METAMORPHISM OF REDUCED GRANULITES IN LOW-CO2 VAPOR-FREE ENVIRONMENT
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Pervasive flooding of CO2 has been proposed as the cause of granulite facies metamorphism that is capable of producing many distinctive characteristics of the deep continental crust: Reduced water activity, orthopyroxene-bearing assemblages, depletion of large-ion lithophile (LIL) elements, and dehydration1-6. The calculations presented here of C-O-H fluid composition for conditions of granulite facies metamorphism show that oxygen fugacity (fO2) estimates from many terranes are sufficiently low that the addition of CO2-rich fluid causes graphite to precipitate. For values of pressure (P) and temperature (T) common to granulites, and with fO2 slightly below the quartz-fayalite-magnetite buffer (QFM), the addition of CO2 sufficient to grow 10 vol. % orthopyroxene requires the precipitation of 1.5 vol. % graphite. As 0.1 vol. % graphite is readily recognizable, but is not reported in most low fO2 granulites, these rocks have not been flooded by CO2, and low fH2O is probably due to extraction of a magma or recrystallization of an already dry rock7,8. Granulite terranes may thus result from a combination of these three processes and the dominance of any one cannot now be demonstrated on a regional basis. 1984 Nature Publishing Group.
