Carbonic fluid inclusions in sapphirine + quartz bearing garnet granulite from the Limpopo Belt, southern Africa

Abstract

Sapphirine + quartz bearing garnet granulite from the Central Zone of the Neoarchean Limpopo Belt contains three categories of fluid inclusions: dominant pseudosecondary inclusions in garnet and plagioclase, secondary inclusions in matrix quartz, and least dominant primary inclusions in the same quartz grains. The melting temperatures of the trapped fluids lie in the range of −58.6 to −56.4 °C. Homogenization of the pseudosecondary inclusions into the liquid phase occurs within the temperature range of +9.1 to +28.4, which correspond to CO2 densities of 0.72 to 0.87 g/cm3. In contrast, primary inclusions homogenized at +6.1 ± 3.1 °C, which translates into slightly higher densities of 0.87-0.91 g/cm3. The fluid densities, when computed into isochores, indicate entrapment of CO2 at lower pressure than the peak P-T condition of the rock. The results imply either influx of CO2-rich fluids during post-peak uplift or density reversal of peak metamorphic carbonic fluid. Lack of aqueous fluid inclusions and abundant pseudosecondary carbonic inclusions in the rock confirmed that low-H2O activity condition is necessary to form unique sapphirine + quartz assemblage.