Elastic Moduli of Single-Crystal Orthoenstatite From Room Temperature to 1450 K

Orthopyroxene [(Mg,Fe)2Si2O6] is commonly considered to be one of the four major minerals in Earth's upper mantle. Thus, data on the elastic properties of orthopyroxene over wide ranges of temperature and pressure are necessary to develop reliable models of the composition and structure of the upper mantle. New elasticity data are provided on the nine independent adiabatic elastic moduli of orthoenstatite (Mg end-member orthopyroxene) from room temperature to 1450 K at ambient pressure. These data were obtained using the resonant ultrasound spectroscopy (RUS) with a natural single-crystal specimen; they extend by 380 kelvin the temperature range for which the elastic moduli of single-crystal orthoenstatite have previously been reported. Broad agreement in the temperature dependences of the nine Cij's is found when comparing our results with those from a Brillouin spectroscopy study of orthoenstatite up to 1073 K (Jackson et al., PEPI, 161, 1- 12, 2007). An earlier report (Jackson et al., Am. Mineral., 89, 239-245, 2004) identified marked non-linear temperature effects in the C33 and C55 moduli and correlated this nonlinearity to high-temperature mode softening in orthoenstatite at high temperature. Similar non-linear effects in the C33 and C55 are seen in the current RUS experiments and are carefully documented up to 1450 K in intervals of 25 kelvin. The current RUS study also reveals conspicuous non-linear trends in the temperature dependences of two of the three off-diagonal moduli, C13 and C23, above 1000 K. These results are interpreted in terms of the high-temperature isotropic properties of orthoenstatite, and their impact on our understanding of the properties of Earth's upper mantle is discussed.