Ferrous iron diffusion in ferro-periclase across the spin transition

We present first-principle calculations on absolute diffusion rates of high- and low-spin ferrous iron in ferro-periclase. The diffusivity of high- and low-spin iron depends on the physical conditions (pressure and temperature), iron concentration and the value of the chosen Hubbard U. We also find that low-spin iron swaps back to high-spin during migration. Our results show that throughout the Earthʹs lower mantle, iron diffuses at a faster rate than magnesium (up to an order of magnitude). Thus, the effect of the spin transition only has a small impact on the rheology of periclase in the Earthʹs mantle. Using reasonable vacancy concentrations, we find that ferro-periclase is much weaker than MgSiO3-perovskite throughout the mantle. We conclude, therefore, that ferro-periclase controls the viscosity of the mantle in regions where it becomes interconnected, such as regions of high strain around slabs or near plumes. At very high pressures, such as in Super-Earths, low-spin iron diffusion is expected to become much slower than magnesium, eventually making ferro-periclase more viscous than pure periclase.