HR: 1340h
AN: MR23B-0055    [Abstracts]
TI: Plastic Anisotropy of MgSiO₃ Post-Perovskite: Implications for Crystallographic Preferred Orientation and Seismic Anisotropy in the D'' Layer
AU: * Cordier, P
EM: Patrick.Cordier@univ-lille1.fr
AF: Laboratoire Structure et Proprietes de l'Etat Solide - UMR 8008, Universite des Sciences et Technologies de Lille, Villeneuve d'Ascq, F-59655 France
AU: Carrez, P
EM: philippe.carrez@univ-lille1.fr
AF: Laboratoire Structure et Proprietes de l'Etat Solide - UMR 8008, Universite des Sciences et Technologies de Lille, Villeneuve d'Ascq, F-59655 France
AU: Ferre, D
EM: denise.ferre@univ-lille1.fr
AF: Laboratoire Structure et Proprietes de l'Etat Solide - UMR 8008, Universite des Sciences et Technologies de Lille, Villeneuve d'Ascq, F-59655 France
AU: Mainprice, D
EM: David.Mainprice@dstu.univ-montp2.fr
AF: Laboratoire de Tectonophysique - UMR 5568, Universite de Montpellier II, Montpellier, F-34095 France
AU: Tommasi, A
EM: Andrea.Tommasi@dstu.univ-montp2.fr
AF: Laboratoire de Tectonophysique - UMR 5568, Universite de Montpellier II, Montpellier, F-34095 France
AB: The recent discovery of the Post-Perovskite phase and prediction of its elastic constants using atomistic modelling has major implications for the interpretation of seismic anisotropy of the D'' layer. However, seismic anisotropy produced by a plastic flow field is a combination of single crystal elastic anisotropy, plastic anisotropy and the resulting crystallographic preferred orientation (CPO). To test the hypothesis that plastic flow of post-perovskite can account for the observed seismic anisotropy in the D'' layer it is necessary to determine the plastic anisotropy that is governed by the activity of slip systems. Considerable insight into the physical properties of minerals at high pressure has been obtained from calculations using atomic scale computational methods. 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) from first-principles. A GSF is produced when a crystal is sheared in a plane by a given displacement. In general it is not stable and must be balanced by a restoring force. It is thus possible to calculate the intrinsic resistance to plastic shear associated with a given slip system: the so-called Ideal Shear Stress (ISS). Several potential slip systems of the post-perovskite structure have been tested: [100] glide in (010), (001) and (011), [010] glide in (100) and (001), [001] glide in (100) and (010) and [110] glide in (001). From our calculations, [100](010), [100](001), [100](011) and [001](100) appear to be the easiest slip systems. The GSF are then used to refine the core structure of dislocations using the Peierls-Nabarro model. A series of Visco-Plastic Self-Consistent (VPSC) plasticity models has been conducted based on our results. The VPSC models predict a strong maximum of [100] parallel to the flow direction and weaker concentration of [001] normal to the flow plane with diffuse distribution of [010] normal to the flow direction. The predicted seismic properties using the elastic constants (136 GPa, 4000K) of Stackhouse et al. (2005) have a fast Vp maximum parallel to the flow direction and low Vp normal to the flow direction with an anisotropy of 4% and high shear wave splitting of more than 5% in flow plane and no shear wave splitting normal to the flow plane. The Vs distribution is compatible to first order with the seismically observed transverse anisotropy for S waves, however the Vp distribution has an orthorhombic distribution, but there are few observations for Vp in the D'' layer.
DE: 3902 Creep and deformation
DE: 3904 Defects
DE: 3924 High-pressure behavior
DE: 8162 Rheology: mantle (8033)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005