Stability of high-density clinoenstatite at upper-mantle pressures

Abstract

SILICATE pyroxenes are major components in mineralogies! models of the Earth's upper mantle^1,2, with the transformation of chain-silicate pyroxenes to denser garnet structures being a possible cause³ of the seismic discontinuity at 400 km depth that divides the upper mantle from the transition zone. At shallower depths assemblages containing two pyroxenes are stable: calcium and sodium components are accommodated in a diopside–jadeite solid solution³, while magnesium and iron form a second, calcium-poor pyroxene. In the absence of experimental data, orthoenstatite was long believed to be the stable polymorph of (Mg, Fe)-pyroxene over the entire upper mantle. More recently, however, petrological experiments^4,5 at pressures and temperatures in excess of 8 GPa and 900 °C have provided evidence for the transformation of Mg-orthopyroxene to a clinopyroxene phase. The thermodynamic and physical properties of this phase are completely unknown. Here we report the results of a high-pressure single-crystal diffraction study which confirm the stability of a high-clinopyroxene phase of MgSiO₃ at high pressures, and allow an initial estimate to be made of the density changes associated with the transformation of the orthopyroxene component in the Earth's upper mantle.