Carbon isotopic equilibrium between calcite and graphite in Skallen Marbles, East Antarctica: evidence for the preservation of peak metamorphic temperatures

Carbon isotope thermometry between calcite and graphite has been applied to the granulite facies marble assemblages in the Skallen region of the Lützow–Holm Bay, East Antarctica. The carbon and oxygen isotope values of calcite from five samples, which are interlayered with pelitic and charnockitic assemblages, suggest within-layer isotopic equilibrium. Textural studies of graphite crystals indicate two types; one smooth, highly reflecting regular shaped graphite and the other rough, dull and irregular shaped graphite. The δ13C values of the dull surfaced graphite are lower by about one per mil than the high reflecting graphite. Carbon isotopic compositions suggest that graphite with lower δ13C values is confined to the surface layer of the graphite crystal. This is indicative of a later-stage overgrowth during retrograde metamorphism. δ13C of dull graphite was excluded in calculating the temperature of peak metamorphism. Temperatures of ∼850°C correspond to the peak metamorphic temperatures for the region. This temperature estimate is consistent with the regional metamorphic conditions based on cation exchange thermometry in pelitic granulites and is interpreted to be more precise.