Putative biosignatures in hydrovolcanic tuffs from a Mars-relevant environment: Western Snake River Plain volcanic field, Idaho

Abstract

Hydrovolcanic tuffs formed through magma-water interactions in lacustrine or littoral environments have been found to contain micro-textural elements, including microtubules and granular textures, within glass fragments. The morphological and associated geochemical characteristics of these micro-textures show evidence that they are formed by microbially-mediated dissolution of basaltic glass in an aqueous setting, establishing them as likely biosignatures. Further, the terrestrial setting of these samples and their basaltic composition is analogous to environmental conditions expected to have existed throughout the history of Mars.

We present the first report of putative biogenic alteration textures in basaltic glass from the Pliocene-Pleistocene hydrovolcanoes of the Western Snake River Plain volcanic field, Idaho, USA. Samples collected from tuff cones, tuff rings, and maars which erupted into paleo-Lake Idaho contain microtubules and granular alteration comparable in morphology (diameter, length, curvature, internal contents, branching) and size distribution to those recorded in other terrestrial hydrovolcanic fields. Petrographic relationships between fresh glass containing microtubules, glass that has been fully altered to palagonite, and mineral-filled vesicles and fractures indicate that tubules formed under low-temperature aqueous alteration conditions during the palagonitization process and ongoing alteration mineral precipitation. Alteration minerals including calcite and zeolites show that these fluids likely did not exceed 80 °C in temperature. Major element mapping of vitric clasts in hydrovolcanic tuff revealed most elements are depleted or neutral within microtubule interiors relative to the surrounding glass, although iron and titanium were notably enriched in many tubules. We found little distinction in major element composition between microtexture-bearing and non-microtexture-bearing glasses, with the exception of some microtexture-bearing samples being depleted in Na₂O compared to the non-microtexture-bearing glass. Western Snake River Plain is relevant to Mars in terms of geochemical, mineralogical, and environmental context; and this setting is capable of preserving putative biosignatures. This work has implications for furthering our understanding of how these microtextures are formed and preserved, and for potential astrobiological investigation of Mars.