Geochemistry of Lunar Granulites and Implications for the Composition and History of the Lunar Crust
Kent, Jeremy J 1982-
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Lunar meteorites originate from random locations on the Moon, and therefore can serve as tools to study lithologies from locations other than the area covered by the Apollo and Luna missions. This study is based on the geochemical compositions of lunar granulites Northwest Africa (NWA) 5744, MacAlpine Hills (MAC) 88105, and Queen Alexandra Range (QUE) 93069. NWA 5744 is a magnesian lunar meteorite with bulk Mg# = 79, and may have been a monomict or genomict crustal rock prior to metamorphism. This meteorite does not fit into the established lunar classification scheme for primary crustal rocks, with major element relationships that best fit the Apollo Mg-suite but trace elements better fitting the Apollo ferroan anorthosite (FAN) suite. NWA 5744 could represent a distinct type of crustal lithology formed on the far side of the Moon. This would imply that bulk lunar crust models primarily based on Apollo samples from the Procellarum region may need revision. Metamorphism appears to have been intense but brief enough in NWA 5744 to prevent complete re-equilibration among the different minerals, and likely occurred near the lunar surface relating to an impact melt sheet or high shock event. Both MAC 88105 and QUE 93069 are polymict lunar regolith breccias containing clasts of granulite. Most of these clasts have Mg#’s < 70 and match the Apollo FAN suite more closely than the Mg-suite in both major and trace element composition. MAC 88105 contains a broad range of FAN granulites and is likely to originate from some depth below the lunar surface in a well-mixed regolith. QUE 93069 has two to four distinct compositional populations among its granulite clasts and is likely to originate at or near the lunar surface in a regolith that was less well-mixed than MAC 88105. Due to the geochemical makeup of the lunar surface, it is more likely that both MAC 88105 and QUE 93069 originated from the nearside of the Moon than the far side. NWA 5744 is conversely more likely to have originated on the far side of the Moon. All three meteorites likely originated far from the Apollo and Luna mission sites.