Sound velocity of MgSiO₃ post-perovskite phase: A constraint on the D″ discontinuity

The discovery of a post-perovskite phase transition in MgSiO₃ has significant implications for seismological observations in the D″ region at the bottom of Earth's mantle. The D″ discontinuity, which is manifested as a sharp positive seismic-wave velocity jump 200-300 km above the core-mantle boundary (at pressure of 119∼ 125 GPa), is one of the most enigmatic seismic features in this region. Whether this velocity increase may be due to the formation of a post-perovskite phase at the D″ discontinuity has not, however, been directly addressed by experiments. Here we present the results of aggregate sound velocity measurements of the MgSiO₃ post-perovskite phase by Brillouin spectroscopy in the diamond anvil cell (DAC) up to a pressure of 172 GPa, in combination with infrared laser annealing of the sample. Based on these results and our recent high-pressure velocity measurements on perovskite, the aggregate shear wave velocity contrast across the perovskite to post-perovskite phase transition is at most 0.5%. This contrast is much smaller than typically observed across the D″ discontinuity, indicating that the formation of an isotropic aggregate of the post-perovskite phase provides an insufficient velocity increase to explain the D″ discontinuity. Lattice preferred orientation (LPO) of post-perovskite is likely to be crucial for explaining the D″ discontinuity.