Volatiles associated with alkaline igneous rift activity: Fluid inclusions in the Ilímaussaq intrusion and the Gardar granitic complexes (south Greenland)

Abstract

Fluid inclusions in minerals from Si-oversaturated and Si-undersaturated alkalic igneous complexes belonging to the Precambrian Gardar Rift Province in South Greenland contain distinctly different carbon-bearing gases: granites and quartz syenites are characterized by CO 2 and only minor CH 4 whereas agpaitic nepheline syenites are dominated by hydrocarbons (essentially CH 4) and contain only insignificant amounts of CO 2 and/or CO. In the alkaline Ilímaussaq Intrusion the hydrocarbon-rich gases may well reflect equilibration with graphite at magmatic temperatures and at oxygen fugacities approaching those defined by the synthetic MW buffer reaction. In general, formation of hydrocarbon gases in agpaitic nepheline syenites is considered to reflect the fulfillment of three basic conditions: (1) low oxygen fugacities; (2) a wide temperature range of crystallization and a low-temperature solidus; and (3) retention of volatiles during crystallization. The characteristic carbon-bearing vapour associated with continental alkaline igneous activity is, however, considered to be CO 2.