Methane in fluid inclusions from granulites: A product of hydrogen diffusion?

Abstract

Many fluid inclusion studies of granulite grade rocks reveal the presence of CO 2rich inclusions that appear to have been trapped near the peak of metamorphism. Final melting temperatures of CO 2 [Tm(CO 2)] reported for these inclusions are often below the CO 2 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 CH 4, and the presence of CH 4 has been confirmed in some cases by Raman or mass spectroscopy. A CO 2 melting temperature near −56.6°C is commonly offered as evidence that the inclusions contain “nearly pure CO 2”. However, significant amounts of CH 4 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 CH 4 may comprise a significant portion of peak metamorphic fluids when graphite is present, but it is never a significant species in CO 2-rich fluids. In most cases the amount of CH 4 should be virtually undetectable with microthermometric or spectroscopic techniques. Only in aqueous fluid inclusions can CO 2 and CH 4 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 ƒO 2 within one log unit of the fayalite-magnetite-quartz equilibrium ( FMQ ± 1), most uplift paths result in external ƒH 2 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 CO 2 by conversion to CH 4 and H 2O, are consistent with the compositions of fluid inclusions reported from granulite terranes. Previous workers have postulated that CO 2-rich fluid inclusions in granulites could originate from post-trapping diffusive loss of H 2O from H 2OCO 2 inclusions in response to an ƒH 2O 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 H 2O-poor compositions. The latter could occur if influx of CO 2-rich fluids occurs during uplift.