Single-crystal elasticity of natural Fe-bearing orthoenstatite across a high-pressure phase transition

Sound velocities and elastic moduli have been measured on Fe-bearing orthoenstatite (OEN) single crystals up to 12 GPa by Brillouin spectroscopy. The ambient adiabatic bulk modulus (K_s0) and shear modulus (G₀) are determined to be 113(1) GPa and 75.9(7) GPa, respectively. A fourth-order finite strain fit to the data yields pressure derivatives of K_s0′ = 8.8(1), K_s0″ = −0.68(6), G₀′ = 2.9(1), and G₀″ = −0.40(2). These values are significantly higher than those for other major mantle minerals up to 10.5 GPa but lower than some previous measurements on OEN. A pronounced increase of shear anisotropy was observed at a pressure of 12.06(9) GPa, coinciding with a phase transition from orthoenstatite to a recently discovered high-pressure phase with space group P2₁/c. A high-pressure phase transition in OEN is unlikely to be the cause of the X discontinuity in the 250–325 km depth range. Rather, a change in seismic anisotropy would be expected to accompany the orthoenstatite-to-P2₁/c phase transition in the upper mantle at greater depths.