Abstract
The carbon isotopic composition of natural graphites from rocks belonging to different facies of metamorphism and in magmatic rocks provides evidence for their formation from sedimentary organic matter (mostly of plant origin). In these rocks subjected to regional and contact metamorphism, a preferential mobilization of the light carbon fraction of organic matter may occur, as a result of which the residual organic substance or graphite is enriched in heavy carbon (13C). Besides, during metamorphism of organic matter in carbonate rocks, an enrichment in13C in the organic substance (or graphite) usually takes place due to 13C/12C exchange with the host rock. The principal reason for the consistent slant of δ13C values of graphites in a positive direction seems to be a repetitive (multiple) removal of carbon in the form of isotopically light carbon dioxide during progressive degradation of fossil plant matter contained in the rock.
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