Orthoenstatite/clinoenstatite phase transformation in MgSiO3 at high-pressure and high-temperature determined by in situ X-ray diffraction: Implications for nature of the X discontinuity

In the upper mantle, the X discontinuity at the depths of 240 to 340 km is observed in many localities, but its origin is under debate. The phase transformation of (Mg, Fe) SiO3 from orthopyroxene to high-pressure clinopyroxene has been proposed to cause this discontinuity. In this study, in situ X-ray diffraction experiments were performed, and the orthoenstatite/high-pressure clinoenstatite (C-2/c) phase boundary in the MgSiO3 system was determined. All the experiments were carried out using a Kawai-type multianvil apparatus installed at BL04B1 in SPring-8, Hyogo, Japan. Two types of experiments were carried out: (1) "phase observation experiments" at varying pressures and temperatures and (2) "quench experiments" at fixed pressure and temperature. The phase boundary, determined precisely by integrating the results of both sets of experiments, was found to be P (GPa) = 0.0035T (degrees C) + 3.7, using a recent MgO pressure scale. The orthoenstatite/high-pressure clinoenstatite phase boundary at 1400 degrees C determined here is 0.7-1.0 GPa lower than previous reports based on quench experiments. The phase transformation of orthopyroxene is expected to occur at 260 km depth along a typical mantle geotherm, and the shallower part of the observed X discontinuity can be explained by this phase transformation.