1、,Chapter 16 The Isotope Geology of Pb (2),Two stage model of Pb evolution (Stacey-Kramers model)1) Pb started evolution with primordial isotope ratios 4.57 Ga ago;2) At 3.70 Ga, the Pb began a diff evolution since the source undergoing a geochemical differentiation,So, in the case:,4.57Ga,3.7Ga,Now,
2、1,2,Stage No.1,Stage No.2,left,t1,t2,This model is based on the following average crustal values:,These numbers came from average isotopic compositions of sedimentary and volcanic rocks in oceans and some old granites,These values were derived from the average crustal Pb compositions And the Pb isot
3、ope ratios of 13 conformable ore deposits,Stage No.1,Stage No.2,Crust values,Q,P,Stage No.2,Crust values,Stage No.1,All Pb removed from the second reservoir must have isotopecompositions on the growth curve;,2) The time for the PB to leave The reservoir is determined by isochron,Pb evolution in the
4、second stage can be described by:,This also can be used to solve both model age and isochron age.,How to solve the equation for a model age?,When using the table to figure out t for common leads, you have to consider the following conditions:,1. They evolved from primordial lead between 4.57 and 3.7
5、0 Ga in a uniform reservoir having =7.192 and =32.208.2. At 3.70Ga theand va1ues of the reservoir were changed but remained constant after that until the Present except for decay of U and Th to Pb.3. Lead was removed from the reservoir at some time t and was incorporated into a common Pb mineral.4.
6、After separation from the reservoir, the isotopic composition of the Pb was not changed by mixing with other leads or by additions of radiogenic Pb formed by decay of U and Th.,What are differences between single-stage model and two Stage model?,Reservoir,Pb isotope compositions,Model age,Pb isotope
7、 in young volcanic rocks,Ascension Island, Gough (Gast et al.,1964); St. Helena (Gast,1967,1969); Tenerife, Canary Islands (Oversby et al,1971); Hawaiian Islands (Tastumoto, 1966a).,Isotopic composition of Pb in the Tertiary igneous rocks of the Isle of Skye, northwest Scotland.,MORB & OIB originate from isotopically distinct mantle sources,Sr-Nd-Pb isotopic constraints onmantle end-members,MORB in the Qinling Mts.,Pb anomaly Southern hemisphereradiogenic Pb (EMI, EMII compositions contained in OIB) that in northern hemisphere. So-called DUPAL Pb anomaly,
