Equilibration temperatures and elastic wave velocities for upper mantle rocks from eastern Australia: implications for the interpretation of seismological models

Abstract

This study integrates available geological, geophysical and petrophysical data for deep-seated lithologies which occur beneath eastern Australia and which are accessible at the surface as xenoliths in basaltic host rocks. It is shown that spinel lherzolite (mantle wall-rock) occurs over a depth range commencing at about 30 km and extending to about 55 km. This represents a temperature range of 850–1000 °C. Acoustic wave velocity measurements were carried out on three types of spinel lherzolites and one mantle-derived garnet pyroxenite. The acoustic velocities range from 7.7 to 7.9 km s −1 for the pressure and temperature conditions beneath eastern Australia derived from xenolith geothermobarometry. These values demonstrate that the use of dunite as a generalised mantle rock results in overestimation of model mantle V p especially when used with a standard continental model geotherm. The measured spinel lherzolites also show variable anisotropy in acoustic wave velocity (5–8%) for these moderately foliated samples. It is demonstrated that by using a combination of the measured acoustic velocities and single-crystal elasticity data for constituent minerals, it is possible to interpolate V p for a wide modal range of rock types relevant to the lower crust and upper mantle. Triangular grids which allow graphical estimation of V p for spinel lherzolites, garnet lherzolites and garnet pyroxenites have been constructed. The petrologic and petrophysical results are used to constrain models of the lower crust and upper mantle profiles based on available seismic data for eastern Australia.