SOUND VELOCITIES IN OLIVINE AT EARTH MANTLE PRESSURES

The independent elastic constants of an upper mantle mineral, San Carl os olivine [(Mg1.8Fe0.2)SiO4], were measured from 0 to 12.5 gigapascal s. Evidence is offered in support of the proposition that the explicit temperature dependence of the bulk modulus is small over the range of temperatures and pressures thought to prevail above the 400-kilometer discontinuity, and thus the data can be extrapolated to estimate the properties of olivine under mantle conditions at a depth of 400 kilome ters. In the absence of high-temperature data at high pressures, estim ates are made of the properties of olivine under mantle conditions to a depth of 400 kilometers. In contrast with low-pressure laboratory da ta, the predicted covariance of shear and compressional velocities as a function of temperature nearly matches the seismically estimated val ue for the lower mantle.