Stable isotope characteristics of hydrocarbon gases in the alkaline Ilímaussaq complex, South Greenland

Abstract

Fluid inclusions in minerals from agpaitic nepheline syenites and hydrothermal veins in the alkaline Ilímaussaq complex (South Greenland) are rich in hydrocarbon gases (predominantly methane), reflecting very low oxygen fugacity conditions during and after crystallization. In addition, small amounts of solid bituminous matter occur disseminated in the rocks. Analyses of the δ13C and δD compositions of methane from fluid inclusions are presented. The δ13C of methane ranges from — 1.0 to — 5. 1 ‰ PDB and a pronounced trend toward isotopically lighter values is observed with increased magmatic differentiation of the complex. The δD of methane ranges from -139 to -144 ‰ SMOW, whereas the δ13C of higher hydrocarbons, extracted together with the fluid inclusion methane, is invariably higher and scatters more than δ13C values for methane from the same sample. The observed differences between methane and other hydrocarbons indicate isotopic disequilibrium between the different hydrocarbon gas species, and it is suggested that some of the hydrocarbons are derived from condensed organic compounds in the rock. The stable isotope results do not permit any definite conclusions regarding the ultimate origin of the carbon in the hydrocarbon gases. A juvenile origin is considered most plausible, although assimilation of (sodium-rich ?) sedimentary or metamorphic carbonates, in conjunction with condensation to form the higher hydrocarbons and the bituminous materials upon cooling, is also a possibility. The δD compositions of the methane could indicate equilibration with magmatic water.