Carbon isotopes in pelites of the Precambrian Uncompahgre Formation, Needle Mountains, Colorado

Carbon isotopic ratios and weight percentages of carbon were measured in 15 samples of slate, phyllite, and schist of the approximately 1500- to 1600-m.y.-old Uncompahgre Formation of the Needle Mountains, southwestern Colorado. Rocks with less than 1 percent total carbon, all of which is reduced, have δC¹³ values of −23 to −28 per mil, whereas those with 1 to 6.4 percent carbon have δC¹³ from −29 to −31 per mil. In general, the slates and phyllites contain more carbon and isotopically lighter carbon than do the schists of higher metamorphic rank. Increasing loss of C¹²-enriched methane with increasing intensity of metamorphism is suggested to account for these differences.