Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics

摘要: Abstract In order to explore mantle heterogeneity through geological time, Pb isotopic compositions have been determined for 8 differentiated-layered intrusions whose ages are between 2.7 and 0.05 b.y.. The Pb-Pb of these the U-Pb characteristics their parent sources discussed. dating method is found be applicable this type basic or ultrabasic rocks agrees satisfactorily with other available methods. Significant differences calculated values bodies intrusions. This could reflect either since Archean times, contamination some by continental crust. Discrimination hypotheses can proposed from positions initial Sr ratios massives in respect supposed “terrestrial” evolution line. two which plot on that closed system line (Muntsche Tundra, U.S.S.R., Skaergaard, Greenland) belong also a simple two-stage model Pb, low first-stage μ value 7.8. If considered as pieces “primitive” closed-system mantle, a4.55 ± 0.01 age earth ratios.