AN OXYGEN BAROMETER WITH THE ASSEMBLAGE GARNET-EPIDOTE

Abstract

An oxygen barometer has been developed using garnet-epidote equilibria and the latest available thermodynamic data via the reaction:2 Ca2FeAl2Si3O12(OH)=2 Ca2FeAl2Si3O12+H2O+1/2 O2epidote (Ps33Czo67)garnet (Alm33Grs67)fluidwhere epidote (Ps33Czo67) reacts to form (Alm33Grs67), water and oxygen. The garnet-epidote oxybarometer has been applied to a wide variety of metamorphic rocks including low to medium-pressure greenschists and amphibolites, as well as high-pressure eclogites. Results indicate that this equation represents a sensitive oxygen monitor with only a minor dependence on pressure. Andradite solid solution of less than 15 mol% has only minor influence in the locus of the curve in fO2/T space. Garnet-epidote equilibria may also provide a thermometer when combined with an independent oxygen buffer. The garnet-epidote oxythermobarometer is a reliable oxygen sensor that can be applied to many low through medium-grade metamorphic rocks. The extensive solid solution found in both garnet and epidote allows this equation to be applied to mineral pairs with a wide range of compositions. The garnet-epidote oxythermobarometer is potentially applicable to any rocks that contain the equilibrium assemblage garnet+epidote.