Nitrogenous Altered Volcanic Glasses as Targets for Mars Sample Return: Examples From Antarctica and Iceland. Issue 2 (15th February 2022)

• Record Type: Journal Article
• Authors: Nikitczuk, M. P.; Bebout, G. E.; Ota, T.; Kunihiro, T.; Mustard, J. F.; Flemming, R. L.; Tanaka, R.; Halldórsson, S. A.; Nakamura, E.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 2(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-15
• Subjects: hyaloclastites -- Mars -- stable isotopes (nitrogen) -- basalt alteration -- biogeochemistry -- planetary sample‐return; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2021JE007052
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: 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 δ 15 Νair values of −20.8‰ to −7.1‰. Iceland‐Oregon basalts contain 1.6–172 ppm N with δ 15 Ν of −6.7‰ to +7.3‰. Correlations between alteration extents, N concentrations, and concentrations of K2 O, 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. 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°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. Key Points: Antarctic/Iceland glass alteration minerals such as phyllosilicates and zeolites can incorporate and store N during fluid rock interactions Textures, chemistry and N (though here not definitely biogenic) in altered glasses may provide environmental and biogeochemical records Altered hyaloclastites, which are widespread on Mars' surface, should be considered when selecting samples for return to Earth