Plesovice zircon—a new natural reference material for U--Pb and Hf isotopic microanalysis - 图文

Available online at www.sciencedirect.comChemicalGeology249(2008)1–35

www.elsevier.com/locate/chemgeo

Ple?ovicezircon—Anewnaturalreferencematerialfor

U–PbandHfisotopicmicroanalysis

Ji?íSlámaa,b,c,?,JanKo?lera,DanielJ.Condond,JamesL.Crowleye,AxelGerdesf,JohnM.Hancharg,MatthewS.A.Horstwoodd,GeorgeA.Morrish,LutzNasdalai,

NicholasNorbergi,UrsSchalteggerj,BlairSchoenej,

MichaelN.Tubrettk,MartinJ.WhitehouselCentreforGeobiologyandDepartmentofEarthScience,UniversityofBergen,Allegaten41,N-5007Bergen,NorwayInstituteofGeology,AcademyofSciencesoftheCzechRepublic,v.v.i.,Rozvojová135,Prague6,16502,CzechRepubliccDepartmentofPetrologyandStructuralGeology,CharlesUniversityinPrague,Albertov6,Prague2,12843,CzechRepublic

dNERCIsotopeGeosciencesLaboratory,KingsleyDunhamCentre,Keyworth,NottinghamNG125GG,UK

eDepartmentofGeosciences,BoiseStateUniversity,Boise,ID83702,USA

fInstituteofGeosciences,JohannWolfgangGoetheUniversity,Altenh?ferallee1,D-60438FrankfurtamMain,Germany

gDepartmentofEarthSciences,MemorialUniversityofNewfoundland,St.John's,NL,CanadaA1C5S7

hDepartmentofGeologyandGeochemistry,UniversityofStockholm,SvanteArrheniusv?g8C,Stockholm,SE-10691,Sweden

iInstituteforMineralogyandCrystallography,UniversityofVienna,Althanstrasse14,Vienna,A-109,AustriajDepartmentofMineralogy,UniversityofGeneva,RuedesMara?chers13,CH-1205Geneva,Switzerland

kMicroanalyticalFacility-INCOInnovationCenter,MemorialUniversityofNewfoundland,230ElizabethAvenue,St.John's,NL,

CanadaA1C5S7

lLaboratoryforIsotopeGeology,SwedishMuseumofNaturalHistory,SE-10405Stockholm,Sweden

baReceived13August2007;receivedinrevisedform16November2007;accepted20November2007

Editor:R.L.Rudnick

Abstract

Matrix-matchedcalibrationbynaturalzirconstandardsandanalysisofnaturalmaterialsasareferencearetheprinciplemethodsforachievingaccurateresultsinmicrobeamU–PbdatingandHfisotopicanalysis.WedescribeanewpotentialzirconreferencematerialforlaserablationICP-MSthatwasextractedfromapotassicgranulitefaciesrockcollectedinthesouthernpartoftheBohemianMassif(Ple?ovice,CzechRepublic).

Datafromdifferenttechniques(ID-TIMS,SIMSandLAICP-MS)andseverallaboratoriessuggestthatthiszirconhasaconcordantU–Pbagewithaweightedmean206Pb/238Udateof337.13±0.37Ma(ID-TIMS,95%confidencelimits,includingtracercalibrationuncertainty)andU–PbagehomogeneityonthescaleusedinLAICP-MSdating.Inhomogeneitiesintraceelementcompositionduetoprimarygrowthzoningpreventitsuseasacalibrationstandardfortraceelementanalysis.ThecontentofUvariesfrom465ppminpristinepartsofthegrainsto~3000ppminactinide-richsectorsthatcorrespondtopyramidalfaceswithahighdegreeofmetamictization(presentinca.30%ofthegrains).Thesedomainsareeasilyrecognizedfromhighintensities

?Correspondingauthor.CentreforGeobiologyandDepartmentofEarthScience,UniversityofBergen,Allegaten41,N-5007Bergen,Norway.Tel.:+4755583529;fax:+4755583660.

E-mailaddress:jiri.slama@geo.uib.no(J.Sláma).0009-2541/$-seefrontmatter?2007ElsevierB.V.Allrightsreserved.doi:10.1016/j.chemgeo.2007.11.005

2J.Slámaetal./ChemicalGeology249(2008)1–35

onBSEimagesandshouldbeavoidedduringtheanalysis.HfisotopiccompositionofthePle?ovicezircon(N0.9wt.%Hf)ishomogenouswithinandbetweenthegrainswithamean176Hf/177Hfvalueof0.282482±0.000013(2SD).TheageandHfisotopichomogeneityofthePle?ovicezircontogetherwithitsrelativelyhighUandPbcontentsmakeitanidealcalibrationandreferencematerialforlaserablationICP-MSmeasurements,especiallywhenusinglowlaserenergiesand/orsmalldiametersoflaserbeamrequiredforimprovedspatialresolution.?2007ElsevierB.V.Allrightsreserved.

Keywords:Zirconreferencematerial;LaserablationICP-MS;Ple?ovice;U–Pbdating;Hfisotopes;BohemianMassif

1.Introduction

IsotopicdatingofaccessorymineralsbymeansofU-andTh-decayisthemostpreciseandaccuratetechniqueforestablishingtheageofhigh-temperatureeventsinmagmaticandmetamorphicrocks.Themethodisalsowidelyappliedtodatingofdetritalmineralsinsedimentaryprovenancestudies.Recentintroductionofnewtechniques,suchasU–Pbzircon(ZrSiO4)datingbylaserablationICP-MS(e.g.,Ko?lerandSylvester,2003andreferencetherein),requiresdevelopmentofreferencematerials,bothformatrix-matchedagecalibrationandforqualitycontrolpurposes.Therequirementsforsuchzirconreferencematerialsare(i)homogeneityandconcordanceofradiogenicPb/Uratios,(ii)lowcommonPbcontent,(iii)moderateUcontent(tenstohundredsppm),(iv)crystalline(non-metamict)structure,(v)sizesuitableforrepeatedlaserablationanalyses(grainsseveralmmtocmindiameter)and(vi)availabilitytothescientificcommunity.Previousattemptstoproducechemicallyhomogeneoussyntheticzirconmaterial(e.g.,Hancharetal.,2001)havefailedbecausePbdoesnotreadilyenterthestructureofzirconandalsobecauseofthestrainimposedonthecrystallattice.

Zirconreferencematerialforin-situHfisotopicanal-ysisbylaserablationICP-MSshouldhave(i)homo-geneousHfisotopiccomposition,bothwithinandbetweenindividualgrains,(ii)moderateHfcontent(low%level)andpreferablyhomogeneousHfconcentration,and(iii)lowLu/HfandYb/Hfvaluesanditshouldalsooccurasmm–cmsizedgrainsandbeavailableinsufficientquantity.

Thereareseveralnaturalzirconsamples(e.g.,91500,Temora,MudTank,GJ-1,SL13)thathavebeenproposedaspotentialcalibrationandreferencematerialsforin-situU–Pbisotopicanalysisbutonlyafewmeetthecriteriafora“good”referencematerialandtheyareoftennotavailableinquantitiesneededforlaserablationICP-MSanalysis.Fromthesethe“91500”zircon(Wiedenbecketal.,1995)hasbeenmostoftenusedasareferencematerialforin-situanalysisofHfisotopes

buttherecentstudies(Griffinetal.,2006;2007;Corfu,2007)havediscusseditsisotopicheterogeneity.Becauseofitslimitedamountandextensiveuseinmicroanalysis,thesupplyofthe91500zirconreferencematerialhasbeenalmostexhausted(WoodheadandHergt,2005).Thisstudypresentsnewisotopicdataforanaturalzirconextractedfromahigh-temperaturepotassicgranulitefromthesouthernBohemianMassif(localityPle?ovice[readPleschovitze]intheCzechRepublic)thatappearstobeasuitablereferencematerialforlaserablationICP-MSU–PbdatingandHfisotopicmeasure-ments.ThiszirconmeetsmostofthecriteriaforacalibrationandreferencematerialanditcanbeobtainedonrequestfromDepartmentofEarthScienceattheUniversityofBergen(http://www.geo.uib.no/ceia).2.Sampleandgeologicalsetting

ThestudiedzirconcomesfromapotassicgranulitethatwasfirstdescribedbyVrána(1989)andrecentlyalsobyJanou?eketal.(2007)fromthePle?ovicequarrysituatedintheeasternpartoftheBlanskylesgranulitebody,ca.5kmNNEofthetownof?eskyKrumlov(Fig.1)inthesouthernBohemianMassif,CzechRepublic.Thepotassicgranuliteformsupto2mthickfoliatedlayersinthenorthernpartofthepresent-day'sfourthlevelofthequarry(Fig.2;N:48°52′17″,E:14°20′28″).Thelayersareorientedconcordantlytothedominantmetamorphicfoliationinthesurroundingfelsicgranulites.Thisfoliationformedasaresultofisothermaldecompressionthatfollowedthepeakgranulitefaciesconditionsdatedinthisareaatca.340Ma(vanBreemenetal.,1982;Wendtetal.,1994;Kr?neretal.,2000).

Classificationofthepotassic,sometimesreferredtoashyperpotassic(upto13wt.%K2O),granuliteintheAPQdiagramforplutonicrocksreflectsitsalkali-feldsparsyenitictoalkali-feldspargraniticcharacter(Vrána,1989;Janou?eketal.,2007).ThegranuliteismadeupmostlyofK-feldspar(upto93%)andgarnetrelativelyrichinpyrope(~30molar%),whichhasbeenpartlyreplacedbybiotiteduringtheretrogradestagesof

Fig.1.SchematicmapoftheEuropeanVariscides(inserted)andBohemianMassifshowingthepositionoftheBlanskylesgranuliteandthesamplinglocationinthePle?ovicequarry.ModifiedafterSvojtkaetal.,2002.

J.Slámaetal./ChemicalGeology249(2008)1–3534J.Slámaetal./ChemicalGeology249(2008)1–35

Fig.2.OutlinegeologicalmapoftheeasternpartoftheBlanskylesmassif(afterKodymetal.,1981)showingthePle?ovicequarry(asinAutumn2006)withmarkedlocationofthezirconbearingpotassicgranulite.

themetamorphicevolution.Apatite,monaziteandzirconareaccessories;rutileispresentintheformoftinyexsolvedneedlesingarnet.Thecontentofzirconinthegranuliteisupto0.6wt.%anditisvariableonthescaleoftensofcentimeters;higheraccumulationsofzirconcrystalswerefoundinthebiotite(originallygarnet)-richdomains.

Thepotassicgranuliteisthoughttorepresentanon-eutecticmelt(Vrána,1989;Janou?eketal.,2007),possiblyderivedfromtheprotolithoftheadjacentfelsicgranulitesintheBlanskylesmassif(Janou?eketal.,2007).TheBlanskylesmassifrepresentspartoftheallochthonousGf?hlunit(FuchsandMatura,1976;Matteetal.,1990;Fialaetal.,1995;Vránaetal.,1995;Franke,2000),whichformstheuppermosttectonicmemberoftheMoldanubianZone,thecorecrystallineunitoftheVariscanorogeninEurope(Dallmeyeretal.,1995).TheGf?hlunitconsistsmainlyoffelsicgranulites,withsubordinatemaficgranulites,serpenti-nizedgarnetperidotites,pyroxenitesandeclogites(FuchsandMatura,1976;Franke,1989;Fialaetal.,1995).Migmatiticgraniticgneisses(Gf?hlgneisses)inthelowerpartoftheGf?hlunitprobablyalsorepresentoverprintedfelsicgranulites(FuchsandMatura,1976;Fialaetal.,1995).

TheBlanskylesmassifcontainsmainlyfelsicgarnetgranulitegneisses(ca.80%)withsubordinatemaficpyroxene±garnetgranulites(ca.10%),melanocraticbiotite–garnetgranulites(ca.5%)andultramaficrocks,suchasareserpentinizedperidotitesandeclogites(ca.5%;Fialaetal.,1987).AlltheserocksunderwentHP–HTmetamorphismduringtheVariscanorogenywithestimatedpeakconditionsof900–1050°Cand1.5–2GPa(CarswellandO'Brien,1993;KotkováandHarley,1997;O'BrienandR?tzler,2003).Followingthepeakofmetamorphism,therewasalmostisothermaldecompressiontomid-crustallevelpressureswithanoverprintat800–900°Cand0.8–1.2GPa(Cooke,2000)andsubsequentnear-isobariccooling.Theretro-gressionofgranulitesisreflectedingarnetbreakdownandformationofsecondary-coronastructures,retro-gressionofgarnettobiotiteandkyanitetosillimanite(OwenandDostal,1996;O'BrienandR?tzler,2003;Slámaetal.,2007).

TheageofgranulitemetamorphismisconstrainedbyanumberofU–Pbzirconagesatca.340Ma(vanBreemenetal.,1982;Kr?neretal.,1988;Aftalionetal.,1989;Wendtetal.,1994;Kr?neretal.,2000;Friedletal.,2003;Kotkováetal.,2003).ThisagehasoftenbeeninterpretedascorrespondingtothetimeofgranuliteHP–HTmetamorphicpeak,butRobertsandFinger(1997)arguedforzirconcrystallizationfrompartialmeltduringtheretrogradeP–Tpath,andKr?neretal.(2000)andJanou?eketal.(2004)proposedthatthezirconsthatgrewundergranulitepeakconditionsandsubsequentlyduringthedecompressionaresimilarin

J.Slámaetal./ChemicalGeology249(2008)1–355

age.SomezirconcoresfromgranulitesoftheBlanskylesgaveaU–Pbageofca.470Ma(Kr?neretal.,2000),consistentwithawhole-rockRb–SrageobtainedbyJanou?eketal.(2004)andinterpretedbytheseauthorsascorrespondingtotheageofthegranuliteprotolith.ThezirconsfrompotassicgranuliteinthePle?ovicequarrywerepreviouslydatedbyAftalionetal.(1989)andgaveaU–Pbageof338±1Ma.

Thestudiedzirconisincludedinmajormineralphasesofthepotassicgranuliteasupto0.5cm(rarelylarger)equantorprismatic(Fig.3),palepinktobrownidiomorphiccrystals.TheexternalshapeofthecrystalscorrespondstozirconsfromperalkalinegranitesandsyenitesintheclassificationofPupin(1980).MineralinclusionsinzirconcompriseK-feldsparandapatite(Fig.4),rarelyalsoquartzandgarnet.

3.SamplepreparationandanalyticalmethodsAsampleofpotassicgranulitecollectedfromthePle?ovicequarrywascrusheddowntoagrainsizeofb7mmandgravimetricseparationinawater-filledsluiceboxwasusedtoobtainaheavymineralfractioncontainingalmostexclusivelyzircon.Approximately250kgoftherockmaterialyieldedca.500gofgoodqualityzirconcrystalsbetween1and6mminsize.Toensurethoroughcharacterizationofthestudiedzircon,thechemicalandisotopicanalyseswerecon-ductedusingarangeofdifferenttechniquesinseverallaboratories(ID-TIMS:MassachusettsInstituteofTech-nology(MIT),UniversityofGeneva(UNIGE),NERCIsotopeGeosciencesLaboratory(NIGL),BoiseStateUniversity(BSU)andETHZürich(ETHZ);SIMS:SwedishMuseumofNaturalHistoryinStockholm(NORDSIM);LAICP-MS:UniversityofBergen(UoB),J.W.GoetheUniversityofFrankfurtamMain(JWG)andMemorialUniversityofNewfoundland(MUN);MCICP-MS:UniversityofBergen,J.W.GoetheUniversityofFrankfurtamMainandNERCIsotopeGeosciencesLaboratory(NIGL);Ramanspectroscopy:UniversityofVienna).Wherepossible,themeasurementswererepro-ducedbysimilartechniquesindifferentlaboratories.

Fig.3.a)Large,shortprismaticcrystalofthePle?ovicezirconinK-feldsparmatrixofthehostpotassicgranulite;b)typicalcrystalshapesofthePle?ovicezirconswithprevailingequantmorphology(top)andlesscommonprismaticmorphology(bottom).

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