VOL. 5,
NO. 6
GEOPHYSICAL RESEARCH LETTERS
THE EFFECT
OF H20 AND CO2 ON PLANETARY
JUNE 1978
MANTLES
PeterJ. Wyllie
Departmentof GeophysicalSciences,
Universityof Chicago
Chicago,Illinois 60637
Abstract. The solidusfor peridotite-HzO-COzis a divariant surface
traversed by univafiant lines that locate the intersectionsof subsolidus
divariantsurfacesfor carbonationor hydrationreactionsoccurringin the
presenceof HzO-COz mixtures. Vapor phasecompositions
are normally
buffered to these lines; the buffering capacityof carbonatesis much
greaterthan that of amphiboleand phlogopite. Near the bufferedcurve
for the solidusof partly carbonatedperidotitc,extendingto higherpres-
suresandlowertemperatures
fromaninvariant
pointnear26 kb-1200øC,
there is a temperaturemaximumon the peridotitc-vapor
solidus.On the
CO2 sideof themaximum,above26 kb, COz/HzO isgreaterin liquidthan
in vapor, and liquids are SlOe-poor;on the HzO side of this maximum
(includingall pressures
below26 kb), HzO/COz is greaterin liquidthan
in vapor,andliquidschangefrom forsterite-normative
to quartz-normative
with increasingHzO/COz in vapor. Even tracesof HzO and CO2, in
mineralsor vapor,lowermantlesolidustemperatures
throughhundreds
of
degreescomparedwith the volatile-freesolidus.
tic liquidsare developedat highertemperatures,nearerto the solidusfor
volatile-freeperidotite,where H20 and CO2 are normallyso dilutedin
the melt that they havelittle effect.
Peridotite-H20
The systemwasreviewedby Wyllie(1977). Therearedifferentresults
from different laboratoriesfor the solidusand amphibolebreakdown
curves
in differentperidotites.
Fig.1 shows
threeexperimentally-based
versionsof the solidus(phlogopite-free)
containinglessthan about0.4
wt. % H20, insufficientto producethe maximumamphibole.The heavy
linesshowwherevapor-absent
amphibole-peridotite
beginsto melt, probablyby an incongruentreaction.
Peridotite-CO2
Fig.2, showing
theeffectof 0.1 wt. %CO2on peridotite,
is validfor a
Introduction
wide rangeof CO2 contents. The CO2 existsasvaporat pressures
below
the carbonationreactionTQ, and as dolomite abovethis curve. More than
Assumingthat the compositionsof planetarymantlescorrespondto
peridotites, the effects of HzO and COz can be evaluatedby extrapolation of publishedexperimentaldata. Detailedreviewof sources,of selection of data, of controversies,
and somejustificationsfor extrapolations
weregivenby Wyllie(1977) in hisfirst treatmentof the systemperidotite-
10 wt. % CO2 wouldbe requiredto convertall of the clinopyroxene
into
dolomite, leavingexcessvapor at pressures
aboveTQ for other carbonation reactions. Calcicdolomite and peridotitemelt togetheralongQR,
producinglow-SiO2carbonatiticliquids. Dolomiteis replacedby mag-
nesite
athigher
pressures.'
H20-CO2. That review must be assumedin this brief Letter, which introducessome additional featuresfor the phasediagram. Determinationof
Peridotite-H2 O-CO2
the phaserelationships
in peridotite-H20-CO2
is a fkst stepfor evaluation of the behaviorof the componentsC-H-Oin planetarymantles. The
oxygenfugacityis a critical factor. The assumptionthat components
C-H-O are presentdominantly as H20 and CO2 is certainly not true for
all mantlesat all depthsat all times.
H20 and CO2 may exist in minerals,in vaporphase,or in melts. The
physicalstateand chemicalbehaviorof volatilecomponent-rich
films in
Fig. 3 showsthat with excessCO2 and H20, the reactionTQ becomes
a divariantcarbonation
surface,with geometryshownby contoursfor
constant
CO2/H20in thevaporphase.Simi'larly,
thesolidus
is a divariant
surfaceconnectingthe solidus curvesfor excessH20, PM, and for excess
mantlesneedsevaluation;
the solutecontentof H20-CO2fluid mayreach
CO2, PQU (QU differsfrom QR in Fig. 2). The solidussurfacefor carbonatedperidotite,QNU, meetsthe subsolidus
carbonationsurfacealong
the univariantline QN. The vaporphasecontoursfor the solidussurface
tens of weight per cent. Amphibole, phlogopite,and carbonatecan be
crystallinehostsfor H20 and CO2 at upper mantle pressures.Other
possibilities,
suchastitanoclinohumite,
dense
hydrated
magnesian
silicates,
-,
and sanidinehydrate are neglectedin this treatment. At temperatures
above800øC,manyperidotires
canbecompletely
hydrated
byabout0.4
30
wt. % H20, with formation of a.mphibole. At pressures
aboveabout
30 kbar, where amphiboleis not stable,it requiresonly about 0.02 wt.
% H20 to producemaximumphlogopitein peridotRecorresponding
to
the earth'smantle. In contrast,about 30 wt. % CO2 is requked to carbonateperidoti•tecompletely,producingan assemblage
of carbonates+ silica
+ alu,minousmineral. There is a seriesof carbonationreactions,and a
carbonate-pyroxeneexchangereaction, in the pressurerange up to 50
kbar.
The solubility of H20 in silicateliquidsincreasesto about 10 wt. %
at 5 kbar and about 20 wt. % at 20 kbar. In contrast,the solubility of
d-
CO2 in silicateliquidsremainsvery low at pressures
to 10 kbar, and
reaches1-4 wt. % (dependingon liquid composition)at 20 kbar. In the
pressureintervalbetweenabout 20 and 26 kbar, however,the solubility
of CO2 in near-solidusliquidsfrom peridotite increasesdramaticallyto
perhapsas much as 35-40 wt. %; this effect is associated
with the subsoliduscarbonationof peridotRe,asdiscussed
below.
For mantleswith smallamountsof H20 and CO2, only smallamounts
of liquid are developedbelow the solidustemperatureof volatile-free
Paper
number 8L0487.
0094-8276/78 / 068L-0487 $01.00
..-' 1
.....'
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III /
I/'mphibole-.p_eri.dotit
//
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I , 4/
10
800
I000
1200
Temperature
ø0
stronglyby the proportionsof CO2 and H20, and the way they are distributedamongvapor,carbonates,
andhydrousminerals.Commonbasal-
1978 by the American Geophysical
,,,,
10....'-"::"'
600
peridotire.The compositions
of the near-solidus
liquidsareinfluenced
Copyright
1,.'
BI.""'MlWl/,G .'"" I
Union.
440
Fig. l. Experimentallybasedand deducedcurvesfor the solidusof
amphibole-peridotite,
with < 0.4 wt. % H20. Depthsandgeotherms
are
for the earth (CR = Clarkand Ringwood,1964; R = Ringwood,1966);
MIW = Millhollen,Irving and Wyllie(1974); B = Boettcher(Mysenand
Boettch•, 1975); G = Green(1973); S = shield;O = oceanic.
441
Wyllie: TheEffect Of H20AndCO
2 OnPlanetaryMantles
4O
6O
5O
IOO
3O
i v'
_
,
'
I /
,
i ' i , /i
i I
lB.
Glosed
syslem
A.
XC02:0
6/
Go+Do+ Cm //
I_COz/(COz+
H•0)--•.6
-
[- Porlly
4O
JcarbonatedJ
I pendotiteJ,
-+V
(var,able)/•
IO0
•
/
50 •
"'
g20
•
i/
)
'er•-dotlte
•
///
'
,50
ii/I
•;v,,
0
800
0
600
800
I000
1200
1400
Fig.2. Peridotite-COz,
in part schematic.SeeEggler(1976, 1978)and
Wyllie(1977, 1978) for datasources.Depthsandgeotherms
arefor the
earth. PSissolidusfor thevolatile-free
perid0tite.Abbreviations:
O1=
olivine,Opx= orthopyroxene,
Ga = garnet,Sp= spinel,P1= plagioclase,
Cd= dolomitesolidsolution,Cm= magnesite
solidsolution,V = vapor.
of uncarbonated
peridotite,
PQNM,illustrate
a shnple
geometry
below
20 kbar (Mysenand Boettcher,1975), but in the regionof the intersection line, QN, the contourson the surfabepassthrougha temperature
maximumon the line mn, before droppingdown to QN. Unlessthereis
enoughCOz presentto carbonateall of the clinopyroxene,
conditions
in
the areaQNU for meltingof carbonatedperidotitesare not reached.Partially carbonatedperidotite beginsto melt along the line QN, with the
vaporphasecompositionbufferedto highvaluesof HzO/COz, asshown
by the contours. In order to illustrate the effect of carbonation,the
reactionsfor hydrationto amphiboleandphlogopitehavebeenneglected
in Figs. 3 and 4.
The shapesof the contoursare more easilyvisualizedin Fig. 4. Fig. 4
showsthe phasefieldsin the presenceof excessvaporof fixed composi-
from
Fig.
3.The
reaction
(6)isthe
carbonation
reaction
from
Fig.
3.
40
, ,,J , .'M/•J",'
,,,,,,,',,:,
JPeridotire
,,/&4'...[U
' .
_J
."
/
'""
.""
• ZU•. .'
•
/
.
,j
',.,,.•;//../2"
I/! I I o:8f
•
....
! I
/ /.e
/
phasecomposition,three of which are reproducedfrom Fig. 3. B.
Closedsystem,reactionsare identicalwith thosein Fig. 4A at pressures
below the contour for reaction (6). With increasingpressureabove
contour(6) carbonationproceedsand vaporis enrichedin H20/CO2;
the solidusis part of the buffer line QN in Fig. 3. Additional abbreviation: Qz - silicapolymorph.
Thereactions
labelled(4) and(2) arethe additionalcarbonation
reactions
that occur if there is enoughCO2 presentto completethe successive
stepsandproducea carbonate-silica
assemblage,
with accessory
garnet.
In a closedsystemwith a smallproportionof total volatiles,andwith
the samevaporphasecomposition
at low pressures,
the chrbonation
reaction beginsalongthe sameline asin Fig. 4A. With continuedreactionof
70 I ' I '
c-•
PerH•O•///4
800
IOOO
12oo
14oo
o
Temperature
øC
Fig. 3. Peridotite-H20-CO2with excessvapor,omittingamphiboleand
phlogopite,piecedtogetherandaveragedfrom variousdata sources(see
Wyllie, 1977, 1978; Eggleret al., 1976). The divariantcarbonation
surfacedividesthe solidussurfaceinto (1) the area PQNM passing
through a temperaturemaximumalongmn, for the melting of peridotite-vhpor,and (2) the areaQNU for the meltingof carbonated
peridorite-vapor.Two other carbonation
reactions(not shown)intersectthe
150
/ •oo•
• %•,4_"•
i---?/%%'"
I /'
J-geotherms
/
600
o_
•, 3o
Fshield
o
2O0
///--
60Ph'"carbønale---'///C•/
Ph
'-"
J
J• • / ' J//'/
J•///
.40
_=
/
1200 1400
actions:(6) is TQ in Fig.3; for details
of (4) and(2) see,Wyllie
and
Drø ,
S_olidus
-{
,,•
•eZ//f.•
/ / / ,.o/ , /• •
I000
Huang(1976) andWyllie(1977). Eachlineis a contourfor fixedvapor-
'"' -I
I//..••'
800
Fig. 4. Peridotite-HzO-COzwith excessvapor,omittingamphiboleand
phlogopite. Largelyschematic. The magnitudeof the temperaturemaximumon the solidusis not known. A. Open system,with vapor
phasecompositionmaintained constantthroughthree carbonarianre-
tion,
X•o
z=0.6.
The
solidus
curve
isthe
corresponding
solidus
contour
otI
1200 1400
Temperolure
øC
Temperature
øC
.
I000
1600
I/I,
H'
b
/
/
./
__ 50
/Peridotite
Per
H20•V/ / no
vaintiles
I Ph,'--"4•.,_•f, I ,
600 800 I000 1200 1400
Temperalure
øC
Fig. 5. Selectedmeltingreactionsfor peridotite-H•O-CO•,partly schematic. Ie corresponds
to Q in Figs.2 and 3. Abbreviations,seeFig. 2'
additional:
Ph= phlogopite,
Do= dolomite
solidsolution
(Cdin Fig.
2), Per = peridotite,volatile-free. Vapor-absentcurvesare shownfor
peridotite
with Hb (Fig. 1), Ph (estimated),
carbonate
(Fig.2), andPhcarbonate.Buffered
vapor-present
curves
areshownfor peridotite
with
latterarea(seeFig.4A). Thetemperature
maximum
mnis consistent
Do, andDo-Ph. Bufferedvapor-present
curvesfor peridotitewith Hb or
with phase relationshipsdeduced in the systemMgO-SiO2-CO2-H20
(Ellis and Wyllie, 1978), but its magnitudeis not known (exceptfor
PmQ,pureCO2). Depthsandgeotherms
arefor the earth.
Phare,respectively,
coincident
with curveHb, andsomewhat
lowerthan
curvePh up to the invariantpoint near Ie (at higherpre•ures Ph is
jointed by Do). Depthsandshieldgeotherms
are for the earth.
442
Wyllie: TheEffect Of H20AndCO
2 OnPlanetaryMantles
CO2 the vapor phasecompositiontracksacrossthe divariant surfacebe-
comingprogressively
enrichedin H20/CO2. The s61idus
is dividedinto
two parts, the low pressurepart beingidenticalwith that in Fig. 4A, but
the highpressure
part corresponding
to the vapor-phase
buffer line, QN in
Fig. 3. The vapør-phaseat the solidusbecomesprogressively
enrichedin
H20/CO2 with increasing
pressure
alongQN.
Eachcarbonatioi•
reaction(Fig. 4A) is represented
in peridotite-HzOCOz by a divariantcarbonationsurfacewhich intersectsthe solidussur-
facein aunivariant
line,similarto QN in Fig.3. Similarly,eachhydration
reaction,involvingthe formation of amphiboleor phlogopite,is representedin peridotite-CO2-H20by a subsolidus
divariantsurfacewhich
intersectsthe solidussurfacein a univariantline where the vapor phase
compositionis buffered, unlessthere is more H20 presentthan that requiredto hydratethe peridotitecompletely:about0.4 wt. % for amphi-
bole,and0.02 wt. % for phlogopite.Thebuffering
capacity
of the
hydrousmineralsis obviouslymuchlessthanthat of carbonates.
The estimatedpositionsof curvesfor the beginningof meltingof
severalmineralassemblages
in the systemperidotite-HzO-CO2
arecømparedin Fig. 5. The bufferedsoliduscurvefor Do+V extendingfrom 16
is the line QN from Fig. 3. A subsolidus
reactioninvolvingdolomiteand
phlogopiteproducesan invariantpoint near 16 which terminates•this
buffer curve, and from this point there extendtwo additionalsolidus
curves:onefor the vapor-absent
eutecticof phlogopite-carbonate-peridotite (Hollowayand Eggler,1976), and the other for the sameeutectic
with excessvapor, buffered to high H20/CO2 becauseof the strong
bufferingcapacityof dolomite. Fig. 1 showsalternativepositionsfor
the curveHb, one of which (G) mightjust overlapwith the Do+V curve,
generatinga meltingreactionfor Hb-Do-peridotitewith bufferedvapor,
in a very limited region.
shields,incipient melting must begin in the depth interval 110-170 km
if the mantleperidotitecontainsany H20+CO2 (Fig. 5). The seismic
low-velocityzone beneathshieldsis weakly developedor absent,indicatingthatmantleperidotitein theseregions
contains
little or no phlogopite,
carbonate,or HzO-CO2 vapor. Kimberlitesand other volatile-richalkalic
magmashavebeen erupted,however,demonstratingthat H20 and CO2
haveexisted, at leastlocally at depth. I concludethat H20 and CO2 are
distributedsparsely
andirregularlyin subcontinental
mantle,with periodic
magmaticflushestransportingthesecomponentsinto the overlyinglithosphere,or throughit if tectonicconditionsare suitable.
Repeatedmagmaticactivity indicatesthat H20 and COz must be re-
plenished,
eitherfromdeepersources
or by recycling
viasubducted
oceanic crust. The oxygen fugacity may control the relationshipsamong
graphiteor diamond,carbonate,andnear-soliduscarbonatitic,kimberlitic,
or nephiliniticmeltsat variouslevelsin the mantle.
Acknowledgments.
Thisresearch
wassupportedby the Earth Sciences
Section,NationalScienceFoundation,NSF GrantEAR 76-20410. Supported in part by the MaterialsResearchLaboratoryProgramof the
National
Scie:nce
Foundation
at theUniversity
of Chicago.
Thispaper
constitutes
ContributionNo. 25 of the BasalticVolcanism
StudyProject,
whichisorganized
andadministered
by the Lunarand PlanetaryInstitute,/
UniversitiesSpaceResearchAssociationunderContractNSR 09-051-001
with the NationalAeronauticsand SpaceAdministration.
References
Brey,G., andD.H. Green,Solubilityof CO2in olivinemelilititeat high
pressure
androle of CO2 in the earth'suppermantle,Contr.Mineral
PetroL, 55, 217-230, 1976.
Applications
Figs. 1, 2, and 5 showthat eventrace amountsof H20, CO2 or mixturesof H20-CO2, whetherpresentasvaporor storedin minerals,lower
the temperatureof beginningof meltingof mantleperidotitesthrough
hundredsof degreescomparedwith the solidusfor volatile-freeperidotitc.
For the peridotitc-vaporpart of the solidussurface,the SiOz content
of the liquidis increased
with H20/CO2 in the vapor.Thereis a boundary on the surfaceseparatingquartz-normativeliquidsfrom forsteritenormatireliquids(Mysenand Boettcher,1975; Eggler,1975). On the
H20 sideof mn (Fig. 3), andat all pressures
below26 kbar,H20/CO2
is greaterin liquid thanin vapor. On the COz sideof mn, above26 kbar,
CO2/H20 is greaterin liquid thanin vapor;this appliesto all buffered
reactions
with carbonate.
At highpressures,
evenlow CO2/H20in the
vaporis sufficientto producecarbonate,and the presence
of carbonate
ensuresgenerationof SiO2-poor,CO2-richliquids. At pressures
greater
than 30 kbar,the vaporphase,if present,is bufferedto veryhighH20!
CO2 (Fig. 3) in meltingof dolomite-peridotite
or phlogopite-dolomiteperidotite. The presenceof phlogopitewould ensurehigh K20 and
probably
increase
MgO/CaO
in thesmallamount
ofliquidproduced.
The
Clark, S.P.,and A.E. Ringwood,Densitydistributionandconstitutionof
the mantle,Rev. Geophys.,2, 35-88, 1964.
Egglet,D.H., CO2 as a volatilecomponentof the mantle: the system
Mg2SiO4-SiO2-H20-CO2,
Phys.Chem.Earth, 9, 869-881,1975.
Egglet,D.H., DoesCO2 causepartialmeltingin thelow-velocity
layerof
the mantle?,Geology,4, 69-72, 1976.
Eggler,D.H., The effectof CO2 uponpartialmeltingof peridotitein the
system
Na20-CaO-A1203-MgO-SiO2-CO2
to 35kb,withananalysis
of
meltingin a peridotite-H20-CO2
system,Arner.Jour.Sci., 278, 305343, 1978.
Eggler,D.H., I. Kushiro,andJ.R. Holloway,Stabilityof carbonate
minerals in a hydrousmantle, CarnegieInst. WashingtonYearbook,75,
631-636, 1976.
Ellis,D., and P.J.Wyllie, A modelof phaserelationsin the systemMgOSIO2-H20-CO2and predictionof the compositions
of liquidscoexistingwith forsteriteandenstatite,Proceedings
oœ2ndInternationalKimberliteConference,
SantaFe, (in press),1977.
Green, D.H., Experimentalmelting studieson a model upper mantle
compositionat highpressureunderwater-saturated
and water-undersaturatedconditions,Earth and Plan. Sc• Lett., 19, 37-53, 1973.
near-solidus
liquid is probablycarbonatitic
for the first assemblage,
and
meliliticor kimberlitic
forthesecond
assemblage
(BreyandGreen,1976).
Holloway,J.R., and D.H. Eggler,Fluid-absentmeltingof peridotitecontainingphlogopiteanddolomite,Carnegie
Inst. Washington
Yearbook,
For a largetemperature-depth
intervalduringthe thermalevolution
of a planet, meltingmustoccurwhereverH20 andCO2 arepresent. The
Millhollen,
G.L.,A.J.Irving,and P.J.Wyllie,Meltingintervalof peridotitc
smallproportionof veryfluid, low-viscosity
magmamustmigrate,flush-
with 5.7 per centwaterto 30 kilobars,J. Geoœ,82, 575-587, 1974.
Mysen,B.O., andA.L. Boettcher,Meltingof a hydrousmantle,J. PetroL,
ing out incompatibleelements,and becomingconcentratedas magma
pocketsor abortedintrusionsat shallower,coolerlevels. This process
would produce inhomogeneities
in trace element contentsof mantle
peridotitesbefore temperaturesbecamehigh enoughfor the generation
of largeproportionsof basalticmagma.
The existence
of a temperature-maximum
on the solidus
surface(Fig.
3) showsthat a magmarisingalongan adiabat,not too highin temperature abovethe solidus(Fig. 4) would evolveits dissolved
H20 and CO2
when it reacheda depth corresponding
to a pressureof about 25 kbar,
if equilibriumwere maintained. This could enhancethe prospectsof
crack propagationand explosiveeruption,if tectonicconditionswere
suitable
(O.L.Anderson,
personal
comm.,1977).
Considernow the calculatedpositionsof geotherms
in the Earth.
Figs.1 and5 showthat the seismic
low-velocity
zonein oceanic
regions
(80-100kin) is compatible
withincipient
melting
associated
withthe
instabilityof amphibole.The loweroceaniclithosphere
mustbecome
enrichedin rocks similar to kaersutiteeclogite. Beneathcontinental
75, 636-639, 1976.
16, 520-593, 1975.
Ringwood, A.E., Mineralogy of the mantle, in: Advancesin Earth
Sciences
(P.M. Hurley,ed.), 357-399,M.I.T. Press,
Cambridge,
Massachusetts,
502 p., i 966.
Wyllie,P.J., Mantlefluidcompositions
bufferedby carbonates
in peridotite-CO2-H20, J. GeoL, 85, 187-207, 1977.
Wyllie, P.J., Kimberlitemagmasfrom the systemperidotite-H20-CO2,
Proceedingsoœ2nd International Kirnberlite Conference,Santa Fe,
(in press),1977.
Wyllie, P.J., andW.I• Huang,Carbonationandmeltingreactionsin the
systemCaO-MgO-SiO2-CO2
at mantlepressures
with geophysical
and
petrologicalapplications,Contrib.Mineral PetroL, 54, 79-107, 1976.
(Received February 17, 1978;
revised April 4, 1978;
accepted April 20, 1978.)