We are conducting a pilot study on the hydrothermal alteration of lavas at Lassen Volcanic National Park as an analog for potential hydrothermal deposits on Mars. Lassen has hot springs and fumaroles that have altered its lavas into silica, sulfates, and phyllosilicates, all mineral types also identified on Mars. Hydrothermal environments were likely common on Mars (due to evidence of early aqueous and a long record of volcanic activity), and such environments could have remained habitable long after the surface cooled and desiccated. However, some hydrothermal environments are more habitable than others, and being able to distinguish between the deposits of hostile acid-sulfate fumarole and more accommodating near-neutral hot spring environments can provide clues to habitability. Lassen hydrothermal environments produce silica by both acid-sulfate leaching and precipitation from neutral hydrothermal waters, both of which have been suggested as potential origins for deposits in Columbia Hills of Gusev Crater on Mars. ObjectivesThe objectives of this study are to:1. Determine the mineralogical and geochemical patterns of lava alteration associated with acid-sulfate leaching at Lassen hot springs and fumaroles, and contrast them with the patterns associated with more neutral, Cl-rich hydrothermal fluids at fumaroles and hot springs within Lassen park. 2. Compare these results to the mineralogy and geochemistry of hydrothermal deposits observed in the Columbia Hills of Gusev Crater, and determine if either scenario is more consistent with the observations. 3. Determine what mineralogical and geochemical evidence for hydrothermal alteration is most likely to be preserved in the rock record by analyzing samples from the hydrothermally altered lavas of Pleistocene Brokeoff Volcano (in Lassen park) and comparing them to samples from the active fields.