Sound velocities in MgSiO3-garnet to 8 GPa

Abstract

Velocities of acoustic compressional (P) and shear (S) waves were measured to 8 GPa at room temperature for dense (99.7% of theoretical density), isotropic polycrystalline specimens of MgSiO3-majorite garnet using the phase comparison method of ultrasonic interferometry in a 1000-ton split-cylinder apparatus (USCA-1000). The pressure derivatives of the adiabatic bulk modulus (Ks) and the shear modulus (G) are Ko′ = (∂Ks/∂P)T = 6.7±0.4 and Go′ = (∂G/∂P)T = 1.9±0.1. Substitution of Si for Mg and Al in these garnets increase Ko′ by 26%, and Go′ by 16%, from Mg3Al2Si3O12-pyrope to majorite. The high values of Ko′ and Go′ from this study lead to much steeper velocity-depth profiles in the transition zone (400-700 km) than those calculated from the Ko′ and Go′ estimates from elasticity systematics by Duffy and Anderson (1989).