Remnants of early Archaean hydrothermal methane and brines in pillow-breccia from the Isua Greenstone Belt, West Greenland

Fluid inclusions containing high-density methane and saline waters (brines), associated with carbonates, have been found in undeformed, annealed quartz-bearing vesicles from pillow-breccia at Isua (West Greenland). Massive quartz veins cementing the pillow fragments contain the same type of carbonate-bearing saline aqueous inclusions as the pillows, but different gaseous inclusions: either trails of low-density methane close to the boundary of the pillow fragment or isolated high-density CO2-rich inclusions, in the centre of the veins. High-density methane and carbonate-bearing aqueous inclusions (brines) are presumed to represent remnants of a sea-floor type hydrothermal alteration system, subsequently re-equilibrated in terms of fluid density during the metamorphic evolution. Low-density methane has been expelled from pillow fragments during post-metamorphic uplift, whereas high-density CO2 probably represents remnants of the peak-metamorphic fluids. Some present-day sea-floor hydrothermal systems, notably the off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30°N, appear to be quite comparable with this hypothetical early Archaean fluid. This setting represents a highly favourable environment for the development of life on Earth.