Ethane- and hydrogen-bearing carbonic fluid inclusions in a high-grade metamorphic rock

Abstract

Ethane- and hydrogen-bearing carbonic fluid inclusions are found within quartz in a leucocratic garnet granulite obtained from the Neoarchean Limpopo Belt in South Africa. The trapped fluids are present as secondary inclusions and show melting temperatures in the range of -60.9 to -56.9 °C, suggesting a dominantly CO2-rich composition. The wide range of estimated CO2 densities (0.611-1.020 g/cm3) indicates entrapment of the fluids at 5 kbar at 700 °C and significant subsequent density decrease due to post-peak decompression. Laser Raman analysis of the inclusions at room temperature confirmed that the trapped fluids are CO2-rich (>95 mol%) with very minor CH4 (2.5-4.6 mol%), N2 (0.1-0.4 mol%), and C2H6 (0.01-0.02 mol%) constituents. A thin carbon film precipitated on the cavity wall, not visible optically, was also identified by Raman spectroscopy. We therefore infer the following carbon-forming reaction during decompression and/or cooling below 700 °C and 5 kbar; CO2 + CH4 → 2C + 2H2O. When the inclusions were heated to 150 °C to homogenize all the fluid phases, H2 (0-0.05 mol%) and H2O (1.7-0.3 mol%) were detected instead of C2H6. Although the processes of the formation of C2H6 and H2 are not known, the presence of CH4, C2H6 and H2 in CO2-dominant fluid is considered as a possible product of a series of reactions in the C-O-H system within the inclusion cavity during decompression/cooling.