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 dehydration^1–6. 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 rock^7,8. 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.