HR: 0800h
AN: MR31A-0117    [Abstracts]
TI: Ab Initio Treatment of the CaSiO3-MgSiO3 Solvus.
AU: * Jung, D Y
EM: daniel.jung@erdw.ethz.ch
AF: Institute for Mineralogy and Petrography, ETH Zürich, Sonneggstr.5, Zürich, 8092 Switzerland
AU: * Jung, D Y
EM: daniel.jung@erdw.ethz.ch
AF: Laboratory for Crystallography, ETH Zürich, ETH Hönggerberg Wolfgang-Pauli-Strasse 10, Zürich, 8093 Switzerland
AU: Oganov, A R
EM: a.oganov@mat.ethz.ch
AF: Laboratory for Crystallography, ETH Zürich, ETH Hönggerberg Wolfgang-Pauli-Strasse 10, Zürich, 8093 Switzerland
AU: Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: Institute for Mineralogy and Petrography, ETH Zürich, Sonneggstr.5, Zürich, 8092 Switzerland
AB: The lower mantle of the Earth extends from about 670 km to 2980 km and consists mainly of MgSiO₃-perovskite (~ 75 vol%), (Mg,Fe)O magnesiowüstite (~ 20 vol%) and CaSiO₃-perovskite (~ 5 vol%). It is possible to calculate thermodynamic properties, structures and energetics of the individual minerals at extreme conditions of the mantle using ab initio methods, such as density functional theory. To obtain a more realistic picture of the lower mantle it is necessary to not only investigate chemically pure minerals, but to consider minerals as solid solutions, as they are in nature.
The density functional theory with the generalized gradient approximation (GGA) and the projector augmented wave (PAW) method, as implemented in the VASP code, was used to calculate the structure and stability of CaSiO₃ perovskite in the pressure range of the Earth's mantle (0-150 GPa), no post-perovskite structure has been found [1].
Here we focus on the two perovskite solvus. We use a subregular solid solution model together with point defect calculations to model the solvus at different pressures in the lower mantle regime. Additionally, the effect of the different symmetries ( Pbnm and I4/ mmm) of the perovskites has to be included. This is important especially for the Ca-perovskite, since the energy differences of the two phases are very small and thus likely to have an influence on the solvus.
We investigated the solvus at different pressures of the lower mantle. At pressures and temperatures of the lower mantle, the solvus in the (Ca,Mg)SiO₃ system remains wide open and solubilities of Ca in Mg-perovskite and Mg in Ca-perovskite low. From these results in the simple system it is highly unlikely that Ca-perovskite will disappear (i.e. fully dissolve in Mg-perovskite) with depths in the lower mantle. Information of the solubility of Ca in MgSiO₃ in more complex systems will elucidate the mineralogical composition of the lower mantle of the Earth. This is the first work to treat this subject with ab initio methods. Presently, calculations on the Ca-Mg-perovskite solvus with aluminium impurities are in progress.
[1] Jung D.Y., Oganov A.R. (2005) Phys. Chem. Minerals 32, 146-153
DE: 3924 High-pressure behavior
DE: 3939 Physical thermodynamics
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005