ARCHEAN CYCLES OF SUBAQUEOUS VOLCANIC ERUPTION, HYDROTHERMAL ALTERATION, AND MICROBIAL COLONIZATION

Sub-greenschist-facies metabasites from the ca. 2701 Ma Blake River Group of the Abitibi Greenstone Belt (AGB) preserve evidence of ancient microbial life in the form of mineralized ichnofossils. Flow units at the Hurd Property (Harker Township, Ontario) repeat in a classic subaqueous eruption pattern: massive holocrystalline waning to lobate and hyaloclastite flows. Two complete cycles occur at the outcrop, and ichnofossils appear in hyaloclastite samples from the top of each. Pervasive hydrothermal alteration in a low temperature (~200ºC), flank-like system replaced many of the primary igneous minerals with an albite-chlorite-quartz-calcite±magnetite±actinolite±augite±hornblende assemblage, as identified petrographically and from bulk rock XRD. Palagonite alteration, observed along rims of volcanic glass shards in modern altered ocean crust (AOC), is preserved in the hyaloclastite samples, but fresh glass is absent and replaced by chlorite. Whole-rock H₂O contents are as high as ~5 wt. %, indicating significant hydration.

For non-hyaloclastite samples, including chlorite-poor massive flows, concentrations of major oxides, and low Zr/Y (~3.7), suggest a tholeiitic andesite protolith. The hyaloclastites are variably enriched in K₂O, Rb, Sr, SiO₂, and Mn, compared with the mean composition of the massive flows. Hyaloclastite δ¹⁸O_silicate values are more variable than those of corresponding massive flows and are elevated relative to values for fresh modern andesite. These trends suggest extensive low-temperature fluid-rock interaction in the more permeable units, as observed for modern seafloor processes. Overall ranges of N concentration (1-6 ppm) and δ¹⁵N_Air (-4 to +7‰) for AGB samples fall within ranges exhibited in modern AOC and interpreted as resulting from a transfer of sedimentary/organic signatures in pore waters (Li et al., 2007).

Volcanic rocks at the Hurd Property likely erupted in an intermediate period of arc construction. Microbial organisms were hosted within the glass shards of the hyaloclastite flows, whose higher permeability allowed hydrothermal fluid circulation and microbial access. Progressive eruption events sealed permeable layers with new massive holocrystalline flows and deposited new hyaloclastites for microbial colonization.