Carbon isotope composition of graphite and carbonate minerals from 3.8-AE metamorphosed sediments, Isukasia, Greenland

Graphite comprises about 2% of some of the 3.8-AE metamorphosed sedimentary rocks of Isukasia, west Greenland. Delta13Csub(PDB) of carbon in this graphite ranges from -16.0 to -9.3%. For those samples that contain both graphite and siderite, Δsub(carbonate-graphite) is about 6; this fractionation is consistent with an inorganic equilibrium between siderite and graphite at roughly 400-5000C. It is likely that graphite found in thise rocks formed by the reaction: 6FeCO3=2Fe3O4+5CO2+C in which case it is of little help in determining whether or not organisms were active 3.8 AE ago. The presence of quartz-magnetite-cummingtonite-iron formation in the Isukasia metasedimentary sequence may, ultimately, be one of the most powerful environmental indicators remaining in these rocks. (Auth.)