Fluid inclusions in an osumilite-bearing granulite from Bunt Island in the Archean Napier Complex, East Antarctica: implications for a decompressional P-T path?

Tsunogae, T., Santosh, M., Osanai, Y., Owada, M., Toyoshima, T., Hokada, T., and Crowe, W.A. (2003)@Polar Geoscience, 16, 61-75.


Abstract

We report high-density CO2-rich fluid inclusions in garnet, orthopyroxene, and quartz from an osumilite-bearing aluminous granulite from Bunt Island of the Archean Napier Complex, East Antarctica. The melting temperatures of fluids lie in the range of -56.8 to -57.8°C, being close to the triple point for pure CO2 (-56.6°C). Homogenization of the CO2-rich fluids into the liquid phase occurs at temperatures in the range of -35.4 to 24.7°C. This translates into CO2 densities in the range of 0.788~1.084 g/cm3. The estimated CO2 isochore for high-density inclusions in garnet intersects the P-T trajectory of Bunt Island at around 10 kbar at 1050°C, which corresponds to the peak metamorphic conditions of the region derived from mineral phase equilibria. We therefore infer that CO2 was the dominant fluid species present during the ultrahigh-temperature metamorphism in Bunt Island. CO2 inclusions with lower density occurring in quartz and garnet provide isochores that intersect the P-T path at <7 kbar and <950°C, indicating density reversal of originally high-density inclusions along a decompressional exhumation path of the ultrahigh-temperature rocks in the Bunt Island.