Methane in Fluid Inclusions from Granulites: A Product of Hydrogen Diffusion?
Geochim Et Cosmochim Acta 1990 54:641-651
Many fluid inclusion studies of granulite grade rocks reveal the presence of co2-rich inclusions that appear to have been trapped near the peak of metamorphism. Final melting temperature of co2 [tm(co2)] reported for these inclusions are often below the co2 triple point of -56.6 Deg. C and some are below -60 deg. 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. We have modeled variation in the fugacities of molecular fluid species in inclusion and matrix fluids during uplift from 6 kb and 800 deg. C, assuming hypothetical uplift pressure-temperature paths that are concave (t-concave) or convex (t-convex) toward the temperature axis. For rocks buffered at fo2 within one log unit of the fayalite- magnetite-quartz equilibrium (fmq +- 1), most uplift paths result in external fh2 overpressures of bars to tens of bars at t>400 deg. C. Previous workers have postulated that co2-rich fluid inclusions in granulites could originate from loss of h2o from h2o-co2 inclusions in response to an fh2o gradient between the inclusion and matrix fluids. Our results suggest that for fluids buffered by fmq +- 1, this is possible only if (1) uplift is t-convex and the matrix fluid composition remains nearly constant or (2) the matrix fluid evolves toward relatively h2o-poor compositions. The latter could occur if influx of co2-rich fluids occurs during uplift.
OP; Final Contract Report;
Geochim. Et Cosmochim. Acta, V. 54, 1990. PP. 641-651
Univ. of CA.--Riverside