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

Scott A. Wipperfurth, Meng Guo, Ondřej Šrámek, William F. McDonough

Research output: Contribution to journalArticlepeer-review

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 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.

Original languageEnglish
JournalUnknown Journal
Publication statusPublished - 2018 Jan 16

ASJC Scopus subject areas

  • General

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