A 3.75 GA SEA-FLOOR HYDROTHERMAL SYSTEM FROM THE ISUA GREENSTONE BELT, WEST GREENLAND

Carbon isotope studies from the Isua greenstone Belt, west Greenland suggest that life on Earth had emerged by 3.75 Ga (Mojzsis et al. Nature, 1996; Rosing, Science, 1999). Here we report petrological and fluid inclusion evidence for the existence of a sea-floor hydrothermal system in this greenstone belt.

Our data come from a well preserved pillow lava sequence in a low-strain zone in the north eastern part of the greenstone belt. Pillow lavas and pillow breccias are altered to quartz mica schists and contain abundant quartz veins and quartz globules. All the rocks in this area are metamorphosed to amphibolite grade. Quartz mica schists with well preserved pillow breccia textures are interpreted as hydrothermally altered basalts, later metamorphosed to amphibolite grade. Quartz veins and globules in the pillow breccia predate the metamorphic fabric, although were annealed and/or recrystallised during the metamorphism. The quartz globules are thought to be infilled vesicles. Some preserve textures reminiscent of the drusy lining of geodic quartz and these textures are thought to be primary and contemporaneous with the hydrothermal alteration.

Despite later annealing, some quartz crystals preserve rare, isolated fluid inclusions. They record two independent fluid systems, which have compositions of pure methane and a highly saline aqueous fluid (25 wt % NaCl), respectively. Metamorphic conditions were calculated from pelitic sediments intercalated with the pillow breccia as 460-480 oC, ~4kb. The measured fluid compositions from the fluid inclusions are difficult to reconcile with their genesis during the amphibolite grade metamorphism. Rather, they bear a strong resemblance to present-day sea-floor hydrothermal fluids. Thus we interpret our findings to represent the remnant of a sea-floor hydrothermal system responsible for the alteration of the pillow breccias and the co-precipitation of quartz and calcite. We believe that our results from Isua represent the oldest known example of a terrestrial sea-floor hydrothermal system.