In-situ study of elasticity of MgSiO₃ pyroxene and its elastic behavior across the phase transition

Jennifer Kung¹, Baosheng Li¹, Takeyuki Uchida², and Yanbin Wang²

¹Mineral Physics Institute, Stony Brook University

²APS-GSECARS

(Mg,Fe)SiO₃-pyroxene with ~10mol% of iron, is a major mineral component of the Earth's upper mantle. The orthoenstatite (Oen) end-member (MgSiO₃) is often used as an analogue for the behavior of more complex mantle pyroxene compositions. Previous high pressure studies have shown that OEn (orthorhombic, space group Pbca) would transform to a high pressure phase of clinoenstatite (HCEn, monoclinic, space group C2/c) at high-pressure and temperature conditions. Unfortunately, HCEn is unquenchable and transform to another low-pressure polymorph (LCEn, monoclinic, space group P2₁/c). To measure the elasticity of HCEn, the in-situ experimental techniques are demanded.

Rapid advances in utilizing ultrasonic techniques in multi-anvil, high-pressure apparatus, in conjunction with in-situ synchrotron X-radiation, have made possible to perform elasticity measurements across such phase transitions; using these techniques, travel times (of P and S waves), length of the specimen and PVT-equation of state, can be collected simultaneously at high pressure and temperature conditions. The high pressure ultrasonic experiments were performed using the 1000-ton press and T-cup apparatus at beamline 13ID at the Advanced Photon Source of the Argonne National Laboratory (supported by GSECARS).

In this meeting, the first measurement of elasticity for unquenchable MgSiO₃ pyroxene at high pressures and temperatures will be presented. In our experiments, we also observed an anomalous elastic behavior across the phase transition in OEn > HCEn. The anomaly is the first time observed experimentally in the mantle mineral. This feature will be reported in details.