Constraints on granulite genesis from carbon isotope compositions of cordierite and graphite

Abstract

Carbon dioxide is a common constituent in the channels of high-grade metamorphic cordierites1–6 Here we show that analysis of the stable isotopes of carbon in cordierite (crd) or graphite (gr) can provide a means to discriminate between two controversial models of granulite formation: (1) large-scale CO2 infiltration7, or (2) magmatic processes8–11, which might result in fluid-absent metamorphism. The fractionation of carbon isotopes between cor-dierites and graphites in high-grade rocks approximates the predic-ted equilibrium value. This suggests that cordierites can preserve peak metamorphic carbon isotope and channel gas compositions. Isotopically light carbon values in many granulite-facies cordierites (δ13Ccrd< −15) and graphites severely restrict theories of massive CO2 infiltration (δ13Cfluid≳−7). CO2rock ratios from a number of granulite terranes are estimated to be ≲0.01 by weight (∼0.016 on a molar oxygen basis), unless the rocks contain sub-stantial additional carbon. These data emphasize the importance of local fluid buffering in the granulite facies, consistent with dehydration by partial melts.