Iron Partitioning and Density Changes of Pyrolite in Earth’s Lower Mantle

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Science  03 Dec 2009:
DOI: 10.1126/science.1181443


Phase transitions and the chemical composition of minerals in Earth’s interior influence geophysical interpretations of its deep structure and dynamics. A pressure-induced spin transition in olivine has been suggested to influence Fe partitioning and depletion, resulting in a distinct layered structure in Earth’s lower mantle. For a more realistic mantle composition (pyrolite), we observed a significant change in the Fe-Mg partition coefficient at about 40 GPa that is explained by a spin transition at much lower pressures. However, only a small depletion of Fe is observed in the major high-pressure phase (Mg-silicate perovskite), which may be explained by preferential retention of Fe3+. Changes in mineral proportions or density are not associated with the change in partition coefficient. The observed density profile agrees well with seismological models, suggesting that pyrolite is a good model composition for the upper to middle parts of the lower mantle.