In Situ Elasticity Measurements of Unquenchable MgSiO3 Clinoenstatite at Upper Mantle Conditions

Introduction A phase transformation from orthorhombic (orthoenstatite [OEN], space group Pbca) to monoclinic symmetry in (Mg,Fe)SiO3, a common constituent of the Earth’s crust and upper mantle, is likely to occur in the deeper portion of the upper mantle [1, 2]. Angel et al. [3] confirmed that the clinoenstatite phase above 8 GPa is an unquenchable, high-pressure, monoclinic phase (highpressure clinoenstatite [HP-CEN], space group C2/c). Because of its unquenchable nature, the HP-CEN phase has to be synthesized within its stability field in order to study its elasticity. To date, the elasticity of HP-CEN has not been measured; such measurements are limited by the techniques available. For determining the elasticity at corresponding conditions by using an ultrasonic technique, the length of the specimen is a key parameter. Recently, the installation of x-radiography for ultrasonic measurement, combined with in situ x-ray diffraction, has made it possible to monitor the length change of the sample during an experiment and to measure the travel times (t(P,S)) and density of the sample simultaneously [4]. By using the state-of-art experimental setup, we were able to measure the elasticity of unquenchable HP-CEN. In this study, we present results on the measured elasticity of HP-CEN at high pressure and temperature for the first time.