Ultrasonic observations of elasticity changes across phase transformations in MgSiO₃ pyroxenes

Using ultrasonic interferometry in conjunction with synchrotron X-radiation techniques in a high-pressure, multi-anvil apparatus, the elastic behavior of MgSiO₃ pyroxene has been continuously monitored as the specimens undergo phase transformations from the orthoenstatite (OEN) to the high-pressure clinoenstatite phase (HP-CEN) and from HP-CEN to the low-pressure clinoenstatite phase (LP-CEN). In the former case, elastic softening and amplitude attenuation is observed for both compresssional (P) and shear (S) waves when the pressure exceeds 9 GPa at room temperature, which we suggest is associated with a transition to a metastable phase intermediate between OEN and HP-CEN. In the latter case, both P and S wave velocities decrease rapidly as the back-transformation from HP-CEN to LP-CEN occurs on decrease of pressure below 4 GPa at room temperature; this is accompanied by an increase in attenuation of the P waves in the specimen, but not the S waves.