Sound velocities in MgSiO3‐garnet to 8 GPa

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).