Distinguishing high‐alumina mare basalts using Clementine UVVIS and Lunar Prospector GRS data: Mare Moscoviense and Mare Nectaris

Abstract

High‐alumina (HA) mare basalts are a unique group of the lunar sample collection. Sample geochemistry indicates that these basalts are derived from sources composed of late‐stage cumulates from the Lunar Magma Ocean (LMO). Their aluminous nature suggests their sources contained significant plagioclase, which has implications regarding the efficiency of plagioclase separation from earlier forming, mafic cumulates in the LMO to form the anorthositic lunar crust, and hence the heterogeneity of the lunar mantle. The Apollo and Luna missions sampled HA basalts from four different locations that are separated by 80 equatorial degrees (∼2400 km). Radiometric age dating of these samples demonstrates aluminous basaltic volcanism spanned over 1 billion years, suggesting HA basalts may be more prevalent on the Moon than implied by the sample population. Knowing their global occurrence would ultimately enhance our understanding of lunar evolution. Aluminous mare basalts occupy a unique location in Th‐FeO compositional space that suggests they can be identified using global remote‐sensing data of the Moon. We present our approach for distinguishing exposures of HA basalts using Clementine ultraviolet‐visible‐infrared (UVVIS) and Lunar Prospector Gamma Ray Spectrometer (LP‐GRS) data with constraints based on the FeO, TiO2, and Th abundances of Apollo and Luna HA samples. We identified 34 regions of interest (ROIs) where HA basalts could be a prominent component of the lunar surface. By analyzing the rims and proximal ejecta from small impacts (0.4–4 km in diameter) into the mare surface we characterized compositionally distinct basaltic units that make up the mare and thus determined which units represent HA basalt lavas. The results were used to generate maps that depict discrete mare units and classify their general basalt type. Here we focus on two ROIs: Mare Moscoviense and Mare Nectaris. Mare Moscoviense is composed of four basaltic units, two of which are HA candidates. Clementine UVVIS data of Mare Nectaris show evidence of up to three mare basalt units. One is the remnants of a mid‐Ti unit that capped earlier low‐Ti flows. The majority of the basin is filled by a compositionally indistinguishable low‐Fe, low‐Ti basalt. However, spectral profiles suggest there are two units. Regardless, the units both fit the criteria for a HA basalt.