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.