INVITED: TRACES OF ARCHEAN MICROBIAL ACTIVITY IN PILLOW BASALTS FROM BARBERTON AND PILBARA

Archean microbial activity can be inferred within the exceptionally well preserved 3.5 Ga seafloor basalts from Barberton, South Africa (BGB) and Pilbara, W. Australia (PWA) by comparison to bioalteration of modern MORB glass. The Archean, formerly glassy rims and inter-pillow hyaloclastites of the lavas contain micron-sized, microbially generated, tubular structures now consisting of titanite. These structures are interpreted to have formed by microbial etching of the original glass and were subsequently mineralized by titanite. Overlapping metamorphic and magmatic dates from the pillow lavas as well as preliminary dating of titanite in one sample imply this process occurred soon after eruption. X-ray mapping shows carbon along the margins of the tubular structures in both BGB and PWA. Disseminated carbonates within the microbially altered BGB pillow rims have C-isotope values depleted by as much as -16 per mil, which is consistent with microbial oxidation of organic matter. In contrast, the crystalline pillow interiors exhibit C-isotope values bracketed between Archean marine carbonate (~0 per mil) and mantle CO2 (-5 to -7 per mil). Carbonates in modern pillow glass margins likewise contrasts those of the adjacent crystalline cores and likely have sequestered metabolic CO2 during alteration. The generally low 13C values (<–7) of disseminated carbonates in basaltic glass are attributed to metabolic byproducts formed by oxidation of DIC in pore waters. A few positive 13C values have been observed in modern MORB. These come from slow-spreading ridges and suggest in those settings lithotrophic utilization of CO2 in which methanogenic Archaea produced CH4 from H2 and CO2. It would seem from the low-13C BGB carbonates that oxidative metabolic pathways were utilized by microbes within the early Archean seafloor. This important inference hinges on the preservation of the C-isotopic signal during devitrification and metamorphism which would be possible only in a closed system. Basalt 18-O analyses shows that pillow fragments with negative carbon isotope values all have near mantle values whereas the pillow interiors are O-18 enriched as by typical seafloor weathering. During the subsequent metamorphism, BGB glasses were not bathed in fluids indicating that their C- isotope values may record Archean activity.