The applicability of garnet–orthopyroxene geobarometry in mantle xenoliths

The various existing garnet–orthopyroxene geobarometers calibrated based on the Al2O3 or Cr2O3 components in orthopyroxene buffered by coexisting garnet for mantle xenoliths, abbreviated as Al2O3-barometer and Cr2O3-barometer, respectively, have been extensively studied, to test their precision by applying them to phase equilibrium experimental data, and to evaluate their validity by applying them to natural pyrolites. The Cr2O3-barometers failed in reconstructing the experimental pressures due to imprecise description of activities of garnet and orthopyroxene end members. Several of the Al2O3-barometers showed perfect ability in reproducing the experimental pressures and successfully discerned the garnet-facies and garnet–spinel transition facies lherzolites as well as the graphite- or diamond-bearing mantle xenoliths. It is recommended that pressure and temperature estimation of mantle xenoliths may be best determined simultaneously by combining the garnet–orthopyroxene barometer and the two-pyroxene thermometer of Taylor (1998). Orthopyroxenes containing Al2O3 contents of less than 0.37 wt.% should not be used in the pressure estimation in order to avoid significant P–T errors propagated from analytical errors.