Comparison of the garnet—orthopyroxene geobarometer with recent experimental studies, and applications to natural assemblages

The garnet-orthopyroxene geobarometer developed by Harley (1984) in the accompanying paper is compared with other recent experimental data on alumina solubility in orthopyroxene, obtained both in end-member and more complex systems. The geobarometer, in part based on recent reversed data in MgO-Al2O3-SiO2 (Perkins et al., 1981), is in reasonable agreement with other recent data including the reversed data of Perkins & Newton (1980) in CaO-MgO-Al2O3-SiO2. Comparison with the experimental data of Wood (1974) in the FeO-MgO-Al2O3-SiO2 system indicates that less alumina enters orthopyroxene at a chosen P—T condition than suggested by that study. Application of the geobarometer to garnet-peridotite xenoliths, and peridotite massifs within gneiss terrains yields broadly consistent and reliable pressure estimates provided the temperature of equilibration is well constrained. Existing data for garnet-peridotite xenoliths in kimberlite yield P—T estimates which lie near steady-state continental geotherms but which lie well to the high-temperature side of such geotherms at high pressures. These data, however, do not necessarily define a simple 'kinked geotherm'. It is suggested instead that the xenolith data must be viewed in terms of polygenetic models of equilibration and cation exchange closure