Z Zajza 2008,元素分配实验研究
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Determinationoffluid/meltpartitioncoefficientsby
LA-ICPMSanalysisofco-existingfluidandsilicate
meltinclusions:Controlsonelementpartitioning
Zolta´nZajacza,*,WernerE.Haltera,ThomasPettkeb,MarcelGuillonga
aInstituteofIsotopeGeochemistryandMineralResources,DepartmentofEarthSciences,ETHZu¨rich,ETHZentrumNW,Clausiusstrasse25,Zu¨richCH-8092,SwitzerlandbInstituteofGeologicalSciences,UniversityofBern,Baltzerstrasse1-3,BernCH-3012,Switzerland
Received16July2007;acceptedinrevisedform24January2008;availableonline4March2008
Abstract
Analysesofco-existingsilicatemeltandfluidinclusions,entrappedinquartzcrystalsinvolatilesaturatedmagmatic
systems,alloweddirectquantitativedeterminationoffluid/meltpartitioncoefficients.Investigationsofvariousgranitic
systems(peralkalinetoperaluminousincomposition,logfO2=NNOÀ1.7toNNO+4.5)exsolvingfluidswithvarious
chlorinities(1–14mol/kg)allowedustoassesstheeffectofthesevariablesonthefluid/meltpartitioncoefficients(D).Partition
coefficientsforPb,Zn,AgandFeshowanearlylinearincreasewiththechlorinityofthsefluid(DPb$6*mCl,DZn$8*mCl,
DAg$4*mCl,DFe$1.4*mCl,wheremClisthemolinityofCl).Thissuggeststhatthesemetalsaredissolvedprimarilyas
Cl-complexesandneitheroxygenfugacitynorthecompositionofthemeltaffectssignificantlytheirfluid/meltpartitioning.
Bycontrast,partitioncoefficientsforMo,B,As,SbandBiarehighestinlowsalinity(1–2mol/kgCl)fluidswithmaximum
valuesofDMo$20,DB$15,DAs$13,DSb$8,DBi$15indicatingdissolutionasnon-chloride(e.g.,hydroxy)complexes.
Fluid/meltpartitioncoefficientsofcopperarehighlyvariable,buthighestbetweenvaporlikefluidsandsilicatemelt
(DCu62700),indicatinganimportantroleforligandsotherthanCl.PartitioncoefficientsforWgenerallyincreasewith
increasingchlorinity,butareexceptionallylowinsomeofthestudiedbrineswhichmayindicateaneffectofotherparameters.
Fluid/meltpartitioncoefficientsofSnshowahighvariabilitybutlikelyincreasewiththechlorinityofthefluid(DSn=0.3–42,
DW=0.8–60),anddecreasewithdecreasingoxygenfugacityormeltperaluminosity.
Ó2008ElsevierLtd.Allrightsreserved.
1.INTRODUCTION
Exsolutionofavolatilephaseisofmajorimportancein
sequesteringelementsfromcrystallizingsilicatemelts.Due
totheirlowerdensityandlowviscositytheexsolvedvola-
tilesmayleavetheirparentalmagmaandmigratetodiffer-
entpartsofthemagmaticreservoirsorintothesurrounding
rocks.Asphysicalandchemicalconditionschange,vola-
tilescontinuouslyreactwiththemagmasorrocksintheir
newenvironmentandenrichordepletethemindifferentchemicalcomponents.Thismayeventuallyleadtothefor-
mationoforedeposits.Alternatively,theymaybedegassed
intotheatmosphereexertingasignificantimpactonthe
atmosphericbudgetofnumerouselements.Thus,volatiles
playacrucialroleinthetransportofchemicalcomponents
betweenactivemagmaticandothergeologicenvironments.
Metalratiosinthemagmaticvolatilephasearedetermined
uponvolatileexsolutionandareimportantindetermining
thetypeoforedepositsformed.Sofarmostofourknowl-
edgeoffluid/meltpartitioninghasbeenbasedonexperi-
mentalwork,whichattemptedtoreproducenatureunder
controlledlaboratoryconditions(e.g.,Holland,1972;
Urabe,1987;Shinoharaetal.,1989;KepplerandWyllie,
1991;Simonetal.,2006).Experimentaldatashowthat
0016-7037/$-seefrontmatterÓ2008ElsevierLtd.Allrightsreserved.
doi:10.1016/j.gca.2008.01.034*Correspondingauthor.Presentaddress:DepartmentofGeology,UniversityofMaryland,CollegePark,MD20742,USA.E-mailaddress:zajacz@umd.edu(Z.Zajacz).
www.elsevier.com/locate/gcaAvailable online at www.sciencedirect.com
GeochimicaetCosmochimicaActa72(2008)
2169–2197thedistributionofdifferentelementsbetweenthemeltand
theexsolvedvolatilephaseisstronglydependentonthe
chemicalcompositionofthesystemandpressureandtem-
perature(CandelaandPiccoli,1995).However,experimen-
talistsarechallengedbynumeroustechnicaldifficulties,
includingtheattainmentofequilibriumandclosedsystem
behaviorwithrespecttotheinvestigatedelements.Numer-
ousexperimentalstudiesinvestigatedpartitioningofsome
economicallyimportantmetalssuchasCu,Pb,Zn,Au,
WandMo;however,onlylittleornoexperimentaldata
existonpartitioningofother,economicallyimportant
elementssuchasSn,Ag,Bi,Sb,As,ThandU.
DatafromHeinrichetal.(1992)andAude´tatandPettke
(2003)indicatethepotentialpoweroftheuseofnatural
fluidandmeltinclusioncompositionstodetermineempiri-
calfluid/meltpartitioncoefficientsinnaturalsystems.How-
ever,nosystematicinvestigationhasbeencarriedoutwith
thismethodcomparingdatacollectedfromvarious
samples.
Inthispaperwepresentanewsystematicapproachto
determinepartitioncoefficientsbetweenmeltsandexsolved
fluidsinnaturalmagmaticsystems.Thisapproachisbased
onanalysesofsilicatemeltandfluidinclusionsentrappedin
quartz,crystallizedinvolatilesaturatedmagmas.Co-exis-
tenceoffluidandsilicatemeltinclusionsinthesameassem-
blagesensuresthattheyrepresentequilibratedfluidand