Bioactivity at Shallower Depth in 2.77 Ga Alteration of Mt. Roe Basalt, Pilbara, Western Australia

The sericite-chlorite zone of the 2.77 Ga Mt. Roe basalt near Whim Creek, Pilbara, Western Australia, is a product by hydrothermally alteration, not paleosol formed under anoxic atmosphere. The zone is interesting in the standpoint of biological activity at late-Archean era, as it has been reported that the black veins in alteration zone and the sediment above it are characterized by the organic carbons with d13C values of methanogen and methanotroph. A fresh and consecutive core of Mt Roe basalt including alteration zones, which was drilled by ABDP (Archean Biosphere Drilling Project), gives new and more detailed insight into this concern. The core is ca. 300 m long, and is composed of amygdaloidal/massive basaltic lavas and tuffs with clastic sediments interbedded. About seventy selected samples were studied mineralogically using microscope, XRD, XGT, ESCA and EPMA, and were analyzed chemically for major and trace elements using XRF and ICP-mass. The results show that (1) the core is composed of at least three lava/tuff units with different Ti/Zr and Zr/Hf ratios which are separated by thick sedimentary units, (2) the sedimentary units are comprised of sandstone, siltstone and shale, all of which are black-colored and rich in organic carbon, (3) the strongly sericitized horizons of several meters were recognized just below these sedimentary units, and thin sericitized layers are interbetted in amygdaloidal basalt and tuff horizons, the major part of which is associated with the fine-grained, black and organic carbon-rich quartz vein, (4) in some parts of sericitized zone, sulfide, Zn-rich and Ti-REE veins were observed, (5) all of the sericitized zone is characterizes by the depletion of Fe, Mg, Ca, Na and P and the enrichment of K, (6) several other kinds of veins, such as quartz, calcite, and chlorite, distribute across the basalt horizon, (7) organic carbons in black vein show d13C values of -40 to -50 per mil (PDB), and (8) methane was the only gas detected from the fluid inclusions in quartz vein. These observations suggest the followings, (1) there might be at least two stages of alteration; (2) the first one could be responsible for the alteration of plagioclase to calcite and mafic minerals to chlorite, for the calcite and chlorite veins, and for filling up amygdules; (3) the second one might relates to submarine hydrothermal activity at shallower depth, (4) the hydrothermal fluid might circulate through depositing sediment and top of erupted basalt/tuff, (5) methanogene could inhabitate in hydrothermal vein producing methane and made the fluid strongly reducing, and (6) methanotroph could inhabitate around seafloor. As the black hydrothermal vein was not observed in uppermost massive lava unit, the timing of this alteration would have been essentially concurrent with the volcanic activity. This is concordant with the previously reported SHRIMP age of around 2.8 Ga for euhedral apatite minerals in hydrothermal vein.