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

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°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°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 δ¹³C 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 δ¹³C 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 δ¹³C 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 δ¹³C 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.