Compositional analyses of small lunar pyroclastic deposits using Clementine multispectral data

Clementine ultraviolet-visible (UVVIS) data are used to examine the composi tions of 18 pyroclastic deposits (15 small, three large) at 13 sites on the Moon. Compositional variations among pyroclastic deposits largely result f rom differing amounts of new basaltic (or juvenile) material and reworked l ocal material entrained in their ejecta upon eruption. Characterization of pyroclastic deposit compositions allows us to understand the mechanisms of lunar explosive volcanism. Evidence for compositional differences between s mall pyroclastic deposits at a single site is observed at Atlas crater. At all sites, compositional variation among the small pyroclastic deposits is consistent with earlier classification based on Earth-based spectra: three compositional groups can be observed, and the trend of increasing mafic abs orption band strength from Group 1 to Group 2 to Group 3 is noted. As redef ined here, Group 1 deposits include those of Alphonsus West, Alphonsus Sout heast, Alphonsus Northeast 2, Atlas South, Cruger, Franklin, Grimaldi, Lavo isier, Oppenheimer, Orientale, and Riccioli. Group 1 deposits resemble luna r highlands, with weak mafic bands and relatively high UV/VIS ratios. Group 2 deposits include those of Alphonsus Northeast 1, Atlas North, Eastern Fr igoris East and West, and Aristarchus Plateau; Group 2 deposits are similar to mature lunar maria, with moderate mafic band depths and intermediate UV /VIS ratios. The single Group 3 deposit, J. Herschel, has a relatively stro ng mafic band and a low UV/VIS ratio, and olivine is a likely juvenile comp onent. Two of the deposits in these groups, Orientale and Aristarchus, are large pyroclastic deposits. The third large pyroclastic deposit, Apollo 17/ Taurus Littrow, has a very weak mafic band and a high UV/VIS ratio and it d oes not belong to any of the compositional groups for small pyroclastic dep osits. The observed compositional variations indicate that highland and mar e materials are also present in many large and small pyroclastic deposits, and they suggest that volcanic glasses or spheres may not be dominant juven ile components in all large pyroclastic deposits.