HR: 16:45h
AN: MR24A-04    [Abstracts]
TI: Plastic Deformation of MgSiO$_{3}$ Perovskite: Cubic or Orthorhombic ?
AU: Ferre, D
EM: denise.ferre@univ-lille1.fr
AF: Laboratoire de Structure et Proprietes de l'Etat Solide, UMR CNRS 8008 - Universite de Lille 1, Villeneuve d'Ascq, 59655 France
AU: * Cordier, P
EM: patrick.cordier@univ-lille1.fr
AF: Laboratoire de Structure et Proprietes de l'Etat Solide, UMR CNRS 8008 - Universite de Lille 1, Villeneuve d'Ascq, 59655 France
AB: The Earth's lower mantle consists mainly of (Mg, Fe)SiO$_{3}$ perovskite and (Mg, Fe)O. Perovskite which takes up at least 70 per cent of the total volume is likely to control the rheology of the lower mantle. Understanding the elementary deformation mechanisms (dislocations, slip systems) of perovskite is thus one of the central issues of deep mantle studies. This knowledge is also needed to interpret seismic anisotropy in the lower mantle. (Mg, Fe)SiO$_{3}$ perovskite is not stable at high temperature under ambient pressure. Deformation experiments on perovskite must be carried out under extreme pressure-temperature conditions, inducing severe limits on our progress in that field. Contrary to the ideal, high-symmetry, structure of CaTiO$_{3}$ which is cubic (space group Pm3m), (Mg, Fe)SiO$_{3}$ perovskite is distorted, resulting in an orthorhombic symmetry (space group Pbnm). Distortions consist in tilted octahedra and Mg atoms being displaced from the centres of their sites. Assessing the influence of these distortions on plastic shear anisotropy in perovskite is thus a major issue as cubic or orthorhombic symmetries can lead to very different crystal preferred orientations and seismic anisotropies. Considerable insight into the physical properties of minerals at high pressure has been obtained from calculations using computational methods. Mechanical properties represent however a complicated case as several length-scales from atomic to macroscopic must be considered. In minerals (with usually complex crystal chemistry), the choice of slip systems is largely controlled at the atomic scale within the dislocation cores. Modelling dislocation cores of minerals at the atomic scale requires many atoms to be simulated and is still very challenging. In this study, we propose an alternative approach based on the calculation of generalized stacking fault (GSF). A GSF is produced when a crystal is sheared in a given plane by a given displacement u. It is not stable in general and must be balanced by a restoring force. It is thus possible to calculate the intrinsic resistance opposed to plastic shear associated to a given slip system. This approach is applied here for the first time to (Mg, Fe)SiO$_{3}$ perovskite to provide a first estimate of plastic anisotropy in this important mineral.
DE: 8162 Rheology--mantle
DE: 5120 Plasticity, diffusion, and creep
DE: 3902 Creep and deformation
DE: 3904 Defects
DE: 3924 High-pressure behavior
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting