Contrib Mineral Petrol (1988) 100:92-106 Contributions to Mineralogyand Petrology 9 Springer-Verlag1988 Calculation and application of clinopyroxene-garnet-plagioclase-quartz geobarometers* D.P. Moecher, E.J. Essene, and L.M. Anovitz** Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA Abstract. Recently published thermodynamic and experi- mental data in a variety of chemical systems have been evaluated to derive Gibbs free energies for hedenbergite and pyrope. These were used to calculate the geobarometric equilibria Hedenbergite + Anorthite = Grossular + Almandine + Quartz: "HD barometer", Diopside + Anorthite = Grossular + Pyrope + Quartz: "DI barometer". We have compared the pressures obtained from these equi- libria for garnet-clinopyroxene-orthopyroxene-plagioclase- quartz assemblages with the geobarometer Ferrosilite + Anorthite = Almandine + Grossular + Quartz: "FS barometer". Pressures calculated for 68 samples containing the above assemblage from a variety of high grade metamorphic ter- ranes indicate that, in general, the HD and DI barometers yield values that are in good agreement with the FS barome- ter, and that the three barometers are generally consistent with constraints from aluminosilicate occurrences. How- ever, in some samples the HD barometer yields pressures up to 2 kbar greater than constraints imposed by the pres- ence of an aluminosilicate phase. Relative to the FS barome- ter, the HD barometer overestimates pressure by an average of 0.2_+ 1.0 (1 a)kbar and the DI barometer underestimates pressure by an average of 0.6 4-1.6 (1 o-) kbar. The pressure discrepancies for the HD and DI barometers are likely to be a result of imprecision in thermodynamic data and activi- ty models for silicates, and not a result of resetting of the clinopyroxene equilibria. The relative imprecision of the DI barometer relative to the FS barometer results from overes- timates of pressure by the DI and FS barometers in Fe-rich and Mg-rich systems, respectively. Application of the HD and DI barometers to high grade Cpx-Gt-Pg-Qz as- semblages yields pressures that are generally consistent with other petrologic constraints and geobarometers. It is con- cluded that the HD and DI barometers can place reasonable constraints on pressure (+ 1 kbar relative to the FS barome- * Contribution No. 447 from the Mineralogical Laboratory of the University of Michigan ** Present address: Department of Geosciences, University of Ari- zona, Tucson, AZ 85721 USA Offprint requests to: D.P. Moecher ter) if not extrapolated to mineral assemblages whose com- positions are extremely far removed from the end member system for which the barometers were calibrated. Introduction Major advances in the accuracy and precision of geobar- ometers have been made in the last decade, in part a result of careful experimental reversal of pressure dependent equi- libria (Bohlen et al. 1980; Bohlen and Boettcher 1982; Boh- len et al. 1983a, 1983b; Gasparik 1984a, 1984b; Gasparik and Newton 1984; Bohlen and Liotta 1986; Koziol and Newton 1988), more precise thermodynamic data (e.g. Ha- selton and Newton 1980; Metz et al. 1983; Bohlen et al. 1983; Robie and Hemingway 1984; Haselton et al. 1987; Robie et al. 1987), and more accurate modeling of activity- composition relations for mineral phases involved in geo- barometric equilibria (e.g., Newton et al. 1980; Newton and Haselton 1981; Ganguly and Saxena 1984; Davidson and Lindsley 1985; Anovitz and Essene 1987a). Reasonably ac- curate and precise geobarometers now exist for most granu- lite facies metabasites, charnockites and high grade metape- lites, and for some upper amphibolite facies metabasites and metapelites. The above experiments also serve as impor- tant constraints on thermodynamic data for mineral phases involved in the particular reaction. In concert with precise thermodynamic data for other phases, the experimentally constrained thermodynamic data may be used to calculate geobarometers that are not easily reversed experimentally. Using this approach we have calculated the location of the pressure dependent reactions Hedenbergite + Anorthite = Grossular + Almandine + Quartz 3 CaFeSizO6 + 3 CaA12Si~Os = 2 Ca3A12Si3012 + F%AIzSi3012 + 3 SiOz (1) (the "Hedenbergite (HD) barometer") and, Diopside + Anorthite = Grossular + Pyrope + Quartz 3 CaMgSi206 + 3 CaA12Si2Os = 2 Ca3A12Si3012 + Mg3AI2Si3012 + 3 SiO2 (2) (the "Diopside (DI) barometer"), in order to expand the range of assemblages for which there exist reasonably pre-