| Popis: |
Absolute diffusion rates in minerals of the Earth's lower mantle are calculated from first principles using density functional theory. The agreement with the available experimental data is excellent and provides confidence in predicting diffusivities in regions of the lower mantle inaccessible to current experimental techniques. I have calculated the diffusivity of all constituting species in ferro-periclase ((Fe,Mg)O), (Fe,Mg)SiO3 perovskite and MgSiO3 post-perovskite. This enables me to put tight constraints on the viscosity of the Earth's lower mantle, and, in particular, on the viscosity of post-perovskite for which no experimental data are available. I find that perovskite deforming in diffusion creep can readily reproduce the Earth's lower mantle viscosity profile that has been inferred from inversion modelling. I also show that postperovskite is either much stiffer (if also deforming in diffusion creep) or up to four orders of magnitude weaker than perovskite (if deforming in dislocation creep). This leads to a new interpretation of the sharp seismic reflector in the lowermost lower mantle, known as D00, as the onset of a sudden rheological transition and the generation of a mineral texture therewith. Finally, I also find that the pressure induced high-to-low-spin transition of iron in ferro-periclase and perovskite has a negligible effect on the rheology of the lower mantle. |
