Computer Science
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58..959m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 2, pp.959-967
Computer Science
2
Scientific paper
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. CO 2 is extracted from scapolite without fractionating carbon isotopes by reaction with phosphoric acid at 25 or 75°C. Thus, partial yields of CO 2 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 CO 3 in the scapolite structure, indicate little fractionation of 13 C / 12 C between scapolite and calcite (0.1 ± 1.2%.) at equilibrium conditions of 650-800°C. The carbon isotope composition of CO 2 extracted from scapolite in twenty-nine samples of regional granulite facies gneisses, amphibolites, calc-silicate gneisses, and crustal xenoliths yield values of 13 C 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 13 C / 12 C relative to the range of isotopic compositions determined here. The 13 C 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 13 C 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 CO 2 from mafic melts emplaced in the lower crust or upper mantle. The values of 13 C for scapolites from calc-silicate gneisses and calc-silicate xenoliths (-10.0 to -2.9%.) may result from depletion of 13 C / 12 C as a result of decarbonation of calcite-bearing protoliths during prograde metamorphism.
Essene Eric J.
Moecher David P.
Valley John W.
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