MICRO-ICHNOFOSSILS IN ANCIENT VOLCANIC ROCKS

The earliest ichnofossils may be micron-sized, microbially generated, tubular structures now consisting of titanite observed in the formerly glassy rims and inter-pillow hyaloclastites of Archean pillow basalts. Tubular structures mineralized by titanite have been observed in exceptionally well-preserved 3.5 Ga seafloor basalts from both the Barberton Greenstone Belt, South Africa (BGB) and the Pilbara craton, W. Australia (PWA). These structures are interpreted to have initially formed by microbial etching of the originally glassy material that was subsequently mineralized by titanite. Overlapping metamorphic and magmatic ages from the pillow lavas as well as direct dating of the titanite by in situ laser ablation multi-collector-ICP-MS demonstrates the titanite is of Archean age and implies the microbial alteration process occurred soon after eruption. Comparison of the Archean tubular structures to bioalteration features observed in modern MORB glass shows striking similarities. X-ray mapping reveals carbon along the margins of the tubular structures in both BGB and PWA as in the modern ones. Disseminated carbonates within the microbially altered BGB pillow rims have C-isotope values depleted by as much as -16 per mil (PDB), 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 contrast those of the adjacent crystalline cores and likely have sequestered metabolic CO2 during alteration. The generally low carbon isotope values (<-7) of disseminated carbonates in basaltic glass are attributed to metabolic byproducts formed by oxidation of DOC in pore waters. The low 13-C of the BGB carbonates suggest that oxidative metabolic pathways were utilized by microbes within the early Archean seafloor. Achean microbial inchnofossils preserved in pillow basalts from greenstone belts may have the potential to elucidate not only the presence of early life on Earth but also possibly the conditions under which life began on our planet.