Diffusion of transition metals in periclase by experiment and first-principles, with implications for core-mantle equilibration during metal percolation

Diffusion of the divalent transition metals in the deep mantle is important for understanding length scales of mantle heterogeneity, transport across the core-mantle boundary, and the degree of equilibration during core formation. Here we report the results of experiments and theoretical calculations on Ni, Co, Fe and Mn diffusion in periclase, the primary repository of these elements in Earth's lower mantle. The periclase used in the experiments was doped with 550ppm Al³⁺ to fix the cation vacancy concentration, and the experiments were performed at 2GPa and temperatures between 1673 and 2073K. The variation in diffusion coefficients among the transition metals was found to be less than a factor of three, and to increase in the order Ni<Co~Mn<Fe. Theoretical results on the diffusion energetics, based on density functional theory with electron correlation effects accounted for using the GGA+U method, correctly predict the relative diffusivities, and yield absolute diffusion coefficients in reasonable agreement with the experimental results. A simple crystal field splitting model does not provide even qualitative guidance for the trends in relative diffusivity, but does show strong correlation with changes in migration energy due to spin transitions. The diffusion coefficients are rapid enough under mantle conditions to allow equilibration during core separation through the solid mantle, if the metal percolates through a grain-scale permeable network with channel spacing of centimeters or less.