Japan Geoscience Union Meeting 2018

Presentation information

[EE] Oral

S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Tectonophysics

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

Mon. May 21, 2018 3:30 PM - 5:00 PM A11 (Tokyo Bay Makuhari Hall)

convener:Hidenori Terasaki(Graduate School of Science, Osaka University), Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University), William F McDonough (共同), George Helffrich(Earth-Life Science Institute, Tokyo Institute of Technology), Chairperson:Ohtani Eiji(Tohoku University), Helffrich George(ELSI, Tokyo Institute of Technology)

4:45 PM - 5:00 PM

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

*William F McDonough1,2, Scott A. Wipperfurth2, Meng Guo2,3, 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 (κPb) values and assess their ratios for the Earth, core, and silicate Earth. Complementary bulk silicate Earth domains (i.e., continental crust CCκPb = 3.94 +0.20-0.11 and modern mantle MMκPb = 3.87 +0.15-0.07, respectively) tightly bracket the solar system initial SSκPb = 3.890 ± 0.015. These findings reveal the bulk silicate Earth’s BSEκPb 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 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 SSκPb 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.