Recent first-principles calculations have proposed that bulk sound speed (Vφ) decreases at the perovskite (Pv) → postperovskite (PPv) transition. Yet some measurements suggest an increase in Vφ at the PPv transition in (Mg,Fe)SiO3 and NaMgF3. Our new measurements on NaMgF3 up to 50 GPa under quasi-hydrostatic stress conditions reveal that the Vφ of PPv remains lower than that of Pv near the PPv transition, consistent with a first-principles study on NaMgF3. Combined with similar observations in (Mg,Fe)SiP3, MgGeO3, and CaIrO3, our result on NaMgF3 suggests that the Vφ decrease is a robust trend in the PPv transition and is controlled more by crystal structure than by chemical composition. Our finding also strengthens the proposal that the elastic properties of PPv can explain the small P-wave increase across the D″ discontinuity and the anticorrelation between the Vφ and S-wave velocity anomalies in the D″ region.
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)