Seismic properties of peridotite xenoliths as a clue to imaging the lithospheric mantle beneath NE Tasmania, Australia

Abstract

Peridotite xenoliths from Northeast Tasmania, Australia, consist of spinel lherzolites and minor dunites that originated from the uppermost mantle at depths of less than 40km, based on the geothermal gradient beneath Tasmania, where the depth of the Moho is 29km. The crystal preferred orientations (CPOs) of olivine and pyroxene were measured in 5 of 13 collected samples (1 dunite and 4 lherzolites) that were large enough to enable measurements. Olivine CPOs indicate the dominance of (010) [100] slip. Geothermobarometric analyses of the xenoliths yield temperatures of 950–1050°C and pressures of 0.7–1.1GPa. Using these data, we calculated changes in the seismic properties of a rock with ideal volume fractions of olivine and pyroxene under conditions of 1000°C and 1GPa, for various orientations of the structural reference frame in the case of three geodynamic models (extension, shear, and upwelling). The model of horizontal extension performs best in explaining the structure of the uppermost lithospheric mantle beneath Northeast Tasmania.