[S-IT18] Planetary cores: Structure, formation, and evolution

[SIT18-10] Thorium and uranium power plate tectonics, but not the geodynamo

*William F McDonough^1,2, Scott A. Wipperfurth², Meng Guo^2,3, Ondřej Šrámek⁴ (1.Department of Earth Sciences and Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan, 2.Department of Geology, University of Maryland, College Park, MD 20742, USA, 3.Institute of Crustal dynamics, China Earthquake Administration, Beijing 100085, China, 4.Department of Geophysics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic)

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 (κ_Pb) values and assess their ratios for the Earth, core, and silicate Earth. Complementary bulk silicate Earth domains (i.e., continental crust ^CCκ_Pb = 3.94 ^+0.20_-0.11 and modern mantle ^MMκ_Pb = 3.87 ^+0.15_-0.07, respectively) tightly bracket the solar system initial ^SSκ_Pb = 3.890 ± 0.015. These findings reveal the bulk silicate Earth’s ^BSEκ_Pb 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. Experimental studies find marked differences in the partitioning of U and Th during core formation. We performed a Monte Carlo simulation to calculate the κ_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 ^SSκ_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 (0.07 ppb by weight, equivalent to 0.014 TW of radiogenic power) were added to the core and do not power the geodynamo.