Sound velocities in MgSiO 3 ‐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 MgSiO 3 ‐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 (K S ) and the shear modulus (G) are K o ′ = (∂Ks/∂P) T =6.7±0.4 and G o ′ = (∂G/∂P) T =1.9±0.1. Substitution of Si for Mg and Al in these garnets increase K o ′ by 26%, and G o ′ by 16%, from Mg 3 Al 2 Si 3 O 12 ‐pyrope to majorite. The high values of K o ′ and G o ′ from this study lead to much steeper velocity‐depth profiles in the transition zone (400–700 km) than those calculated from the K o ′ and G o ′ estimates from elasticity systematics by Duffy and Anderson (1989).