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 Th-232/U-238 recording planetary differentiation, atmospheric oxidation, and biologically mediated processes. We report several thousand Th-232/U-238 (kappa) and time-integrated Pb isotopic (kappa(Pb)) values and assess their ratios for the Earth, core, and silicate Earth.
Complementary bulk silicate Earth domains (i.e., continental crust kappa(CC)(Pb) = 3.95(-0.13)(+0.19) and modern mantle kappa(MM)(Pb) = 3.87(-0.07)(+0.15)) tightly bracket the solar system initial kappa(SS)(Pb) = 3.890 +/- 0.015. These findings reveal the bulk silicate Earth's kappa(BSE)(Pp) 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 kappa(Pb) 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 kappa(SS)(Pb) 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 (similar to 0.03 TW) the geodynamo. (C) 2018 Elsevier B.V. All rights reserved.