Primary carbonate/CO2 inclusions in sapphirine-bearing granulites from central Sri Lanka

High-density CO2-rich fluid inclusions from a sapphirine-bearing granulite (Hakurutale, Sri Lanka) have been studied by microthermometry, Raman spectr ometry and SEM analysis. Based on textural evidence, two groups of inclusio ns can be identified: primary, negative crystal shaped inclusions (group I) and pseudo-secondary inclusions, which experienced a local, limited post-t rapping modification (group II). Both groups contain magnesite as a daughte r mineral, occurring in a relatively constant fluid/solid inclusion volume ratio (vol(solid) =0.15 total volume). CO2 densities for group I and II dif fer only slightly. Both groups contain a fluid, which was initially trapped at peak metamorphic conditions as a homogeneous (CO2+MgCO3) mixture. Therm odynamic calculations suggest that such a fluid (CO2+15 vol% MgCO3) is stab le under granulite facies conditions. After trapping, magnesite separated u pon cooling, while the remaining CO2 density suffered minor re-adjustments. A model isochore based on the integration of the magnesite molar volume in the CO2 fluid passes about 1.5-2 kbar below peak metamorphic conditions. T his remaining discrepancy can be explained by the possible role of a small quantity of additional water.