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 similar to 60 degrees C-100 degrees C (Iceland) and similar to 60 degrees C-170 degrees 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 delta N-15(air) values of -20.8 parts per thousand to -7.1 parts per thousand. Iceland-Oregon basalts contain 1.6-172 ppm N with delta N-15 of -6.7 parts per thousand to +7.3 parts per thousand. 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 N-2 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. Plain Language Summary Finding evidence of past or present life beyond Earth involves identifying physical features and/or chemistries preserved within rocks. Our knowledge of how the signs of life are preserved, and how to identify them, depends on our understanding of Earth biology and how similar rocks on other planetary bodies can preserve such evidence. Volcanic rocks containing natural glass that are subject to chemical modification by waters are relevant to studying alteration at/near the surface of Mars. We have analyzed examples from Iceland and Antarctica to identify textures, minerals, and chemistry, particularly nitrogen, because it is required for all life on Earth, and other elements that can become concentrated in minerals during rock alteration. Minerals formed as a result of rock-water interactions indicate their formation in fluids with pH < 9 and temperatures of <170 degrees C. Relatively high concentrations of nitrogen correlated with those of other elements (i.e., potassium) indicate that nitrogen is stored in these minerals. Although much of the N chemistry here is not definitively the result of biological activity, chemically modified glassy volcanics containing nitrogen-bearing minerals may be useful targets to aim for, especially with other observations, when selecting samples on Mars in the search for ancient or modern life.