2.76 GA SUBMARINE HYDROTHERMAL SYSTEM ASSOCIATED WITH LIFE IN THE MT. ROE BASALT, PILBARA, AUSTRALIA

2.76 Ga Mt. Roe basalt in the Pilbara craton, Australia, erupted terrestorially in large part, but the minor evidences of pillow lava, clastic sediments, and evaporite of calcite suggest that the area has occasionally descended to shallow submarine environments. Hyderothermal alteration system, diaspore-pyrophyllite-rutile-phosphate veins in sericite zone enveloped by chlorite zone, has developed on the Mt. Roe basalt beneath thin crastic sediments and evaporites. This hydrothrmal system is characterized by shallow depth (<50m), high salinity (10-15wt% NaCl equiv.), existence of organic carbon both in the veins and in the overlying sandstone, enrichment of so-called "insoluble elements", such as Al, Ti, Hf, Zr and REE, reduced solution with CH4 and without CO2, and similar age of the veining and basalt eruptions. These evidences suggest that the hydrothermal system had developed under the subsequent submarine condition after the eruption of basaltic lava and tuff. We examined the features of organic matters extracted from vein and sandstone. The H/C and N/C atomic ratios are 0.05-0.1 and 0.01-0.015, respectively. There is no difference in ratios between sandstone and vein. On the other hand, the carbon isotopic ratio ranges from -48 to -40 permil for the carbon extracted from sandstone, and from -42 to -26 permil from hydrothermal veins. As methane gas is observed in the fluid inclusions in the hydrothermal veins, these values suggest that methanogene has lived in the underground and methanotroph has lived on sea floor when the hydrothermal system functionated. Methanotroph needs oxygen, which might be expected to dissolved in sea water.