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 experimentaldata on alumina solubility in orthopyroxene, obtained both inend-member and more complex systems. The geobarometer, in partbased on recent reversed data in MgO-Al₂O₃-SiO₂ (Perkins etal., 1981), is in reasonable agreement with other recent dataincluding the reversed data of Perkins & Newton (1980) inCaO-MgO-Al₂O₃-SiO₂. Comparison with the experimental data ofWood (1974) in the FeO-MgO-Al₂O₃-SiO₂ system indicates thatless alumina enters orthopyroxene at a chosen P-T conditionthan suggested by that study. Application of the geobarometer to garnet-peridotite xenoliths,and peridotite massifs within gneiss terrains yields broadlyconsistent and reliable pressure estimates provided the temperatureof equilibration is well constrained. Existing data for garnet-peridotitexenoliths in kimberlite yield P-T estimates which lie near steady-statecontinental geotherms but which lie well to the high-temperatureside 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 viewedin terms of polygenetic models of equilibration and cation exchangeclosure. Garnet-peridotite bodies enclosed in gneisses in the Alps andNorway yield pressure estimates in the range 20–30 kilobars.Such pressure estimates favour a mantle origin and tectonicemplacement model for these bodies. The most recent data relevantto the ‘country-rock’ orthopyroxene-eclogites fromNorway give pressure estimates (18 ? 8 kb) which do not allowdiscrimination between the in situ model of formation and thetectonic emplacement models. Some suites of crustal granulites yield pressure estimates whichare internally consistent, consistent with the presence of characteristicaluminosilicate polymorphs, and similar to pressures calculatedusing other reliable geobarometers where these are available.However, the geobarometer may underestimate pressures in comparisonwith a previous calibration (Harley & Green, 1982), particularlywhere measured values of K^ga-opx _DFe-Mg are high. This discrepancyresults from the sensitivity of the geobarometer equation torelatively small changes in garnet composition, independentof variations in alumina contents of the orthopyroxenes. A diversesuite of samples from a single locality in Enderby Land, Antarctica,illustrates some of these problems and uncertainties.