In-situ Measurement of Elastic Velocity Contrast between MgSiO3 Enstatite Polymorphs at High Pressure

Introduction Orthopyroxene is an important mineralogical constituent of petrological models of the Earth’s upper mantle. Therefore, the reconstructive (or first-order) phase transformation between orthoenstatite (OEN, orthorhombic symmetry, space group Pbca) and highpressure clinoenstatite (HP-CEN, monoclinic symmetry, space group C2/c) is of great interest for geophysical applications. In particular, this phase transition was shown to be likely to occur in the deeper portions of the upper mantle [1-3], where the depth is coincident with the reflector observed at ~300 km [4, 5]. However, the HP-CEN phase is unquenchable. The elastic property, therefore, has to be measured within its stability field. Restricted by the techniques that are available, measuring the elasticity has never been done experimentally. The absence of direct measurements of sound velocities of the HP-CEN has made it difficult to ascertain whether the magnitude of the acoustic velocity or the impedance contrast between the OEN and HP-CEN phases of enstatite is sufficiently strong to produce an observable seismic signature. Contrary to most high-pressure mantle phases (e.g., wadseylite, ringwoodite, and Mg-rich perovskite), HPCEN cannot be recovered at ambient conditions because it transforms to a low-pressure monoclinic form (P21/c) upon the release of pressure [3, 6]. Thus the HP-CEN phase has to be synthesized in situ, followed by acoustic experiments performed within its stability field. Such acoustic experiments, which basically measure the travel times of sound waves in specimens, require techniques for monitoring the specimen length under elevated pressure and temperature conditions, to obtain velocity data. This is especially important when the specimen is undergoing a first-order phase transformation involving a volume change. Recently, our group (ultrasonic group at SUNY) has adapted x-radiographic imaging techniques for use in simultaneous ultrasonic and in situ x-ray diffraction experiments in order to permit direct measurements of the specimen length [7, 8]. The stability field of high-pressure clinoenstatite extends to the transition zone (~15 GPa). The 1000-ton multianvil press (T-25) [9] newly installed at beamline 13-ID at the APS has given us the opportunity to make ultrasonic measurements that cover the pressuretemperature (P-T) space of high-P clinoenstatite in the mantle conditions. Through the collaboration between our group and GSECARS, we have been able to measure elastic velocity across the OEN and unquenchable HPCEN phases at mantle conditions.