Sound velocities and single-crystal elasticity of orthoenstatite to 1073 K at ambient pressure

Single-crystal Brillouin spectroscopy measurements have been carried out to 1073 K and ambient pressure to determine the elastic modulus tensor of a natural orthopyroxene, nearly pure Mg end-member (Mg_0.994Fe_0.002Al_0.004)₂(Si_0.996Al_0.004)₂O₆ orthoenstatite. Three single-crystal specimens with orthogonal crystallographic orientations were prepared for Brillouin measurements to determine all the single-crystal elastic moduli (C_ij). The single-crystal elastic moduli were calculated using the measured velocities of sound and our independent measurement of the volume thermal expansion. These are, to our knowledge, the highest temperatures at which the complete single-crystal elastic modulus tensor of orthoenstatite has been measured. The elastic moduli at ambient conditions obtained in this study are in excellent agreement with previous measurements on Mg₂Si₂O₆ orthoenstatite (OEN). The elastic anisotropy of OEN is approximately the same at all measured temperatures. The velocities in different directions change at different rates, with C₃₃ showing the largest temperature dependence (decreasing with temperature almost twice as fast as C₁₁ or C₂₂). The variation of elastic moduli with temperature is linear up to approximately 673 K, above which some of the moduli decrease with a quadratic dependence on temperature. The non-linear behavior is in accord with recent elastic mode softening observations in orthoenstatite at higher temperatures and could add support to account for a thermally induced solid-state mechanism for the low velocity zone in Earth's upper mantle.