Dark mantles over domes and rilles on the Moon: Evidence for pyroclastic materials

Abstract

A collection of dark mantles covering volcanic domes and surrounding rilles were examined. Remotely sensed information on FeO and TiO2 content, clinopyroxene (CPX) and olivine (where olivine is an assumed proxy for glass) abundance, band center positions near 1 and 2 μm, and a parameter related to OH and/or H2O content were assessed for these mantling deposits. Based on our examination of these dark mantles associated with domes and rilles we found that all of the examined dark mantles had higher FeO abundances than the surroundings. The Birt E pit of Rima Birt and the mantle over the Manilius-1 dome are higher in glass and lower in CPX than their surroundings and with deeper band depth near 3 μm: all these indicators suggest higher pyroclastic glass abundance. The mantle at Rima Calippus is also nominally glass-rich although its band depth near 3 μm is comparable to nearby mare basalts and thus is not taken to be unusually hydrated. The dark mantles associated with the Yangel-1 and Cauchy-5 domes are higher in CPX, lower in glass, and higher in TiO2 content with low band depth near 3 μm indicating they are glass-poor and nominally rich in ilmenite. The Hyginus crater of Rima Hyginus is distinct from the other studied mantles in that it has higher TiO2 content, lower CPX abundance, higher glass or olivine, and weak 3 μm band depth. Also, the band minimum near 1 μm deos not occur at a longer wavelength than surrounding mare suggesting that in this instance the higher olivine (as a proxy for glass) might actually be due to olivine rather than glass. Thus, in the study of dark mantles associated with domes and rilles each represents deposits and eruptions that need to be characterized on a case-by-case basis rather than as a collective group.