Radioactive decay of potassium (K), thorium (Th), and uranium (U) power the Earth's engine, with variations in 232Th/238U recording planetary differentiation, atmospheric oxidation, and biologically mediated processes. We report several thousand 232Th/238U (κ) and time-integrated Pb isotopic (κpb) values and assess their ratios for the Earth, core, and silicate Earth. Complementary bulk silicate Earth domains (i.e., continental crust κpb CC = 3.94 +0.20-0.11 and modern mantle κpb MM = 3.87 +0.15-0.07, respectively) tightly bracket the solar system initial κpb SS = 3.890 ± 0.015. These findings reveal the bulk silicate Earth's κpb BSE is 3.90 +0.13-0.07 (or Th/U = 3.77 for the mass ratio), which resolves a long-standing debate regarding the Earth's Th/U value. We performed a Monte Carlo simulation to calculate the Kpb of the BSE and bulk Earth for a range of U concentrations in the core (from 0 to 10 ng/g). Comparison of our results with κpb SS constrains the available U and Th budget in the core. Negligible Th/U fractionation accompanied accretion, core formation, and crust - mantle differentiation, and trivial amounts of these elements (0.07 ppb by weight, equivalent to 0.014 TW of radiogenic power) were added to the core and do not power the geodynamo.
|Publication status||Published - 2018 Jan 16|
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