Earth's chondritic Th/U: Negligible fractionation during accretion, core formation, and crust–mantle differentiation
Earth's chondritic Th/U: Negligible fractionation during accretion, core formation, and crust–mantle differentiation
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DOI：
10.1016/j.epsl.2018.06.029
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发表时间：
2018-01
影响因子：
5.3
通讯作者：
S. Wipperfurth;Meng Guo;O. Šrámek;W. McDonough
中科院分区：
文献类型：
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作者：
S. Wipperfurth;Meng Guo;O. Šrámek;W. McDonough
钾（K）、钍（Th）和铀（U）的放射性衰变为地球的引擎提供动力，232 Th/238 U的变化记录了行星分化、大气氧化和生物介导的过程。我们报告了数千个232 Th/238 U（κ）和时间积分Pb同位素（κ P B）值，并评估了地球、地核和硅酸盐地球的比值。互补的块状硅酸盐地球域（即大陆地壳κ P B C C= 3.95− 0.13+ 0.19和现代地幔κ P B M M= 3.87− 0.07+ 0.15）紧紧地包围了太阳系的初始值κ P B S S= 3.890±0.015。这些发现揭示了块状硅酸盐地球的κ P B B SE为3.90− 0.08+ 0.13（或Th/U= 3.77的质量比），这解决了关于地球Th/U值的长期争论。我们进行了Monte Carlo模拟计算的κ P B的BSE和散装地球的范围内的U浓度的核心（从0到10 ng/g）。我们的结果与κ P B S S的比较限制了有效U和Th的预算在核心。可忽略的Th/U分馏伴随着吸积、核形成和壳幔分异，微量的这些元素（< 0.2 ng/g U）被添加到核中，并没有显著的动力（<0.03 TW）。
Radioactive decay of potassium (K), thorium (Th), and uranium (U) power the Earth's engine, with variations in 232 Th/238 U recording planetary differentiation, atmospheric oxidation, and biologically mediated processes. We report several thousand 232 Th/238 U (κ) and time-integrated Pb isotopic (κ P b) values and assess their ratios for the Earth, core, and silicate Earth. Complementary bulk silicate Earth domains (ie, continental crust κ P b C C= 3.95− 0.13+ 0.19 and modern mantle κ P b M M= 3.87− 0.07+ 0.15) tightly bracket the solar system initial κ P b S S= 3.890±0.015. These findings reveal the bulk silicate Earth's κ P b B S E 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 κ P b 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 κ P b S S 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.