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Redox Control on Sulfur Abundances Among Terrestrial Planets.

Abstract : Sulfur constitutes several wt% of chondrite meteorites, the accretion material of terrestrial planets. Although classified as a volatile element, sulfur has a significant solubility at high temperature in both molten Fe-metal and silicates, which dominantly depends on the prevailing redox conditions. Redox conditions of planetary bodies during accretion seem extremely heterogeneous as revealed by variable proportions of iron in the metal (Fe) and oxidized (FeO) forms preserved in meteorites and terrestrial planets. Meteorites collections display completely metallic to completely oxidized samples, which probably make of the redox potential, expressed hereafter as oxygen fugacity, the most variable chemical parameter in the primitive solar system (6 log units of variation in fO2). We have calculated the sulfur distribution between molten silicate and metal as a function of oxygen fugacity based on available thermodynamic models. We fixed the initial composition of the system in terms of major elements including constant Fe and S contents. We simply allow changes in the ratio of metal over silicate, by simulating variable fO2 conditions. Increasing fO2 decreases sulfur solubility in molten silicate because sulfur and oxygen anions compete for a single anionic site in the melt structure. However, at metal-Fe saturation, increasing fO2 increases FeO content in the silicate, which increases the sulfur solubility in the silicate. This results in rather complex and non-monotonic trends in sulfur partitioning between metal and silicate as fO2 changes. At very reducing fO2, typically prevailing in Enstatite chondrite and probably during Mercury's accretion, nearly all sulfur partitions into the silicate portion. Sulfur content in the range 0.1-2 wt% dissolves in the silicate magma ocean. Under relatively oxidized conditions of accretion, probably prevailing in carbonaceous chondrites and on Vesta and Mars, sulfur also strongly partitions in the silicate with concentration in the range 0.08-0.1wt%. In contrast, at intermediate fO2 conditions of accretion such as on Earth, sulfur content in the silicate portion shows a minimum vs. a maximum in S content in the metal. Therefore, accretion of materials homogeneous in S content but differing in their redox state reproduce the high sulfur content in Mars mantle, which exceed the S content in Earth mantle by a factor of 3-4. The volcanic gases therefore emitted on Mars were much more sulfur-rich than on Earth, which must have played a critical role on the development of life on Mars.
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https://hal-insu.archives-ouvertes.fr/hal-00078671
Contributor : Nathalie Pothier <>
Submitted on : Wednesday, June 7, 2006 - 10:02:45 AM
Last modification on : Friday, April 5, 2019 - 8:08:24 PM

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  • HAL Id : hal-00078671, version 1

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Fabrice Gaillard, Bruno Scaillet. Redox Control on Sulfur Abundances Among Terrestrial Planets.. AGU, 2006, Baltimore, United States. ⟨hal-00078671⟩

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