放射性同位素地球化學上課件

放射性同位素地球化學上中國科學院地質與地球物理研究所中國科學院研究生院2010年放射性同位素地球化學上)提綱放射性同位素地球化學(上)放射性同位素演化的基本原理和同位素示蹤的主要方法1】基本原理12RbSr體系13SmNd體系14U-Th-Pb體系15LuHf體系16ReOs體系1.1基本原理Thebasicequationofradioactived衰變定律2isthedecayconstant,whichwedefinedastheprobabilitythatatomwoulddecayinsometimedt.Ithasunitsoftime".Let'srearrangeequation8.4andwhereNeisthenumberofatomsoftheradioactive,orparent,isotopepresentattimet=0.Integrat-e-AtorN=N.ArSupposewewanttoknowtheamountoftimeforthenumberofparentatomstodecreasetohalfiginalnumberi.e.fwhenN/N,=1/2.SettingN/Noto1/2,wecanrearrange8.8toget:8.10Thisisthedefinitionofthehalf-,fANowthedecayoftheparentproducessomedaughter,Drradiogenic,nuclides.ThenumberofremainingaftertimesimplythedifferencebetweentheinitialnumberofparentsandthenumberlaughtersprodRearranging8.9toisolateNoandsubstitutingthatinto8.11,weobtain:histellsusthatthenumberofdaughtersproducedisafunctionofthenumberofparentspresentandtime.Sinceingeneraltherewillbesomeatomsofthedaughternuclidearoundtobeginwith,Lewhent=0,amoregeneralexpressionis:D=D+N(41)D=D+NeDIwhereDoisthenumberofdaughtersoriginallypresent.exponentialfunctioncanbeexpressedLIon14Providedccl,thehigherorderbermseverysmallandcanbeignored;hencefortimethatareshortcomparedtothedecayconstant(ic,fort<1/),equation5.13canbewrittenas:放射性衰變放射性衰變定律∝N或dN-個N瓣十女中氙時間一不穩(wěn)定核素及其半衰期1468m175uu地球化學常用Aqe。fUni的衰變體系BaseofcambrkzTboundary(BcsofQuaternary)BeUrstablenuclideFig.1.2.Unstablenuclideswithhalf-lives(tve)over0.5Myr,inorderofdecreasingstability.Geologicallyusefulparentnuclidesaremarked.Someverylong-tiveddionuclideswithnogeologicalapplicationarealso地球化學常用衰變體系的一些參數(shù)TAblE8.2LoNG-LivEdRAdioAcTivEDECAYSysTEMSofGEocHEMiCALINTERESTParentDecayModeHalf-lifeβ+,ecB5543x1028x10ydDAr/Ar142x10y4.8x10SSr/bSr6.54x1006x10yr143Nd/Nd1.94x103.6x1064x10-123x10Os.1874948x10y1.4x10y208Pb,4He20Pb/2aPb,He/He9849x10y7.07x10y207Pb,4He207Pb/2aPb,He/He551ⅹ10447x10DePb.tHe20Pb/20Pb.He/Holote:thebranchingratio,ie,ratiosofdecaystortototaldecayofaKis0.117.TheproductionofHefrom"SmdecayisinsignificantcomparedtothatproducedbydecayofUandTh.同位素比值的測量Fg25.Thearrangementoffilamentribbonsoncommotsingle-andtriple-filamentbeadassembles.NotelonsourceCollector樣品離子化和引入系統(tǒng)lits檢測系統(tǒng)磁場fig.2.1.Schematicillustrationofthebasicfeaturesofamagneticsectormassspectrometer.Solidandopencirclesrepresentthelightandheavyisotopesofanelement,respectively.AfterFaul(1966)12Rb-Sr體系TheGeochemicalperiodictableRuIrhlp!cisesta:reii:·Rb是強不相容元素,Sr是中等不相容元素,在上地幔分異過程中,他們都傾向于進入熔體;Rb的行為類似K,傾向于賦存在白云母,鉀長石中;Sr的行為類似Ca,易進入斜長石,磷灰石中(不包括單斜輝石·隨著巖漿演化,超基性→基性→中性→酸性,Rb/Sr(8Rb