Elastic moduli and anisotropy of dunite to 10 kilobars

Abstract

Longitudinal and transverse wave velocities are reported for several directions at pressures to 10 kb in two samples of dunite from the Twin Sisters peaks, Washington. For both dunites, elastic-wave propagation is controlled to a large extent by olivine fabric. Dunite A, which has a strong concentration of olivine a crystallographic axes and girdles of b and c axes, is uniaxial in elastic properties. The longitudinal wave velocity at 10 kb for propagation parallel to the olivine a axes maximum is 8.76 km/sec. For propagation normal to the a axes maximum, longitudinal wave velocities are low (Vp=7.98 km/sec at 10 kb) and two transverse waves (Vs=4.41 and 4.69 km/sec at 10 kb) are clearly transmitted through the rock. Dunite B, with strong concentrations of all three olivine crystallographic axes, is similar in elastic properties to orthorhombic crystals with a high longitudinal wave velocity (9.15 km/sec at 10 kb) along the olivine a axes maximum and a low longitudinal wave velocity (7.83 km/sec at 10 kb) along the olivine b axes concentration. Elastic stiffnesses and compliances were computed from the velocities, and the physical properties of isotropic aggregates of the two dunites were calculated using the Voigt and Reuss averaging techniques. Primarily because of the presence of accessory minerals in the dunites, the Voigt and Reuss velocities are lower than values computed from olivine single-crystal data. The high pressure gradient (∂Vp/∂P=17.0 km sec−1 mb−1) observed for the longitudinal velocity of dunite B at 8 kb is interpreted as being due to the effect of grain boundary cracks.