Many fluid inclusion studies of granulite grade rocks reveal the presence of CO2rich inclusions that appear to have been trapped near the peak of metamorphism. Final melting temperatures of CO2 [Tm(CO2)] reported for these inclusions are often below the CO2 triple point of -56.6°C, and some are below -60°C. This freezing-point lowering is usually attributed to the presence of a second volatile component, such as CH4, and the presence of CH4 has been confirmed in some cases by Raman or mass spectroscopy. A CO2 melting temperature near -56.6°C is commonly offered as evidence that the inclusions contain "nearly pure CO2". However, significant amounts of CH4 may be present but cause seemingly insignificant freezing point depressions. C-O-H fluid speciation calculations for conditions representative of granulite facies metamorphism indicate that CH4 may comprise a significant portion of peak metamorphic fluids when graphite is present, but it is never a significant species in CO2-rich fluids. In most cases the amount of CH4 should be virtually undetectable with microthermometric or spectroscopic techniques. Only in aqueous fluid inclusions can CO2 and CH4 both be significant species. Post-trapping speciation changes to fluid inclusions at constant mass cannot account for the reported compositions unless graphite precipitates in the inclusions. Thus, the observed compositions require post-trapping compositional changes due to loss or gain of components. We have modeled variations in the fugacities of molecular fluid species in inclusion and matrix fluids during uplift from 6 kb and 800°C, assuming hypothetical uplift pressure-temperature paths which are concave or convex toward the temperature axis (T-concave and T-convex, respectively). Our results suggest that for rocks buffered at f{hook}O2 within one log unit of the fayalite-magnetite-quartz equilibrium (FMQ ± 1), most uplift paths result in external f{hook}H2 overpressures of bars to tens of bars at temperatures > 400°C. The highest overpressures are generated during T-concave uplift. Compositional changes resulting from equilibration of such gradients, via diffusive addition of hydrogen to peak metamorphic fluid inclusions and concomitant reduction of CO2 by conversion to CH4 and H2O, are consistent with the compositions of fluid inclusions reported from granulite terranes. Previous workers have postulated that CO2-rich fluid inclusions in granulites could originate from post-trapping diffusive loss of H2O from H2OCO2 inclusions in response to an f{hook}H2O gradient between the inclusion and matrix fluids. The results of the present study suggest that for fluids buffered by FMQ ± 1 this is possible only if 1. (1) uplift is T-convex and the matrix fluid composition remains nearly constant, or 2. (2) the matrix fluid evolves toward relatively H2O-poor compositions. The latter could occur if influx of CO2-rich fluids occurs during uplift. © 1990.
