Lead isotope systematics and the evolution of the core,mantle, crust and atmosphere

A new analysis of the isotope systematics of sulphide common leads can be made on the basis of examining the deriations of the data from a simple single-stage evolution. Δt, the age discrepancy between the single-stage lead model age and the geologic age, increases systematically from 3.8 Ga to the present. This trend appears to reflect an increase in the μ of the primitive mantle due to incorporation of a large portion of the earth's lead into the core, early in the earth's evolution. Leads associated with shale-hosted lead-zinc deposits show a rapid increase in Δt beginning at 2.5 to 2.0 Ga. This deviation of shale-hosted leads from the general trend is interpreted as a response to concentration of uranium in organic-rich shales subsequent to the evolution of an oxidizing atmosphere. Comparison of common leads in alkali feldspars with the volcanogenic sulphide data suggests that they have a similar evolution of Δt with time. Numerical simulations reveal that even substantial increases in real μ over the last 2.0 Ga are not reflected in significant increases in the single-stage model μs.