ZEOLITES AND OTHER ALTERATION PHASES OF HYDROVOLCANIC BASALTIC GLASSES: CHARACTERIZATION THROUGH VISIBLE – THERMAL INFRARED SPECTROSCOPY AND X-RAY DIFFRACTION

Basaltic glass produced through volcanic eruptions or impacts into basaltic substrates can alter following any one of several different pathways. In our research, we have sought to determine mineralogic clues to aid in determining which alteration pathway has been followed. We have also been using tools that are, or could be available, for Mars rovers to aid in defining the geologic history of the martian surface. We have examined hydrovolcanic basaltic glasses from several tuff rings and tuff cones in Idaho, New Mexico, and Utah, glaciovolcanic hyaloclastites from Washington state and Iceland, along with basaltic ash from the Haleakala volcano in Maui, Hawaii, as well as basaltic impact glasses from Lonar Crater in India. The visible through mid-wave infrared (MWIR) reflectance of these samples have been measured as has been their thermal infrared emissivity. We have also examined petrographic thin sections and collected XRD scans of powdered samples. Several trends have become apparent. As noted by Schiffman et al. (2000 and 2002), basaltic glass subjected to ambient weathering alters through a “pedogenic” pathway in which the ash remains disaggregated, there is incipient palagonitization of the basaltic glass grains, but smectites and zeolites do not develop. Hydrovolcanically produced basaltic ashes are deposited with steaming hot water and can undergo rapid syndepositional hydrothermal alteration or post-depositional hydrothermal alteration from hydrothermal cells heated by the magma body. These alteration products are well indurated, develop smectites and often have associated zeolites. The zeolites are detectable in petrographic thin sections, XRD, and also through the observation of a 4.5 µm water overtone absorption in the MWIR. The zeolites are less prominent to absent in the glaciovolcanic hyaloclastites- perhaps due to more rapid quenching by surrounding ice. Smectites are also readily detectable, even in poorly-crystalline states, based on metal-OH overtone features in the 2.2 to 2.3 µm region and in XRD. Rietveld refinement of XRD scans and of altered Lonar Crater impact glasses are on-going and are expected to provide approximate abundances of crystalline and amorphous phases.