Post-perovskite transition in (Al,Fe)-bearing bridgmanite*

The major mineral phase of the Earth’s lower mantle, (Al,Fe)-bearing bridgmanite, transitions to a seemingly layered structure known as post-perovskite at approximately P=125 GPa and T = 2500 K. This transition must produce seismic features in the Earth’s deep lower mantle. Despite extensive investigations by experiments and ab initio calculations, there are still important aspects of this transformation that need clarification. Here, we systematically address this question in (Al,Fe³⁺)-, (Fe²⁺)- and (Fe³⁺)-bearing bridgmanite using ab initio calculations. We particularly address the phase boundary dependence on the chemistry and acoustic velocity changes across this transformation. These results are important to constrain the nature of the D” layer, the deepest layer of the mantle that should be dominated by the post-perovskite phase. For instance, both the topography of and seismic velocity jumps at the D” discontinuity depend on the local composition and temperature. Knowledge of the effect of composition variation on the phase diagram allows us to validate hypotheses concerning the nature of the enigmatic D” layer.