Abstract 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, Δ 199 Hg and Δ 201 Hg) and even (even-MIF, Δ 200 Hg) 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 ( δ 202 Hg = −3.3 ± 0.9‰; 1 standard deviation) than sedimentary rocks that have previously been considered to represent the terrestrial Hg isotope composition ( δ 202 Hg = − 0.7 ± 0.5 ‰ ; 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 Δ 199 Hg versus Δ 201 Hg 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.

中文翻译：

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地球的球粒陨石汞同位素组成和在真晶石形成过程中蒸发平衡脱气的证据

摘要 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未解决。元素汞是 MVE 中最易挥发的元素，并且是一种强亲硫元素。它是为数不多的对奇数 (odd-MIF, Δ 199 Hg 和 Δ 201 Hg) 和偶数 (even-MIF, Δ 200 Hg) 自然界中的 Hg 同位素，传统上用于追踪地表环境中的 Hg 生物地球化学循环。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，我们提供了陆地玄武岩的 Hg 同位素数据，粗岩和花岗质火成岩样品。这些岩石在同位素上比以前被认为代表陆地 Hg 同位素组成的沉积岩（δ 202 Hg = −3.3 ± 0.9‰；1 个标准偏差）更轻（δ 202 Hg = − 0.7 ± 0.5 ‰；1 个标准偏差） . 我们展示了岩浆侵位过程中的脱气诱导 MIF，这与这些样品中的动力学分馏一致。还提供了更完整的球粒陨石（碳质、普通和顽火石）陨石数据集，与之前对碳质球粒陨石（正奇数-MIF）和普通球粒陨石（无 MIF）的研究一致，并证明一些顽火球粒陨石表现出正奇数-MIF，类似于碳质球粒陨石。陆地火成岩属于 MIF 和 MDF 的球粒陨石组成范围。鉴于行星分化（地核形成、蒸发）会导致地球硅酸盐部分的汞损失，并且可能会从球粒陨石成分中分离汞同位素，我们认为地幔汞的预算主要由球粒陨石材料的晚期吸积支配到地球，也建议其他挥发性亲硫元素（S、Se、Te）。考虑到 Hg 同位素特征，具有类似于 CO 球粒陨石或普通球粒陨石成分的材料是最有可能的晚期吸积源候选者。最后，挥发性元素高度贫化的 eurite 陨石在同位素上比球粒陨石重，并且表现出负的奇数 MIF。对 eurite 中挥发性消耗的起源一直存在激烈争论。