Isotopic Evidence of Cr Partitioning into Earth’s Core

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Science  24 Feb 2011:
DOI: 10.1126/science.1199597


The distribution of chemical elements in primitive meteorites (chondrites), as building blocks of terrestrial planets, provides insight into the formation and early differentiation of Earth. The processes that resulted in the depletion of some elements (e.g., Cr) in the bulk silicate Earth relative to chondrites, however, remain debated between leading candidate causes: volatility versus core-partitioning. We show through high-precision measurements of Cr stable isotopes in a range of meteorites, which deviate by up to ~0.4‰ from the bulk silicate Earth, that Cr depletion resulted from its partitioning into Earth’s core with a preferential enrichment in light isotopes. Ab initio calculations suggest that the isotopic signature was established at mid-mantle magma ocean depth as Earth accreted planetary embryos and progressively became more oxidized.