Iron isotope fractionation in the Earth's lower mantle

Understanding the distribution of iron isotopes within planetary bodies can help constrain their histories of accretion and differentiation(1-3). 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 mantle(4). 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.