New insights into lunar petrology: Distribution and composition of prominent low-Ca pyroxene exposures as observed by the Moon Mineralogy Mapper (M3)

Abstract

[1] Lunar geochemical groups such as Mg suite, ferroan anorthosite, and alkali suite rocks are difficult to distinguish from orbit because they are defined by both modal mineralogy and elemental composition of their constituent minerals. While modal mineralogy can be modeled, only specific minerals or elements can be directly detected. At near-infrared (NIR) wavelengths, pyroxenes are among the most spectrally distinctive minerals, and their absorption bands are sensitive to structure and composition. Pyroxenes thus provide important clues to distinguish these geochemical groups and to understand lunar crustal evolution. Using Moon Mineralogy Mapper data, we search for lithologies dominated by strong low-calcium pyroxene (LCP) signatures. We compare the NIR absorptions of 20 LCPs to a suite of synthetic pyroxenes to determine which lunar pyroxenes appear magnesian enough to be candidate Mg suite norites. We detail three prominent regions of LCP (1) in South Pole–Aitken Basin (SPA), (2) south of Mare Frigoris, and (3) north of Mare Frigoris. The absorption band positions suggest that the LCPs north of Mare Frigoris and those in SPA are compositionally similar to one another and of ∼Mg50–75, implying that the mafic material excavated by the SPA impact was relatively iron-rich. Modified Gaussian modeling results suggest that the Apollo basin may have tapped different composition material than is exposed in much of SPA. The LCPs located in the highlands south of Mare Frigoris exhibit absorption bands at short wavelengths consistent with Mg > ∼80. The coincidence of these Mg-rich LCPs with the thorium measured by Lunar Prospector make them good candidates for KREEP-related Mg suite pyroxenes.