Stable carbon isotopes in selected granitic, mafic, and ultramafic igneous rocks

Abstract

Noncarbonate (combustion) and carbonate (acid decomposition) carbon were separately analyzed in 18 granitic rocks from a group of related Tertiary intrusions near Crested Butte, Colorado, and 14 mafic and ultramafic rocks from various localities in the western United States. Among the granites, carbonate carbon ranges from nil to 0.76 per cent with δC 13-values from −5.6 to − 9.0‰ (vs PDB); noncarbonate carbon varies from 32–360 ppm with δC 13-values from −19.7 to −26.6‰, The mafic and ultramafic rocks have carbonate carbon contents ranging from 53 ppm to about 2 per cent with δC 13-values from + 2.9 to −10.3‰; noncarbonate carbon varies from 26 to 150 ppm with δC 13-values of −22.2 to − 27.l‰ For these samples, carbonate carbon ranges from 12.0 to 29.4‰ heavier than coexisting noncarbonate carbon. This consistent difference between δC 13 of carbonate and noncarbonate carbon may be an isotopic fractionation effect. Because the specific indigenous form of noncarbonate (combustion) carbon is in doubt, conclusive interpretations regarding isotopic equilibration and fractionation cannot be made. These results have bearing on the assessment of the isotopic composition of mantle carbon and consequently are germane to the question of the origin (source) and history of crustal carbon. If mantle carbon is isotopically similar to noncarbonate (combustion) carbon, i.e. δC 13-values from −19.7 to − 27.1‰, then a simple mantle degassing source for crustal carbon is improbable. Such a result would indicate an additional source of crustal carbon such as from a primitive atmosphere or extra-terrestrial accretion.