Carbonic fluid inclusions in ultrahigh-temperature metamorphic rocks from Tonagh Island in the Napier Complex, East Antarctica: a preliminary report

Tsunogae, T., Santosh, M., Osanai, Y., Owada, M., Toyoshima, T., Hokada, T., Crowe, W.A., and Miyano, T. (2001)@Polar Geoscience, 14, 25-38.


Abstract

We report here the occurrence of high density CO2-rich fluid inclusions in ultrahigh-temperature metamorphic rocks from Tonagh Island of Archean Napier Complex, East Antarctica. A study on the different lithological units from Tonagh Island (garnet gneiss, sapphirine granulite, garnet-orthopyroxene gneiss, and magnetite-quartz gneiss) shows the common presence of carbonic fluid inclusions entrapped within various minerals. The melting temperatures of fluids in the first three rock types lie in the range of -56.3 to -57.2°C, close to the triple point for pure CO2 (-56.6°C). Fluid inclusions in magnetite-quartz gneiss show slight depression in their melting temperatures (-56.7 ~ -57.8°C) suggesting traces of additional fluid species such as CH4 and/or N2 in the dominantly CO2-rich fluid. Homogenization of pure CO2 inclusions in the garnet gneiss, sapphirine granulite, and garnet-orthopyroxene gneiss occurs into the liquid phase at temperatures in the range of -34.9 ~ +4.2°C. This translates into high CO2 densities in the range of 0.9-1.1 g cm-3. The estimated CO2 isochore intersects the counterclockwise P-T trajectory of Tonagh Island at around 6-9 kbar at 1100°C, which corresponds to the peak metamorphic conditions of Tonagh Island derived from mineral phase equilibria. We therefore infer that CO2 was the dominant fluid species present during the ultrahigh-temperature metamorphism in Tonagh Island, and interpret that the fluid inclusions preserve traces of the syn-metamorphic fluid. The stability of anhydrous minerals such as orthopyroxene in the study area might have been effected by the lowering of H2O-activity through the influx of CO2 at peak metamorphic conditions (>1100°C).