Lunar Differentiation: Insights from Lunar Samples

Apollo samples, lunar meteorites, and remote sensing data reveal that both the maria and the ancient highlands of the Moon are compositionally diverse. Mare basalts erupted from about 4.3 to perhaps as recently as 1 Ga. They vary in their concentrations of Al, Ti, and incompatible trace elements, implying a heterogeneous lunar mantle. The highlands are strikingly different, but no less diverse in lithologic assemblages. The southern nearside and most of the farside highlands average 75 vol% plagioclase, and a major suite of rocks, the ferroan anorthosites, averages 96 vol% plagioclase. The feldspathic composition and Sm-Nd isotopic data reflect plagioclase floatation in a magma ocean 4.46 Ga ago. Late-stage REE-rich magma pooled in the Procellarum region of the lunar nearside. The concentration of heat-producing elements in this region triggered mantle melting and overturn of the cumulate pile, forming two other suites of chemically-distinct highland rocks, the magnesian and alkali suites. These suites of rocks appear to be confined to the Procellarum region, although it is possible that REE-poor versions occur on the farside. Their parent magmas were diverse in composition and experienced extensive fractionation, as shown most prominently by the variation in Mg/Fe and concentrations of incompatible elements among magnesian and alkali suite samples. Highlands magmatism was as diverse as mare basalt magmatism. The lunar mantle, the complex cumulate pile formed from the magma ocean and then overturned, was (and is) heterogeneous on a large scale. There is evidence from granulitic breccias in the Apollo collection and from lunar meteorites that rocks significantly more magnesian than ferroan anorthosites exist in the farside highlands (their abundance is not known), perhaps demonstrating even more complexity to the interior. We need additional orbital, landed, and sample-return data to understand lunar igneous differentiation.