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.
| Quand ? |
Le 18/11/2011, de 11:00 à 12:30 |
|---|---|
| S'adresser à | Razvan Caracas |
| Téléphone | +33 (0) 472 728 967 |
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