Fabric-related seismic anisotropy in upper-mantle xenoliths: evidence from measurements and calculations

Abstract

The seismic properties and their directional dependence have been measured in four mantle peridotites from picritic tuffs (Vitim, Baikal region, Russia). Geothermobarometric data suggest a source depth for the xenoliths corresponding to the spinel-garnet transition zone (40–90km). The experimental data include the measurement of P- and S-wave velocities and densities at confining pressures to 600 MPa and temperatures to 600°C, and the determination of velocity anisotropy and shear-wave splitting. The measured velocities are compared with velocity calculations based on lattice preferred orientation (LPO) of olivine (orthopyroxene), the modal composition of the rocks and the single crystal stiffness coefficients. Macroscopic velocity anisotropy is strongly controlled by the LPO of olivine. Measured (600 MPa, room temperature) and calculated P-wave anisotropies compare fairly well, although the experimentally determined velocities are markedly lower than the calculated velocities. The correspondence between measured and calculated shear-wave splitting data is less pronounced. This is attributed to grain boundary effects (retrogressive reactions, interstitial glass, incomplete crack closure) and to the fact that the LPO of the cpx fraction has not been included in the calculation. The magnitude of P-wave anisotropy corresponds to that observed in mantle peridotites exposed at the surface (Ivrea zone, northern Italy: Ronda, southern Spain).