Iron isotope fractionation in the Earth|[rsquo]|s lower|[nbsp]|mantle

doi:doi:10.1038/ngeo546

Nature Geoscience 2, 514 - 518 (2009)
Published online: 14 June 2009 | doi:10.1038/ngeo546

Subject Categories: Geochemistry | Structural geology, tectonics and geodynamics | Volcanology, mineralogy and petrology

Iron isotope fractionation in the Earth's lower mantle

James R. Rustad¹ & Qing-Zhu Yin¹

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Understanding the distribution of iron isotopes within planetary bodies can help constrain their histories of accretion and differentiation^1,^2,³. A large fraction of the iron in the silicate Earth is dissolved in ferropericlase and ferroperovskite—mineral phases that make up the bulk of the lower mantle⁴. These phases have distinct crystallographic structures and iron resides in them in multiple spin states; they are therefore likely to partition iron isotopes differently. Here we use density functional methods to calculate the equilibrium iron isotope composition of these phases at a range of temperatures and pressures, including those thought to exist near the core–mantle boundary. We find that the iron isotopic composition of ferropericlase strongly depends on the spin state of iron. At pressures near the base of the mantle, the low-spin state is enriched in heavy isotopes relative to the high-spin state. In contrast, for ferroperovskite, our calculations suggest that the isotopic composition is almost independent of spin state. Our results warrant a careful search for a pressure-dependent isotopic signature in samples brought up by mantle plumes and in materials subjected to lower mantle pressures and temperatures in the laboratory.

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