Chemical Alteration and Nitrogenous Biogeochemical Signatures in Altered Volcanic Glasses as Astrobiologically Relevant Targets for Mars Sample-Return

Abstract:

Mars exploration is focussed on seeking evidence of ancient (and modern) habitable environments and microbial life. On Earth, N is tied to life and the formation of secondary minerals during volcanic glass alteration can be mediated by microbes. Many of the same mineral phases occur in variably altered basaltic lithologies on the Mars surface. Altered oceanic basalts on Earth have been shown to preserve organic N isotope signatures and we expand upon this result in a study of terrestrial volcanics. Our goal is to identify a combination of N systematics with other chemical/physical tracers that can serve as a compelling biosignature. We are determining the specific minerals (and other materials such as siliceous hydrothermal deposits) on the Mars surface that could contain useful N records of biogeochemical processes. Textures and major/trace element and N geochemistry of ancient Antarctic basaltic andesitic and modern subglacial and continental lacustrine basaltic hyaloclastites (from Iceland, Oregon) were studied using a combination of in-situ micro-imaging and chemical methods and non-in situ mass spectrometry. Whole-rock hyaloclastites and alteration phases therein (clays, zeolites) have N contents higher than those of fresh MORB/OIB, reflecting enrichment beyond that attributable to igneous processes. Antarctic basaltic breccias are significantly more enriched in N (51.7 to 1143 ppm) and have more negative δ¹⁵N_air (-19.8‰ to -6.1‰) compared to the younger Iceland-Oregon samples (1.6 to 171.9 ppm; -5.7 to +8.2‰). Alteration intensities and contents of Cs, Ba, B, K₂O and Rb are in general correlated with N contents and δ¹⁵N. In Antarctic samples, the most important authigenic hosts for N are heulandite-clinoptilolite, nontronite and possibly chalcedony/quartz. In younger basalts, saponite-nontronite-montmorillonite are the most important N hosts. Positive relationships of N with the LILE indicate that N is sited as structurally fixed NH₄⁺. The Mars atmosphere is ¹⁵N-enriched and lower in its N₂ fraction, compared with Earth’s atmosphere, and recent discoveries of nitrate in Gale Crater confirm that fixed N exists on the Mars surface. NH₄⁺ could be present in alteration phases in glasses, thus the search for biosignatures and assessment of the Mars N budget should in part be focussed on hyaloclastites.