Effect of Spin Crossover on Diffusivity of Fe2+ in (Mg,Fe)O Ferropericlase

Iron (Fe) is one of the major impurities in the Earth’s mantle and can have important effects on the elastic anisotropy, viscosity, diffusivity, and rheology of the deep Earth. Fe partitioning is a key factor in understanding how Fe will impact lower mantle and core phases. In order to predict Fe partitioning it is necessary to know the kinetics of Fe, as adequate Fe transport is necessary for partitioning to take place. Of particular interest is the effect of the recently identified Fe spin crossover on Fe diffusion. In the present study we combine first principles methods with the five-frequency statistical model to compute the dilute Fe2+ diffusivity in ferropericlase (Mg,Fe)O as a function of pressure, temperature, and spin state. The results are integrated into a thermodynamic model for the spin changes to predict the effective diffusivity of Fe throughout the spin crossover regions of the mantle. The results are compared with existing experimental diffusion data and changes in the diffusion rate are interpreted in terms of the changes in electronic structure with the spin transition. Implications of the Fe diffusivity for lower mantle properties are discussed.