Prasanta Sanyal

Abstract

The carbon isotope composition of graphite and carbon and oxygen isotope composition of associated calcite from different locations of the Eastern Ghats Mobile Belt (EGMB) of Orissa have been measured in order to understand the origin of graphite. The d13C values of graphite range from -2.4‰ to -26.6‰. Forty-four of sixty-one samples have d13C values less than -20‰. Most of these low d13C values graphite corresponds to schists and disseminations in khondalite and calc-silicate granulites, thus suggesting graphitization of organic matter. The remaining light-carbon–graphite occurs as veins which is the result of graphitization of transported organic matter. The graphite with intermediate d13C value

(-13‰ to -19‰) suggests carbon contributions from both organic and carbonates sources and/or mantle sources. The higher d13C values graphite (-2.4‰to -8.8‰) represent mantle carbon and/or carbonate sources without significant contribution from organic carbon.

The temperatures of metamorphism have been estimated using carbon isotope ratios of graphite and associated calcite of calc-silicate granulites, where typical cation exchange thermometer assemblages are lacking and significant mineral reaction textures used to calculate pressure–temperature of metamorphic events are absent. Metamorphic temperatures obtained 945 deg C are close to the ultrahigh-temperature reported from the EGMB. The minimum temperature estimated using the graphite–carbonate carbon isotope ratio is 90 deg C. The lower estimates of temperatures probably suggest changes in the carbon isotope ratio of calcite by decarbonation reaction or armoring of carbonaceous matter in silicates during metamorphism preventing continuous exchange with calcite.