Single-crystal elasticity of gamma-(Mg0.91Fe0.09)(2)SiO4 to high pressuresand to high temperatures

Sound velocities and elastic moduli have been measured on gamma-(Mg0.91Fe0. 09)(2)SiO4 (ringwoodite) single-crystals to 16 GPa and to 923K by Brillouin spectroscopy. A third-order finite-strain fit to the data yields pressure derivatives of 4.1(2) and 1.3(1) for the adiabatic bulk modulus (K-S) and s hear modulus (mu) respectively, Mg-rich alpha, beta, and gamma (Mg,Fe)(2)Si O4 have identical values of partial derivativeK(S)/partial derivativeP and of partial derivative mu/partial derivativeP within the experimental uncert ainties. partial derivativeK(S)/partial derivativeT and partial derivative mu/partial derivative Tfor gamma-(Mg,FC)(2)SiO4 are -0.021 and -0.016 GPa/K respectively. Our results indicate that the shear and compressional impeda nce contrasts associated with beta- --> gamma-(Mg,Fe)(2)SiO4 transition are sufficient to produce a visible discontinuity at 520 km even with a modera te (30-50%) amount of olivine. The generally low visibility of the 520 km d iscontinuity is likely due to its width and large amplitude topographic var iations.