TY - JOUR
T1 - Earth's chondritic Th/U
T2 - Negligible fractionation during accretion, core formation, and crust–mantle differentiation
AU - Wipperfurth, Scott A.
AU - Guo, Meng
AU - Šrámek, Ondřej
AU - McDonough, William F.
N1 - Funding Information:
Support for this study was provided by The U.S.–China Fulbright Program grant 15161401 (to M.G.), NSF grant EAR1650365 (to W.F.M.), and Czech Science Foundation grant GAČR 17-01464S (to O.Š.).
Funding Information:
Support for this study was provided by The U.S.–China Fulbright Program grant 15161401 (to M.G.), NSF grant EAR1650365 (to W.F.M.), and Czech Science Foundation grant GAČR 17-01464S (to O.Š.).
Publisher Copyright:
© 2018 Elsevier B.V.
PY - 2018/9/15
Y1 - 2018/9/15
N2 - 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.
AB - 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.
KW - BSE composition
KW - Th/U
KW - core formation
KW - core heat production
KW - crust–mantle differentiation
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U2 - 10.1016/j.epsl.2018.06.029
DO - 10.1016/j.epsl.2018.06.029
M3 - Article
AN - SCOPUS:85049555784
SN - 0012-821X
VL - 498
SP - 196
EP - 202
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
ER -