Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

M. Santosh ¹, D. H. Jackson ², D. P. Mattey ², N. B. W. Harris ²

Affiliations
1 Centre for Earth Science Studies, P. B. 7250, Akkulam, Trivandrum 695031, IN
2 Department of Earth Sciences, Open University, Milton Keynes, MK7 6AA, Buckinghamshire, GB

Abstract

Carbon dioxide-rich inclusions commonly occur in banded charnockites and khondalites of southern Kerala as well as in incipient charnockites formed along oriented zones by desiccation of gneisses. Combined pressure-temperature data from solid phases and CO₂ equation of state barometry from fluid inclusion isochores depict that high density carbon dioxide was the dominant ambient fluid species in all the three rocks. Carbon stable isotope analysis of the fluids using a stepped heating technique in samples collected from Ponmudi, Kottavattom, Manali and Kadakamon show that the banded charnockites and gneisses contain ca. 50-60ppm carbon with δ¹³C values in the range of -8 to -12.3‰. The incipient charnockites are characterised by abundant CO₂ (upto 200 ppm) with δ¹³C in the range, -7.5 to -10.3‰. The carbon stable isotope value of + 1.2‰ obtained for calc-silicate is in ccntrast to the value for the intercalated banded charnockite from Kadakamon (-10‰) depicting that decarbonation was not the source of CO₂, Whereas the heavier carbon-enriched nature of incipient charnockite from Manali may suggest a juvenile source, the carbon isotope values in Ponmudi and Kottavattom suggest a mixed source, possibly due to the interaction of externally derived CO₂ with graphite in the rocks.

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