Retrograde methane-dominated fluid inclusions from high-temperature granulites of Rogaland, southwestern Norway

Abstract

Non-aqueous inclusions in the high-grade (800–1000°C; 4 kbar) metamorphic Rogaland region, southwestern Norway, consist of mixtures of CO2-CH4-N2. In particular the fluid inclusions in quartz veins in migmatites are characterized by high CH4 concentrations and they were re-equilibrated at temperatures below 500°C during isobaric cooling. Observations by microthermometry demonstrated several complicated sequences of phase transitions, including partial and metastable homogenization (at lower temperature than melting), and S-L or S-V transitions. The phase behavior reflects a wide variation in composition and molar volume. Fluid compositions were measured by Raman microspectrometry. By this method, also small amounts of graphite have been detected in CO2-CH4 inclusions. The instantaneous formation of graphite in a CO2-CH4 inclusion by induction of the argon laser beam has been observed which points to the metastability of the CO2-CH4 mixture. The calculated densities of the observed fluid mixtures are relatively low, necessitating a revision of the earlier interpretation of these inclusions as containing pure high-density fluids. Inclusions in quartz, trapped after the peak of metamorphism, record fluid heterogeneity which may present evidence for fluid-deficient metamorphism during the retrograde M2-M3 metamorphism.