Seminar - Michael W. Ammann: "Diffusion in the Earth's lower mantle: looking beyond the experimental barrier with DFT"
Solid state diffusion is one of the most important processes in geosciences. It controls rates of chemical reactions and phase transformations, is responsible for ionic conductivity and, most importantly, controls the rheology of the minerals deep inside the Earth. However, the two known phase transitions from MgSiO3 perovskite to post-perovskite and the high-spin to low-spin transition of iron are currently out of reach for diffusion-experiments. We therefore resort to DFT calculations and harmonic transition state theory to compute absolute diffusion rates in MgSiO3 perovskite and post-perovskite (a CaIrO3 structured phase) and ferro-periclase. Our diffusion rates are in excellent agreement with the few available experimental data at shallow depth (at 30 GPa, ~700 km). We find that diffusion in post-perovskite is strongly anisotropic, with up to 10 orders of magnitude difference between the fastest and slowest direction. These results allow us to constrain the viscosities of these minerals, implying that post-perovskite is much weaker than perovskite. This finding provides a simple explanation for some of the complex seismic features at the bottom of the Earth's mantle. The spin-transition, in contrast, is found to have a very small effect on the diffusivity of magnesium and iron and therefore on rheology.
de 11:00 à 12:30
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