Earth's chondritic Th/U: Negligible fractionation during accretion, core formation, and crust–mantle differentiation

Abstract

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 κPbCC=3.95−0.13+0.19 and modern mantle κPbMM=3.87−0.07+0.15) tightly bracket the solar system initial κPbSS=3.890±0.015. These findings reveal the bulk silicate Earth's κPbBSE is 3.90−0.08+0.13 (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 κPb 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 κPbSS 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 (<0.2 ng/g U) were added to the core and do not significantly power (∼0.03 TW) the geodynamo.