Elasticity and anisotropy of dunite and bronzitite

Compressional and shear wave velocities in 13 directions at pressures to 10 kb were measured in dunite from the Twin Sisters peaks, Washington, and in bronzitite from the Stillwater Complex, Montana. Dunite is characterized by a strong preferred orientation of olivine grains, leading to anisotropy of velocity of 15% at 10 kb. The bronzitite is more nearly isotropic, the greatest difference of P wave velocities reaching 6%. The comparison of elasticity diagrams, obtained at pressures of 0.5 kb and 10 kb, shows that the system of pores, more open at low pressure, is of minor influence; anisotropy is controlled mainly by the preferred orientation of olivine and bronzite in both rocks. Most of the compressional and shear wave velocities, calculated according to Voigt's scheme on the basis of petrofabric determinations and published single-crystal data for olivine and bronzitite, are in good agreement with the ultrasonic velocities measured at 10 kb. The results confirm the possibility of using this procedure to determine anisotropic elasticity of simple aggregates and suggest a possibility of using ultrasonic measurements in petrostructural analysis.