نبذة مختصرة : Hydrovolcanic tuff deposits contain abundant, hydrothermally-altered glass and act as a habitable niche for microorganisms. Similar environments identified on Mars are considered potential targets for astrobiological investigation. Tuffs sometimes contain evidence of possible microbially-mediated alteration, including microtubules (~1 µm diameter and ~10 – 20 µm length) and granular alteration in glass. These microscopic textures are considered putative biosignatures based upon their morphology, size distribution, and geochemical characteristics. In this thesis, a database of volcanic fields with characteristics relevant to Mars and with potential to preserve putative biosignatures in glass is presented. The database contains 45 volcanic fields distributed globally. At seven sites, there is published evidence of microbial biosignatures in the glass. The remaining volcanic fields were compared to these biosignature-bearing sites and Martian volcanic rocks and 11 are recommended as high-priority targets for future investigation. Two sites were explored in-depth. We present the first report of putative biogenic alteration in glass from the Pliocene-Pleistocene hydrovolcanoes of Western Snake River Plain, Idaho, USA. New biosignatures from the Fort Rock Volcanic Field, Oregon, USA are also provided. Samples collected from tuffs erupted into paleo-Lake Idaho and paleo-Fort Rock lake have glass which has partially altered to palagonite, a low-temperature alteration product. Some samples contain microtubules and granular alteration in the glass. Optical microscopy and µXRD were used to determine the textural relationships between fresh glass, glass containing microtubules, palagonite, and other alteration minerals in void spaces (calcite, zeolites). These textural relationships indicate that tubules formed under low-temperature hydrothermal conditions during palagonitization, amidst alteration mineral precipitation. Geochemical measurements and element mapping with an electron microprobe were used to compare ...
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