Anisotropy of Earth's D" layer and stacking faults in the MgSiO3 post-perovskite phase

TL;DR

This study uses first-principles metadynamics to explore the stacking-fault structures and slip mechanisms in MgSiO3 post-perovskite, revealing insights into seismic anisotropy and deformation in Earth's D'' layer.

Contribution

It introduces a novel simulation approach to identify stacking faults and slip planes in post-perovskite, challenging previous assumptions and linking microscopic mechanisms to geophysical observations.

Findings

Identifies (110) slip planes in post-perovskite as key to seismic anisotropy.

Shows stacking faults facilitate phase transition and deformation.

Predicts smaller lattice orientation needed to explain seismic data.

Abstract

The post-perovskite phase of (Mg,Fe)SiO3 is believed to be the main mineral phase of the Earth's lowermost mantle (the D" layer). Its properties explain numerous geophysical observations associated with this layer - for example, the D'' discontinuity, its topography and seismic anisotropy within the layer. Here we use a novel simulation technique, first-principles metadynamics, to identify a family of low-energy polytypic stacking-fault structures intermediate between the perovskite and post-perovskite phases. Metadynamics trajectories identify plane sliding involving the formation of stacking faults as the most favourable pathway for the phase transition, and as a likely mechanism for plastic deformation of perovskite and postperovskite. In particular, the predicted slip planes are (010) for perovskite (consistent with experiment) and (110) for postperovskite (in contrast to the previously expected (010) slip planes). Dominant slip planes define the lattice preferred orientation and elastic anisotropy of the texture. The (110) slip planes in post-perovskite require a much smaller degree of lattice preferred orientation to explain geophysical observations of shear-wave anisotropy in the D" layer.