Metamorphism of reduced granulites in low-CO2 vapour-free environment

Abstract

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 (f O2) 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 f O2 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 f O2 granulites, these rocks have not been flooded by CO2, and low f H2O 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.