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

14 Citations (Scopus)

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

Original languageEnglish
Pages (from-to)196-202
Number of pages7
JournalEarth and Planetary Science Letters
Volume498
DOIs
Publication statusPublished - 2018 Sep 15

Keywords

  • BSE composition
  • Th/U
  • core formation
  • core heat production
  • crust–mantle differentiation

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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