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

TL;DR

This study shows that Earth's Th/U ratio remained nearly unchanged during accretion, core formation, and differentiation, indicating minimal element fractionation and negligible radiogenic power contribution from the core.

Contribution

It provides the first comprehensive analysis of Th/U ratios across Earth's components, constraining core U content and confirming negligible fractionation during planetary formation.

Findings

Earth's Th/U ratio is consistent across different Earth reservoirs.

Core U content is constrained to be very low, contributing minimally to Earth's radiogenic heat.

Negligible Th/U fractionation occurred during Earth's formation processes.

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 $^{232}$Th/$^{238}$U ($\kappa$) and time-integrated Pb isotopic ($\kappa$$_{Pb}$) values and assess their ratios for the Earth, core, and silicate Earth. Complementary bulk silicate Earth domains (i.e., continental crust $\kappa_{Pb}^{CC}$ = 3.94 $^{+0.20}_{-0.11}$ and modern mantle $\kappa_{Pb}^{MM}$ = 3.87 $^{+0.15}_{-0.07}$, respectively) tightly bracket the solar system initial $\kappa_{Pb}^{SS}$ = 3.890 $\pm$ 0.015. These findings reveal the bulk silicate Earth's $\kappa$$_{Pb}^{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 $\kappa_{Pb}$ 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$$_{Pb}^{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.