PEDOGENIC VERSUS PALAGONITIC ALTERATION OF HYDROVOLCANIC BASALTIC GLASSES: CHARACTERIZATION THROUGH VISIBLE – THERMAL INFRARED SPECTROSCOPY AND X-RAY DIFFRACTION ANALYSES: RELEVANCE FOR EARTH AND MARS

Oxidized and hydrated basaltic glasses have previously been characterized as “palagonites”. However, Schiffman et al. (2000 and 2002) drew a distinction between palagonitically and pedogenically altered basaltic glass. In their study area on Kilauea volcano, those authors noted that palagonitic alteration resulted from hydrothermal alteration of basaltic glass and resulted in the formation of well-indurated materials containing palagonite and poorly crystalline smectites. The pedogenically altered basaltic glass resulted from ambient weathering from meteoric water and produced poorly consolidated materials containing ferrihydrite, allophane, and kaolinite. Our examination of hydrovolcanic tuff rings and tuff cones at a number of sites in Idaho, New Mexico, and Utah supports the findings of Schiffman et al. (2000 and 2002). We have used visible and near infrared (VNIR) reflectance and thermal infrared (TIR) emission spectroscopy along with X-ray diffraction (XRD) to characterize the mineralogy of tephra beds associated with these features. Non-linear unmixing of reflectance spectra allows for the discrimination of different Fe³⁺-bearing and phyllosilicate phases which can be used to distinguish between these two alteration pathways. We note that the palagonitized materials in the studied tuff rings and tuff cones were palagonitized in-place suggesting deposition with steam or hot water and relatively rapid palagonitization as opposed to the external sources of hydrothermal heating referenced by Schiffman et al. (2000 and 2002). Pedogenic alteration products at these locations were relatively minor in comparison to palagonitically altered tephras. The methods used in this study are translatable to orbital or rover-based investigations of the martian surface and will allow for better characterization of the alteration environments extant at the time of generation of observed hydrated minerals. On-going work is also comparing the alteration of these hydrovolcanically produced basaltic glasses with impact-generated basaltic glasses.