The Effects of Spin Transition on Iron Diffusion in Ferropericlase

Knowledge of iron (Fe) composition in deep Earth lower mantle minerals is essential to a complete understanding of the Earth's interior. Fe cation diffusion potentially controls many aspects of the distribution of Fe in the Earth's lower mantle, including mixing of chemical heterogeneities, element partitioning and the extent of core-mantle communications. Fe in ferropericlase has been shown to undergo a spin transition starting at about 40 GPa, a pressure too high to have allowed direct experimental study of spin effects on Fe diffusion. This work combines first-principles and statistical modeling to predict diffusion of Fe in ferropericlase over the entire lower mantle, including the effects of the Fe spin transition. We find that spin state dramatically impacts migration barriers both through direct electronic structure effects and changes in elastic properties. However, we also find that the impact of the spin crossover on diffusion is greatly reduced due to the mixed spin state allowing diffusion of both high- and low-spin Fe.