On dislocation glide in MgSiO 3 bridgmanite at high-pressure and high-temperature

Abstract

Dislocation glide in MgSiO3 bridgmanite with Pbnm perovskite structure is modeled at 30 and 60 GPa for the [100](010) and [010](100) slip systems. The velocity of screw dislocations is calculated in the thermally activated regime based on the kink-pair mechanism. We show that the dislocation velocity determination can rely on the atomic scale calculations of a limited amount of parameters: the Peierls stress τp, and the formation enthalpy of a single kink Hk. From the dislocation velocities, the evolution of stress as a function of temperature can be derived from the Orowan equation at any strain rate. Calculations performed at laboratory strain-rates of 10−5 s−1 reproduce well the high stress levels found experimentally. This demonstrates the influence of lattice friction in the mechanical properties of bridgmanite. The same calculations are performed at mantle strain-rate (10−16 s−1). They demonstrate that in the lower mantle, bridgmanite would always be in the thermally activated regime and that stresses close to 1 GPa are still necessary to move dislocations in bridgmanite. In the uppermost lower mantle, dislocation glide is inhibited and other deformation mechanisms, involving diffusion, are needed.