Ultrasonic Interferometry Measurements on Bridgmanite up to Mid–Lower Mantle Conditions

The Fe and Al-bearing MgSiO₃ bridgmanite is the most predominant mineral in the lower mantle, constituting more than approximately 75% of its volume. Given the lack of direct access to the Earth’s deep interior, the composition and mineralogy of the lower mantle are primarily estimated by comparing compressional- (Vp), and shear-wave velocity (Vs) profiles determined from seismological observations with those calculated for candidate mineral assemblages, under pressure and temperature conditions that correspond to those of the lower mantle. In this study, we conducted ultrasonic interferometry measurements on MgSiO₃ and (Mg,Fe)(Si,Al)O₃ bridgmanite in a large volume press up to 42 GPa and 1500 K using advanced multi-anvil techniques, towards the conditions of middle lower mantle. This is a radical extension in the conditions at which the high-pressure ultrasonic interferometry technique has been used, and the temperature dependency of bridgmanite’s sound velocity at high pressures has been evaluated with unprecedented accuracy. Using the new data, we constructed an integrated thermoelastic model for bridgmanite, providing improved constraints for interpreting seismological observations and for refining models of lower-mantle composition.