Extraction and carbon isotope analysis of CO2 from scapolite in deep crustal granulites and xenoliths

Abstract

Carbon isotope compositions of scapolite from granulite facies gneisses and lower crustal xenoliths document the composition and constrain the source of carbon in scapolite from the lower crust. CO2 is extracted from scapolite without fractionating carbon isotopes by reaction with phosphoric acid at 25 or 75[deg]C. Thus, partial yields of CO2 from scapolite are sufficient for accurate carbon isotopic analysis. Isotopic compositions of coexisting scapolite and calcite in high-grade calc-silicate gneisses and marbles, and consideration of the crystal chemical environment of CO3 in the scapolite structure, indicate little fractionation of 13C/12C between scapolite and calcite (0.1 +/- 1.2%.) at equilibrium conditions of 650-800[deg]C.The carbon isotope composition of CO2 extracted from scapolite in twenty-nine samples of regional granulite facies gneisses, amphibolites, calc-silicate gneisses, and crustal xenoliths yield values of [delta]13C that range from -10 to -1%. (PDB). High-grade marbles and graphitic paragneisses are precluded as major sources of carbon for scapolite in the high grade rocks analyzed in this study, as the former are isotopically enriched, and the latter isotopically depleted in 13C/12C relative to the range of isotopic compositions determined here. The [delta]13C values for mafic granulites and amphibolites in granulite terranes composed of supracrustal sequences (-10.1 to -4.0%.) may reflect the isotopic composition of diagenetic carbonate present in their basaltic protoliths. The values of [delta]13C for scapolite in mafic xenoliths and some granulite facies orthogneisses (-8.2 to -1.2%.) are consistent with crystallization of the scapolite from a mafic melt or derivation of CO2 from mafic melts emplaced in the lower crust or upper mantle. The values of [delta]13C for scapolites from calc-silicate gneisses and calc-silicate xenoliths (-10.0 to -2.9%.) may result from depletion of 13C/12C as a result of decarbonation of calcite-bearing protoliths during prograde metamorphism.