Preserved Flora and Organics in Impact Melt Breccias: Implications for Capturing Past Life on Mars

At least seven impact glass-bearing deposits have been documented in the Argentine stratigraphy, each recording separate events between the Holocene and late Miocene [1,2]. Detailed evidence for their origin by impact includes: planar deformation features (PDFs in quartz, feldspars, pyroxene, olivine, etc.), asymmetric isotropization (i.e., alternate-twin deformation) in plagioclase, diaplectic phases, ultra-high temperature melting (e.g., lechatelierite, molten rutile) and decomposition (e.g., baddeleyite), and quench textures around minerals, e.g., beta-crystobalite, etc. [1-3]. Incorporation of materials from depth indicates that this was not an airburst but a series of crater-forming impacts. Many hand samples also contain relicts of extant biota. Scanning electron microprobe (SEM) images reveal preservation of delicate forms including: striated layers between vesicular impact glass and parallel vein-like features at higher magnification. The striated patterns resemble vascular bundles of the mesophyll (ground tissue) of a plant. Identifiable parts of the plant anatomy, e.g. papillae and cell walls, contain skeletal magnetite crystals and high-temperature, i.e., phases indicating that vitreous fossilization occurred at extremely high temperatures and rapid quench rates. The morphology is generally similar to contemporary regional grasses (pampas grass) including small spherules (papilla). The intricate forms (20nm to 20mm) indicate features rapidly preserved rather than simple impressions. Reaction zones (vesiculation and quenched minerals) along the interface between the melt and entrained plants indicate rapid quenching. Compositional mapping reveals the presence of insignificant levels of carbon, but chemical analyses confirm the high silica content (> 60%). Various analytical techniques (micro-Raman, TEM/SEM, and μltra-L2MS) further reveal the preserved organic materials, including tetracyclic pyrrolines, essential members of the group of porphyrin species that are produced through the thermal degradation of chlorophyll with heterogeneous survival of abundant C and N. The survival and preservations of organics within hydrous pockets in rapidly quenched impact glasses may provide a new strategy for identifying biomarkers of possible early life on ancient Mars. Impact melt could encapsulate and preserve this record. It is likely that the porous nature of the target (loess), which characterizes much of the Martian surface, would ensure flash-heat preservation under highly reducing conditions [4]. [1] Schultz, P. H. et al. (2004), EPSL, v. 219, 221-238; [2] Schultz et al. (2006), Schultz, P. H. et al. (2006), MAPS, v. 41, 749-771; [3] Harris, R. S. and Schultz, P. H. (2007), GSA abstracts 39, 371 [4] Harris, R. S., and Schultz, P. H. (2007), LPSC 38, no. 2306.