Nitrogenous Altered Volcanic Glasses as Targets for Mars Sample Return: Examples From Antarctica and Iceland

Mars exploration is focused on seeking evidence of habitable environments and microbial life. Terrestrial glassy basalts may be the closest Mars-surface weathering analog and observations increasingly indicate their potential to preserve biogeochemical records. The textures, major and trace element geochemistry, and N concentrations and isotopic compositions of subaerial, subglacial and continental lacustrine hyaloclastites from Antarctica, Iceland, and Oregon, respectively, were studied using micro-imaging and chemical methods, including gas-source mass spectrometry. Alteration by meteoric-sourced waters occurred in circum-neutral, increasingly alkaline low-temperature conditions of ∼60°C–100°C (Iceland) and ∼60°C–170°C (Antarctica). Incompatible large ion lithophile element (LILE) enrichments compared to mid-ocean ridge basalt (MORB) are consistent with more advanced alteration in Antarctic breccias consisting of heulandite-clinoptilolite, calcite, erionite, quartz, and fluorapophyllite. Granular and tubular alteration textures and radial apatite represent possible microbial traces. Most samples contain more N than fresh MORB or ocean island basalt reflecting enrichment beyond concentrations attributable to igneous processes. Antarctic samples contain 52–1,143 ppm N and have δ¹⁵Ν_air values of −20.8‰ to −7.1‰. Iceland-Oregon basalts contain 1.6–172 ppm N with δ¹⁵Ν of −6.7‰ to +7.3‰. Correlations between alteration extents, N concentrations, and concentrations of K₂O, other LILEs, and Li and B, reflect the siting of secondary N likely as NH₄⁺ replacing K⁺ and potentially as N₂ in phyllosilicates and zeolites. Although much of the N enrichment and isotope fractionation presented here is not definitively biogenic, given several unknown factors, we suggest that a combination of textures, major and trace element alteration and N and other isotope geochemical compositions could constitute a compelling biosignature in samples from Mars' surface/near-surface.