Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites

1,2Frédéric Moynier,3,1Jiubin Chen,3Ke Zhang,3Hongming Cai,1Zaicong Wang,4Matthew G.Jackson,5James M.D.Day
Earth and Planetary Science Letters 551, 116544 Link to Article [https://doi.org/10.1016/j.epsl.2020.116544]
1State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
2Institut de Physique du Globe de Paris, Université de Paris, CNRS, 1 rue Jussieu, Paris 75005, France
3Institute of Surface-Earth System Science, Tianjin University, China
4Department of Earth Science, University of California, Santa Barbara, CA 93106, USA
5Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0244, USA
Copyright Elsevier

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, Hg and Hg) and even (even-MIF, 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 (delta202Hg = −3.3 ± 0.9‰; 1 standard deviation) than sedimentary rocks that have previously been considered to represent the terrestrial Hg isotope composition (delta202Hg=-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 Hg versus 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.

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