Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites
Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites
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DOI：
10.1016/j.epsl.2020.116544
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发表时间：
2020-12-01
影响因子：
5.3
通讯作者：
Day, James Md
中科院分区：
文献类型：
--
作者：
Moynier, Frederic;Chen, Jiubin;Day, James Md
中等挥发性元素（MVE）丰度的变化是类地行星之间最基本的地球化学差异之一。这些变化是否是星云过程、行星挥发、分化或晚期吸积的结果，仍然没有解决。汞元素是MVE中最易挥发的元素，是一种强亲铜元素。它是自然界中少数几种对奇数（odd-MIF，Delta Hg-199和Delta Hg-201）和偶数（even-MIF，Delta Hg-200）Hg同位素都表现出大的质量相关（mass dependent，MIF）和质量无关（mass independent，MIF）同位素分馏的元素之一，传统上用于追踪地表环境中Hg的地球化学循环。然而，地球和陨石的汞同位素组成并没有得到很好的限制。在这里，我们提出了陆地玄武岩，粗面岩和花岗质火成岩样品的汞同位素数据。这些岩石的同位素轻（Δ Hg-202 = -3.3 +/- 0.9 ‰; 1个标准差），而沉积岩以前被认为代表陆地汞同位素组成（Δ Hg-202 = -0.7 +/- 0.5 ‰; 1个标准差）。我们表明，在岩浆侵位脱气诱导MIF，在这些样品中的动力学分馏是一致的。还提出了一个更完整的数据集为顽火辉石（碳质，普通和顽火辉石）陨石，这是与以前的工作碳质顽火辉石（正奇-MIF）和普通的顽火辉石（无MIF），并证明，一些顽火辉石表现出积极的奇-MIF，类似于碳质顽火辉石。陆源火成岩的MIF和EHF均属于碱性成分范围。考虑到行星分化（核心形成，蒸发）将有助于汞损失从地球的硅酸盐部分，并可能fractured汞同位素从南极组合物，我们认为，地幔汞的预算主要是由南极物质到地球的后期吸积，也建议为其他挥发性亲硫元素（S，Se，Te）。考虑到汞同位素的签名，材料的组成类似于CO的钙钛矿或普通的钙钛矿是最有可能的晚期吸积源的候选人。最后，eucrite陨石，这是高度亏损的挥发性元素，同位素重于陨石和表现出负奇MIF。eucrites挥发性亏损的起源一直在激烈的辩论。我们发现，三角洲Hg-199与三角洲Hg-201的关系指向一个平衡的核场转移效应，这表明挥发性损失发生在岩浆海洋阶段的表面的eucrite母体，可能是小行星4-灶神星。(C)2020作者（S）由爱思唯尔公司出版
Variations in the abundances of moderately volatile elements (MVE) are one of the most fundamental geochemical differences between the terrestrial planets. Whether these variations are the consequence of nebular processes, planetary volatilization, differentiation or late accretion is still unresolved. The element mercury is the most volatile of the MVE and is a strongly chalcophile element. It is one of the few elements that exhibit large mass-dependent (MDF) and mass-independent (MIF) isotopic fractionations for both odd (odd-MIF, Delta Hg-199 and Delta Hg-201) and even (even-MIF, Delta Hg-200) Hg isotopes in nature, which is traditionally used to trace Hg biogeochemical cycling in surface environments. However, the Hg isotopic composition of Earth and meteorites is not well constrained. Here, we present Hg isotopic data for terrestrial basaltic, trachytic and granitic igneous samples. These rocks are isotopically lighter (delta Hg-202 = -3.3 +/- 0.9 parts per thousand; 1 standard deviation) than sedimentary rocks that have previously been considered to represent the terrestrial Hg isotope composition (delta Hg-202 = -0.7 +/- 0.5 parts per thousand; 1 standard deviation). We show degassing during magma emplacement induces MIF that are consistent with kinetic fractionation in these samples. Also presented is a more complete dataset for chondritic (carbonaceous, ordinary and enstatite) meteorites, which are consistent with previous work for carbonaceous chondrites (positive odd-MIF) and ordinary chondrites (no MIF), and demonstrate that some enstatite chondrites exhibit positive odd-MIF, similar to carbonaceous chondrites. The terrestrial igneous rocks fall within the range of chondritic compositions for both MIF and MDF. Given the fact that planetary differentiation (core formation, evaporation) would contribute to Hg loss from the silicate portion of Earth and would likely fractionate Hg isotopes from chondritic compositions, we suggest that the budget of the mantle Hg is dominated by late accretion of chondritic materials to Earth, as also suggested for other volatile chalcophile elements (S, Se, Te). Considering the Hg isotopic signatures, materials with compositions similar to CO chondrites or ordinary chondrites are the most likely late accretion source candidates. Finally, eucrite meteorites, which are highly depleted in volatile elements, are isotopically heavier than chondrites and exhibit negative odd-MIF. The origin of volatile depletion in eucrites has been vigorously debated. We show that Delta Hg-199 versus Delta Hg-201 relationships point toward an equilibrium nuclear field shift effect, suggesting that volatile loss occurred during a magma ocean phase at the surface of the eucrite parent body, likely the asteroid 4-Vesta. (C) 2020 The Author(s). Published by Elsevier B.V.