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1、HydrodynamicfractionationofzirconagepopulationsRebeccaL.Lawrence1,RónadhCox1,,RussellW.Mapes2,§,andDrewS.Coleman21GeosciencesDepartment,WilliamsCollege,Williamstown,Massachusetts01267,USA2DepartmentofGeologicalSciences,UniversityofNorthCarolinaatChapelHill,ChapelHill,NorthCarolina275
2、99,USAABSTRACTINTRODUCTIONpopulationsarelikely,therefore,tohavediffer-ent(andunpredictable)sizecharacteristics.ZirconsintransportinthemodernAma-LargedatasetdetritalzircongeochronologyDetritalzircongeochronologyoftendoeszonRiverrangefromcoarsesilttomediumhaschangedthewayweinvestigatep
3、aleogeog-revealsizedifferencesamongzirconpopula-sand.Oldergrainsaresmalleronaverage:raphy,permittingustoexaminesource-sedimenttions.Forexample,Sager-KinsmanandParrishMesozoicandCenozoicgrainshaveaver-connectionsandtoreconstructthedistribu-(1993)useddetritalzirconagedatatodiffer-ageeq
4、uivalentsphericaldiameter(ESD)tionofcrystallineterranesonregionalandentiatemetasedimentarysequencesfromtwo122±42µm(lowerfinesand),whereasgrainscontinentalscales(Rainbirdetal.,1992;Sir-tectonostratigraphicterranes,butthedistinct>2000MahaveaverageESD67±14µm(up-combe,1999;Rainbirdetal.,2
5、001;Gehrelsetagedifferencesbetweenthetwosequencesalsopercoarsesilt).AsafullWentworthsizeclassal.,2003;Milleretal.,2006).Althoughrobustcorrelatewithsedimentgrainsize:thefinestzir-separatesthetwovalues,zirconsintheseagestatisticsareprovidedbythetenstohundredscongrains(pre-analyticalweig
6、hts)yieldedthepopulationsarehydraulicallydistinct.ofzirconsanalyzedpersample,thenumberofyoungestages.DatainChenetal.(1995)like-Hostsandsizeiscorrelatedwithaveragesamplesusedtocharacterizeeachlocation,wiseshowacorrelationbetweengrainsizeandsizeofco-transportedzircons,implyingstratigra
7、phicinterval,orrockunitisoftenveryzirconage.MillerandSaleeby(1995)sortedzir-hydrodynamicfractionation.Zirconsizeissmall:commonlyjustonesamplepercompari-consintosizefractionsthatyieldedsignificantlypositivelycorrelatedwithpercentmediumsongroup.Thisapproachassumesthatzirconsdifferentage
8、characterist
