Lunar cryptomaria: Mineralogy and Composition of Ancient Volcanic Deposits

Abstract

Ancient lunar volcanic deposits, known as cryptomaria, have been detected by remote telescopic and orbital measurements since the 1970s. Cryptomaria are most easily identified by the presence of dark-halo impact craters and are associated with a mare basalt mineralogy, which is indicated by two pyroxene spectral absorption features near 1 µm and 2 µm in the visible to near-infrared (VNIR) wavelengths. However, there are many early igneous lithologies that have been identified in the Apollo sample collection that have a similar VNIR spectral signal, implying a pyroxene-dominant mineralogy. In this study we use high resolution Moon Mineralogy Mapper (M3) VNIR spectral data and the Modified Gaussian Model (MGM) to determine cryptomare mineralogy as well as Lunar Prospector gamma-ray spectrometer (LP GRS) FeO and Th compositional measurements to evaluate which ancient igneous rocks (e.g., low-Ti mare basalt, high-Ti mare basalt, Mg-suite rocks, dunite, high-Al mare basalt, KREEP basalt) are consistent with our mineralogical observations. In addition, spectra from different M3 optical periods were compared to determine how the MGM-derived absorption band centers vary between datasets. Band center differences between optical periods are on the order of ~6±4 nm and ~25±10 nm for the 1 µm and 2 µm features, respectively. Cryptomare mineralogies are dominated by clinopyroxene and are consistent with measurements from locally exposed mare basalts. LP GRS measurements support a mare basalt rock type when regolith mixing is taken into account.