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Terrestrial Accretion Under Oxidizing Conditions

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Science  08 Mar 2013:
Vol. 339, Issue 6124, pp. 1194-1197
DOI: 10.1126/science.1227923

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Earth's Ingredients

What was the composition of the earliest terrestrial starting blocks? The answer lies in understanding how Earth's interior separated into mantle and core components. Siebert et al. (p. 1194, published online 10 January) performed a series of high pressure and temperature experiments to track how chromium and vanadium, which have a slight affinity for iron, partition into metal and silicate fractions. Combined with accretionary models, the data suggest that Earth accreted under the same relatively oxidizing conditions under which the most common types of meteorites formed. Transferring oxygen in the form of FeO from the mantle to the core could have gradually reduced the mantle to its present-day oxidation state.

Abstract

The abundance of siderophile elements in the mantle preserves the signature of core formation. On the basis of partitioning experiments at high pressure (35 to 74 gigapascals) and high temperature (3100 to 4400 kelvin), we demonstrate that depletions of slightly siderophile elements (vanadium and chromium), as well as moderately siderophile elements (nickel and cobalt), can be produced by core formation under more oxidizing conditions than previously proposed. Enhanced solubility of oxygen in the metal perturbs the metal-silicate partitioning of vanadium and chromium, precluding extrapolation of previous results. We propose that Earth accreted from materials as oxidized as ordinary or carbonaceous chondrites. Transfer of oxygen from the mantle to the core provides a mechanism to reduce the initial magma ocean redox state to that of the present-day mantle, reconciling the observed mantle vanadium and chromium concentrations with geophysical constraints on light elements in the core.

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