Chapter 16 The Isotope Geology of Pb

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1、Chapter16TheIsotopeGeologyofPbLead:plumbum(plumbo-)PbPlumbologyplumbotectonicsSomebasicconcepts:1)Radiogeniclead:decayedfromUandTh.2)Commonlead:containedinmineralsandrockswithlowU/PbandTh/PbratiosWhatisdifferencebetweencommonandradiogeniclead??Basedonor

2、iginandoccurrencePrimordialPb:formedinnuclearsynthesis.Mainly204PbPrimevalPb:PbattimetheEarthformed.=primordialPb+radiogenicPbsincenuclearsynthesis.RepresentedbyCanyonDiabloironmeteorite206Pb/204Pb=9.307;207Pb/204Pb=10.294;208Pb/204Pb=29.4763)InitialPb:

3、attimemineralcrystallization.=PrimevalPb+radiogenicPbsincetheformationoftheEarth4)MixedPb:leadatomicweightincommonleadinPb-containedmineralsandinPb+Uminerals:Aston(1927):PbatomicweightinPb+Umineral<

4、ingalenasfromdiff.sourcesvary.However,theatomicweightisunchangeable.Why?Nier(1941):diffPbisotopiccompositionsresultedfrommixingradiogenicPb+“primeval”PbpriortotheformationofgalenasThecommon-Pbdatingmethod:single-stagemodels--Holmes-HoutermansmodelAnumbe

5、rofassumptions:TheEarthwasliquidandhomogeneous,longtimebefore,andatthetime,U,ThandPbwereevenlydistributed;TheisotopiccompositionofprimevalPbwasthesameeverywhere;SmalldifferencesofU/PbratiooccurredwhentheEarthbecamerigid.AnychangeofU/Pbrationisanonlyresu

6、ltofU-PbdecayWhencommonleadmineral(e.g.,galena)formed,PbwasseparatedfromUandThandhasremainednochangeinitsisotopiccompositionsincethenWhatistheHolmes-model?Isotopiccompositionofanysampleofcommonleadwasderivedfromasingle-stagehistory.Themodelassumesthatth

7、eradiogenicPbisdecayedfromUandThinthesourceregions,andthenthePbleftthesourceandincorporatedintooredepositsasgalena.ThePbisotopiccompositioningalenadoesnotchangesincethemineralhasnoUandTh.(16.1)(16.2)IfPbwaswithdrawnfromasystem,the206Pb/204Pbratio:（16.3）

8、TTIsotoperatioofcommonPbofagetiPrimevalPbintheEarthTago238U/204Pb=rationinsourceregionofcommonPbintheEarthnow,,,232Combinetheabovetwoequations:ForbothmodelageandisochronT=(4.54±0.07)*109yFromtheaboveequations,wecancalculatetheage