Biosignatures in Terrestrial Altered Volcanic Rocks — Focus on Nitrogen as a Key Biogeochemical Tracer

Here we synthesize work conducted at Lehigh University and the Pheasant Memorial Laboratory in Misasa, Japan (Institute for Planetary Materials, Okayama University), focusing on nitrogen (N) behavior in altered basaltic glasses and related secondary minerals that serve as terrestrial analogs for Martian surface/subsurface alteration. Initial proof of concept work demonstrated N enrichment in aqueously altered seafloor volcanic glasses with biotic influence suggested by δ¹⁵N signatures and microtubular textures (Bebout et al., 2018). Recently, this approach has been applied to study of hyaloclastites from Antarctica and Iceland that serve as better analogs for Martian hydrothermal alteration processes. This pursuit, employing advanced microanalytical and microscopic techniques, has extended knowledge of the modes of incorporation and isotopic signatures of N as a valuable tracer of biogeochemical processes in such materials (Nikitczuk et al., 2022a,b).

 

In new studies, we have investigated Icelandic amygdules in altered basalts that are mineralogical and geochemical analogs for those on the Noachian Mars surface (Ehlmann et al., 2012; Weisenberger and Selbekk, 2009). In addition, we examined erupted basaltic tephra from Surtsey Island, Iceland which, together with the amygdules, provide records of the alteration of very young erupted mafic volcanics (for Surtsey, <50 years; Jackson et al., 2019). These studies combine N concentrations and isotope compositions with microscopic and microanalytical techniques (SEM, SIMS, XRD, XRF), other isotopic tracers (δ¹³C, δD, δ¹⁸O) and organic geochemistry (GC-MS and Orbitrap work ongoing).

 

Collectively, our work demonstrates ubiquitous N enrichment of one to two orders of magnitude beyond initial concentrations of unaltered equivalents (MORB and OIB), during aqueous alteration of basaltic glass and associated secondary phases. Alteration phases include palagonite and clay, composed mainly of phyllosilicates (e.g., celadonite, illite, chlorite, smectite, saponite, nontronite among others) and zeolites (e.g., analcime, phillipsite, mesolite/scolecite, heulandite, stilbite, thomsonite and chabazite), amorphous silica (e.g., opal) and sulfates (e.g., jarosite and alunite), with enrichment most likely occurring during very early stages of aqueous alteration. Furthermore, their textural features (granular and tubular), trace element abundance, isotopic signatures (δ¹⁵N and δ¹³C) and organic chemistry (presence of n-alkanes and fatty acids with short C chains) indicate the likelihood of past microbial activity and incorporation of bioprocessed N.

 

Through this comprehensive approach, we highlight aqueously altered basaltic rocks and their associated phases, as high-priority targets for biosignature exploration, with a specific focus on N, in alignment with Mars Exploration Program Analysis Group (MEPAG) science goals.  

 

References: Bebout et al. (2018) Astrobiology; Nikitczuk et al. (2022a) Astrobiology; Nikitczuk et al. (2022b) Journal of Geophysical Research: Planets; Ehlmann et al. (2012) Journal of Geophysical Research: Planets; Weisenberger and Selbekk (2009) International Journal of Earth Sciences; Jackson et al. (2019) Scientific Drilling.