The Earth’s Thorium and Uranium Abundance and Distribution
McDonough, William F
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The abundance and distribution of thorium (Th) and uranium (U) in the Earth can provide important data for constraining its composition, heat budget, and processes of differentiation. This project seeks to constrain the 232Th/238U (К) ratio in different domains of the Earth. We reports more than one hundred thousand 232Th/238U ratios and more than ten thousand time-integrated Pb isotopic ratios (КPb) for rocks from the continental crust (CC) and modern mantle (MM). The results reveal that these two complementary reservoirs MMКPb = 3.87 +0.15-0.07 and CCКPb = 3.94 +0.20-0.11 tightly bracket the solar system (SS) initial SSКPb = 3.890 ± 0.015 (Blichert -Toft et al., 2010), defining a bulk silicate Earth (BSE) composition of BSEКPb = 3.90 +0.13-0.07. The CCКPb, MMКPb and BSEКPb are indistinguishable statistically, which indicates that negligible Th/U fractionation accompanied crust-mantle segregation, accretion and core-mantle segregation. Open system crustal growth modeling suggests that the changing incompatibility of Pb during the formation of the continents could on its own account for the kappa conundrum (i.e., К < КPb). The timing of Great Oxidation Event (GOE) coincidently overlapped with the peak of continental crust recycling, but may have no causal relationship or trivially contribution to the kappa conundrum.