STABILITY OF HIGH-DENSITY CLINOENSTATITE AT UPPER-MANTLE PRESSURES

SILICATE pyroxenes are major components in mineralogical models of the Earth's upper mantle1,2, 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 experiments4,5 at pressures and temperatures in excess of 8 GPa and 900-degrees-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.