The involvementof this third componentis also
supportedby the "hybrid" characterof the host dacite [cf.
Kouchi and Sunagawa, 1985], as evidencedby its mixed
crystal population and the presence of fragments with
A
0.5122
0.707
0.709
0.710
87Sr/86Sr
87Sr/86Srindistinguishable
fromthe bulk inclusion
withinit
(Figures5 and 6).
The only clues as to the possible composition of the
precursorsilicic endmembercome from (1) the coresof both
the large plagioclasexenocrystsin the inclusion and those
of R-type plagioclases in the host and (2) the core
0.708
A
Fig.7. 143Nd/144Nd
- 87Sr/86Sr
isotope
diagram
summarizing
results
of traversein comparisonwith bulk analysesof Purico-Chascon
dacitespublishedby Hawkesworth
et al. [1982], and the Puric0
ignixnbrite
[de Silva,1987]. A calculated
mixingcurvebetween
the
basaltic andesitcof the inclusion and the Purico ignimbriteis
shown,suggesting
that the isotopicdata can be explained
by
composition
of the hornblende
xenocryst.
The 87Sr/86Sr mixing
of the maficliquid into the rhyodaeite,
with the maximum
ratios of the plagioclasecores (-0.709) arguablyreflect the amountof contamination
beingpreserved
in themarginal
zone(chip
composition
of thesilicicendmember.
This 87Sr/$6Sr
ratio E). The marie and silicic magmaswere subsequently
mixedto
is typical of the 4.2 Ma to -1 Ma dacitic ignimbrites generate
the hostdacite. Crosses
on the mixingcurvearefor
(87Sr/86Sr = 0.708 - 0.711) which form the basementto increments
of 10%ignimbrite
mixedwiththeinclusion
compositi0a
Cerro
Chascon [de Silva, 1989b].
Furthermore, the (80% and 90% not indicated).
Fig. 8. Schematic
modelfor generating
theobserved
relationships
in sample
P8815.(a) Stage1: Mafiemagma
invades
theignimbrite
beneath
thePurico-Chascon
complex,
partiallymeltingit to forma capof rhyoliticmagmawith"felsic
restitc"
(particularly
plagioclase)
characterized
bytheisotopic
composition
(highS7Sr/S6Sr)
oftheignimbrite.
(b)Stage
2: Further
inputsof maficmagmaresultin convection
in therhyoliteandbreakdown
of themafic/silicie
magma
interface
as marieblobsare entrainedinto the rhyolitethroughviscouscouplingat the interface.Exchangeof crystalphases
occurs,
producing
phases
whichareno longerin equilibrium
(chemical,
isotopic,
or textural)withthelocalhost,i.e.,
xenocrysts.
Theinclusions
chill witha contaminated
marginrepresenting
a mixturebetween
therhyoliteandinclusion
magma. The inclusion-forming
magmais chemicallymodifiedto the basalticandesitccomposition
that we now
observe.
(c) Stage3; Morethorough
mixingbetween
themarieandsiliciemagmas
occurs,
astheratioof maficto silcic
magma
increases
anddensitystratification
completely
breaks
down,to forma hybriddacite(nowseenasthehost).
Inclusions
formedin stage2 are chilledandnot significantly
affected,although
furthercoolingandcrystallization
occurs
in both host and inclusions.
DAVIDSON
ETAL.:SMALL-SCALE
HOST-INCLUSION
DISEQUILIBRIUM
17,673
A
Mafic magmainvadesbasementandmeltspre-existingcrust
Contaminated
margin
B
i:i:i:i:!:i:i:!:i.
':i"'-i::
::i:i:i:i:i:i:i:i:i:i:i:'-::'iiO:'-::i:i:i:i:i:i:
:i:i:i:•.-::i:
i:i:.-':i:i:!:i:!::
"':':'"'"'"•:•ji•
'"''"'"'
"'"'•::':•':•••••••.:--'i,
Xenocryst
87Sr
/ 86Sr--0.709
Pillowsof maficmagmaareentrainedin silicicmelt,with
exchange
of crystalsandcontamination
of inclusionmargins
.
c
.,
ß
lxture
87Sr/ 86Sr--0.7068
Mixingof maficmagmaandsilicicmeltproduces
dacitehybrid-
-the host in which the inclusions are now found
17,674
DAVIDSON ET AL.: SMALL-SCALEHOST-INCLUSIONDISEQU!LIB•M
During the mingling and mixing event, crystals from
both the mafic and silicic systemswere exchanged,giving
rise to common"xenocrysts"within the host and occasional
examplesin the inclusion.Thermal equilibrationwas likely
to have been rapid [Sparks and Marshall, 1986; Frost and
Mahood,
1987], resulting in moderate degrees of
undercooling.
Precipitation of overgrowth rims in
equilibrium with the new melt environmentin each system,
crystallizationof the "hybrid" dacite host, and formationof
plagioclasemicrolites (An>60) within the inclusionwould
have occurred at this stage. The groundmassof the host
representsthe last phase of crystallizationprobably under
conditionsof acceleratedcooling and decompressionduring
eruption and emplacement. Finally, the similar glass
compositionsin host and inclusion indicate that thermal,
but not neccesarilychemical, equilibration between the two
phases had been reached: these are residual glass
Stage2 (Figure8b). As furtherinputsof maficmagma
occur,the interfacebreaksdownas smallblobsof themafic
magmaare entrainedinto the rhyolitedue to convection
and
viscouscouplingat the interface.Xenocrystexchange
occurs
during this process.The marginal zone of the inclusion
mingles
andmixes
withthehigh87Sr/86Sr
andincompatible
elementenrichedmelt at smalllengthscales,resulting
in a
hybrid margin compostion.The texture of the groundmass
within this marginal zone (i.e., few skeletal crystalsbut
large numbersof small crystals(Figure 3d)) suggests
that
the temperaturedifference between upper rhyolite andthe
marie magma was not large, enhancing the likelihoodof
moreintimatemixingas theratioof maficto silicicmagma
increased[e.g., Sparksand Marshall, 1986].
Stage 3 (Figure 8c). Overturn of the magma chamber
causes intimate mixing of mafic magma (basaltic an&site
inclusion precursor) with the rhyolite [cf. Sparks and
Marshall,
1986], to produce the hybrid host dacitewith
compositionsand are the end result of crystallizationfrom
during
the respectivesystems.Our observations
are similarto those inclusionsof basalticandesitc. Limitedexchange
from other "mixed" systemssuch as Chaos Crags [Heiken pillowing/chilling and physical exchange of material
and Eichelberger, 1980] where abundant evidence for (crystals) between host and inclusionsproducesthe basaltic
disequilbriumis found in the cores of large crystals. In andesitc inclusion compositionfrom an initially more rnafic
these systems,similar glass compositionsand different rim magma, as discussedby Sparks and Marshall [1986]. The
and microlite compositions in host and inclusion are dacite contains both "xenocrysts"from the basaltic an&site
and "felsicrestitc"(largehigh87Sr/86Srplagioclase,
and
believedto indicatethermal, but not chemical,equilibrium.
Mg poor hornblendes)from the ignimbrite precursor. The
microlite compositionssuggestthat they crystallizedwithin
6. PETR()GENETIC
MODEL
their respective sub-systems late in the crystallization
The presence of mafic inclusions argues for the sequence,during which time resorptionof xenocrystsand
interactionof a mafic magma with a more silicic one, and the subsequentovergrowthwould have occurred. The tiny
the juxtapositionof crystalsderivedfrom the two reservoirs fragmentsof inclusion identified in the dacite may have
within a hybrid dacite requires that the mixing event was resulted from disruption of larger inclusions due to
vigorous.Further,the formationof a "hybrid"requiresthat a vesiculation. Unlike at Lassen Peak [e.g., Clynne, 1989],
significantvolumeratio of mafic magmawouldhaveto have wholesale disruption of inclusions after microlite formation
interacted with the silicie magma [Sparks and Marshall, is not indicated. Emplacement of the domes may have
1986; Frost and Mahood, 1987]. Our crude bulk mixing followed closely after the mixing event due to an increase
in
model (Figure 7) suggeststhat between40 and 20% mafic fluid pressure.
magma (represented
by the inclusions)was mixed into the
silicic melt to producethe hybrid host (the Purico-Chascon
7. IMPLICATIONS
dacitesshown in Figure 7). These estimatesare maxima, as
theoriginal
maficmagma
mayhavehadhigher143Nd/144Nd
One
of
the main
contributions
of
this
work is to
andlower 87Sr/86Srthan the basalticandesitcinclusionnow demonstrate the value of fine-scale determinations of trace
in geologicalmaterials,
preserved. End-memberdynamicmodelsmay be envisaged elementand isotopecharacteristics
ion
as either the direct injection of the mafic magma into an an area which the new generationof high-resolution
community
in the
existingfelsic magmareservoiror the overturnof a density probesshouldopenup to the geosciences
in
and compositionallystratified chamber.Both mechanisms next few years. This type of micro-sampling,
detailedpetrographic
wouldbe expectedto yield a hybridhost on a local scale. conjunctionwith more conventional
of
However, at this stagewe lean towardsthe latter scenario,as and microprobework, allows a gre.aterresolution
processes.
Althoughwe interprettheresults
from
a two stageprocesswhere the silicic endmemberis first magmatic
we believethatthistypeof
produced
by partialmeltingof ignimbriteand theninteracts thisinitialstudywith caution,
in
with the mafic magma is prefered. Figure 8 shows a data is a very neccesaryadjunctto broaderstudies
modelsof magmatic
processes
andmayprovide
schematicmodel for the origin of the host and inclusions. constraining
models. Further
The details are neccesarilyspeculative,but the model can be the fine tuningto alreadyestablished
measurements
of
this
type
on
other
inclusion-host
paixsaxe
regardedas a good workinghypothesisconstrained
by the
mineralogicaland isotopic data.
in progress
andwill help increase
our understanding
of
Stage1 (Figure8a). Mafic magma,likely somewhat
more mafic-felsic magma interactions.
magnesianthan the presentinclusioncompositions,
invades
Acknowledgments.
J.P.DandS.d.SthankTimDruitt,
ignimbritein the sub-Puricocomplex crust. The mafic
WOrner,
MikeClynne,
andCharlie
Bacon
forfruitful
magmapondsand cools, crystallizingolivine, augitc, and Gerhard
discussions
duringtheIAVCEI congress
in
plagioclase (An>60). Partial melting of the ignimbrite andinstructive
for loanof hisLandRover
and
results,producinga melt with a eutecticcomposition
with SantaFe, andPeterFrancis
to takeon the Puricoproject.TheTurner
enrichedincompatible
elementsand with the sameisotopic encouragement
of theUniversity
of Michigan
(I.P.D)andthe
compositionas the ignimbrite. Some of the phenocrysts Foundation
of London
(S.d.S)supported
thefieldwork.
from the ignimbritewill remainunmelted(i.e., large high RoyalSociety
87Sr/86Srplagioclases,
low Mg# hornblendes
andpossibly Isotopic analysisat the University of Michiganwas
byNSFgrantEAR8720564.
Charlie
DeWolf
is
biotite and quartz) and will provide a record for the supported
forproviding
Pbisotope
analyses.
We thank
Tom
involvementof the ignimbrite component.These may be thanked
termed "restitc"in the manner of Wyborn and Chappell
[1986], although"felsicrestitc"may be moreappropriate.
Vogel,JohnReid,andGerhard
WOmer
(official)
andJohn
WolffandBenSchuraytz
(unofficial)
for thoug.htful
and
DAVIDSONET AL.: SMALL-S•
HOST-INCLUSION
DISEQUILIB•
1?, 675
helpful
reviews
of themanuscript.
S.d.Sis a visiting
post- Grove, T.L.,
and LM. Donnelly-Nolan, The evolution of young
silicic lavas at Medicine Lake Volcano, California: implications
for the origin of compositionalgaps in calc-alkaline series
supported
byNASAgrantNAGW-1167,
andwould
liketo
lavas, Contrib. Mineral. Petrol., 92, 281-302, 1986.
thank
VincentYang at the Johnson
SpaceCenterfor his Halliday,
A.N., G.A. Mahood, P. Holden, LM. Metz, TJ. Dempster,
help
andadvice
withmicroprobe
work.Thisis Lunarand
and I.P. Davidson, Evidence for long residence times of
Planetary
Institute contributionnumber 733, and a
rhyolitic magma in the Long Valley magmatic system: The
contribution
to IGCP project249 "AndeanMagmatism
and
isotopic record in precaldaralavas of Glass Mountain, Earth
itsTectonic
Setting".
Planet. $ci. Lett., 94, 274-290, 1989.
Hawkesworth, C.I., M. Hammill, A.R. Gledhill, P. van Calsteren,
and G. Rogers, Isotopeand traceelementevidencefor late-stage
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51, 297-308, 1982.
Bacon,
C.R., Magmaticinclusionsin silicic and intermediate Heiken, G., and LC. Eichelberger,Eruptionsat ChaosCrags,Lassen
volcanicrocks, J. Geophys.Res., 91, 6091-6112, 1986.
Volcanic National Park, California, J. Volcanol. Geotherm.
doctoral
fellowat theLunarandPlanetary
Institute,
Houston,
Bacon,
C.R.,andT.H. Druitt, Composfional
evolutionof the zon•
calcalkalinemagma chamberof Mount Mazama, Crater Lake,
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Holden, P., A.N. Halliday, and W.E. Stephens, Neodymium and
Oregon,
Contrib.Mineral.Petrol.,98, 224-256,1988.
strontium isotope content of microdiorite-enclavespoints to
Bacon,
C.R., and I. Metz, Magmatic inclusionsin rhyolites,
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Nature, 330, 53-56, 1987.
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Kouchi,A., and I. Sunagawa,A modelfor mixing basalticand dactie
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17-23, 1985.
Clynne,
M.A., Disaggregation
of quenched
magmatic
inclusionsKoyaguchi, T., Evidence for two-stage mixing in magmatic
contributes
to chemical diversity in silicic lavas of LassenP•ak,
California(abstract),IAVCEI General Assembly.Bull. N.M.
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R.S.J., and L.A. Marshall, Thermal and mechanical
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magma chamber,Contrib. Mineral. Petrol., I00, 470-483,
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J.C., Origin of andesitc and dacite: evidence of
The petrogeneticsignificanceof
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Francis,
P.W., W.F. McDonough, M. Hammill, LJ. O'Callaghan, Sciences,
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A. N. Hal!iday,Department
of GeologicalSciences,University
Orpington,U.K., 1984.
of Michigan,Ann Arbor, MI 48109.
Frost,T.P., and G.A. Mahood, Field, chemical and physical
(ReceivedAugust11, 1989;
constraints
on mafic-felsic magma interactionin the Lamarck
revisedJanuary30, 1990;
Granodiorite,Sierra Nevada, California, Geol. Soc. Am. Bull.,
AcceptedFebruary2, 1990.)
99, 272-291, 1987.