(Panahi)Behavior of major and trace elements (incl

PIIS00167037(99)00420-2Behaviorofmajorandtraceelements(includingREE)duringPaleoproterozoicpedogenesisanddiageneticalterationofanArcheangranitenearVilleMarie,Que´bec,CanadaALIREZAPANAHI,*GRANTM.YOUNG,andROBERTH.RAINBIRDDepartmentofEarthSciences,UniversityofWesternOntario,London,OntarioN6A5B7Canada2GeologicalSurveyofCanada,Ottawa,Ontario,K1A0E9Canada(ReceivedJuly29,1999;acceptedinrevisedformNovember12,1999)Abstract—Thispaperpresentsacompletesetofchemicaldatafromapre-2.0-GaweatheringprofilethatformedontheArcheangranitenearVilleMarie,Que´bec.ExaminationofdatafromtwosectionsofthepaleosolindicatesthatNa,Ca,P,andSrwerelostfromthepaleosol,whereasTiNb,Th,Zr,Hf,andTaremainedimmobileduringchemicalweatheringandallsubsequentalteration.RareearthmobilityduringgenesisoftheVilleMariepaleosolcloselyfollowsthepatternofearlystagesofchemicalweatheringinmodernprofiles.AverageconcentrationofREEinbothprofilesis20–40%lowerthanwhatisfoundinthefreshgranite,suggestingmobilizationduringpedogenesis.HREEsseemtobelessmobilizedthantheLREEs.(La/Sm)Nratiosgraduallydecreasetowardthetopofthepaleosolbutthevaluesremainveryclosetothatoftheprotolith.(Gd/Yb)Nvaluesremainunchangedintheweatheringprofile.(La/Yb)Nvaluesintheweatheringprofilearequitevariableandgenerallylowerthanthatoftheprotolith.Eu/Eu*andCe/Ce*valuesareessentiallythesameasthoseoftheprotolith,buttwosamplesfromtheuppermostpartoftheprofilehavedevelopedpositiveceriumanomalyof'0.3relativetotheprotolith.Copyright©2000ElsevierScienceLtd1.INTRODUCTIONPrecambrianpaleosolshavebeenusedtocharacterizetheredoxstateoftheatmosphereunderwhichtheyformed.Ithasbeensuggestedthatmanyolderpaleosolsdifferfromtheirmoderncounterpartsinshowingasignificantlossofironcom-paredwiththeirparentrocks.RyeandHolland(1998;forareviewofthesubject)paidparticularattentiontopaleosolsthatdevelopedbetween2.75and0.45Ga.TheysuggestedthatatmosphericpO2waslow(#0.002atm)before2.2Gabuttherewasadramaticchangeat'2.0Ga,whenpO2increasedtoatleast0.03atm(RyeandHolland,1998).Ohmoto(1996)chal-lengedthisideaandsuggestedthatallpaleosols,regardlessofage,havesufferedalossintotalironandheattributedthistoreductivedissolutionofferrichydroxideduringoraftersoilformationbyhydrothermalfluidsororganicacids.Erosion,diagenesis,andmetamorphismmaymodifypa-leosolsintherockrecord,makingthemverydifficulttoiden-tifyandinterpret.Chemicalpatternspreservedthroughoutpa-leoweatheringprofilesareproductsofweathering,diagenesis,andmetamorphism.Toassessaprofilewiththehopeofpro-vidinginformationaboutatmosphericevolution,thesectionmustbeshowntobeaninsituproductofweatheringandnotallochthonous(cf.Maynard,1992).Second,thechemicalpat-ternsresultingdirectlyfromweathering(i.e.,reactionofrainwaterwiththerock)shouldbecharacterizedandseparatedfromthoseduetodiagenesis.Erosionoftheexposedweather-ingprofilebeforedepositionofoverlyingsedimentsmaycom-plicateoriginalchemicalpatterns.Erosionoftheuppermostpartoftheprofile(leachedzone),forexample,mayexposetheintermediatezone(zoneofaccumulation).Continuedchemicalweatheringmayestablishanewleachedzone,whichissuper-imposedonthepreviouszoneofaccumulation(NesbittandMarkovics,1997).Translocation(transportationfromsurfacetodepth)offine-grainedsoilmaterialshasbeenreportedfrommanysoilprofiles(Nesbittetal.,1980).Diageneticchemicalalterationhasastronginfluenceonthenatureofexistingweatheringproducts(e.g.,precipitationofcements,dissolutionofpreexistingminerals,oxidationofironmineralsandforma-tionofnewminerals).Theobservedchemicalcompositionofpaleoweatheringprofilesis,therefore,thesumoftheeffectsofvariousstagesofweathering(whichdirectlyinvolvedtheat-mosphere)anddiagenesis(SchauandHenderson,1983).Thebest-preservedpaleosolsinCanadaarethoseassociatedwiththesub-Huronianunconformities.Theyformedon.2.5GaArcheangraniticandgreenstonebasementandsubse-quently,werecoveredbythesedimentsofthePaleoproterozoicHuronianSupergroup.AsectionofsuchpaleosolsispreservedonArcheangraniteinsouthwesternQue´bec,nearVilleMarie(Fig.1).TheVilleMarieprofilewasoriginallystudiedbyRainbirdetal.(1990)whocollectedandanalyzedasuiteofsamplesformajorandsometraceelementsusingX-rayfluo-rescencespectrometry.Thepresentstudyhopestoimprovetheresultsofthepreviousstudybyprovidingacompletehigh-qualitydatasetthatwillpermitaquantitativeassessmentofbehavioroftraceelements(includingREE)duringPrecam-brianpedogenesis,andhelpsaddresssomepressingissuesregardingPrecambrianatmosphericoxygen.1.1.GeologicalBackgroundRegionalandlocalgeologyoftheareawasdescribedbyRainbirdetal.(1990).TheArchean('2.5Ga;Stockwell,1982;Kroghetal.,1984)graniticbasement,knownlocallyastheVilleMariegranite,isunconformablyoverlainbyflat-lying*Authortowhomcorrespondenceshouldbeaddressed([e-mail]).PergamonGeochimicaetCosmochimicaActa,Vol.64,No.13,pp.2199–2220,2000Copyright©2000ElsevierScienceLtdPrintedintheUSA.Allrightsreserved0016-7037/00$20.001.002199PaleoproterozoicsedimentaryrocksoftheLorrainFormationintheCobaltGroupoftheHuronianSupergroup.FormationsofthelowerHuronian(QuirkeLake,HoughLake,andElliotLakeGroups)arenotpresentinthearea,suggestingthatthe