N2CH4z.sbnd;CO2 fluids during formation of the Dôme de l'Agout,France

The Dôme de l'Agout consists of an ellipsoidal mass of gneisses and migmatites emerging from a large area of chlorite-sericite schists. Fluid inclusions in syn-metamorphic quartz segregations are typically one-phase at room temperature. Analyses by gas chromatography indicate that their main constituents are N₂, CH₄ and CO₂; such compositions are confirmed by freezing studies on individual inclusions. Nitrogen contents of the inclusions range from 2 to 72 mol% and tend to increase with increasing degree of metamorphism. CH₄ and CO₂ show an erratic distribution with the exception of the lowest grade samples, which invariably have very high CO₂ contents. The origin of the nitrogen remains unsolved; however, it seems likely that the nitrogen was not produced by mineral reactions in the presently exposed rocks but came from an external source and moved from the centre of the dome outwards.