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