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Deep magma ocean formation set the oxidation state of Earth’s mantle

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Science  30 Aug 2019:
Vol. 365, Issue 6456, pp. 903-906
DOI: 10.1126/science.aax8376

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Deep divide in fate of iron

A large component of Earth's atmosphere comes from the interior, where the gas species are dictated by the redox state of the mantle. After formation of Earth's iron core, the mantle became several orders of magnitude more oxidized. Armstrong et al. conducted a set of experiments looking at the redox state of silicate melt representative of Earth's early magma oceans. They found that at some depth, iron oxide disproportionates into iron(III) oxide and metallic iron. The reduced iron sinks to the core, leaving an oxidized rocky mantle that emits carbon dioxide and water instead of more reduced species.

Science, this issue p. 903

Abstract

The composition of Earth’s atmosphere depends on the redox state of the mantle, which became more oxidizing at some stage after Earth’s core started to form. Through high-pressure experiments, we found that Fe2+ in a deep magma ocean would disproportionate to Fe3+ plus metallic iron at high pressures. The separation of this metallic iron to the core raised the oxidation state of the upper mantle, changing the chemistry of degassing volatiles that formed the atmosphere to more oxidized species. Additionally, the resulting gradient in redox state of the magma ocean allowed dissolved CO2 from the atmosphere to precipitate as diamond at depth. This explains Earth’s carbon-rich interior and suggests that redox evolution during accretion was an important variable in determining the composition of the terrestrial atmosphere.

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