On the Origin of the Moon’s Feldspathic Highlands, Pure Anorthosite, and the Feldspathic Lunar Meteorites

Results reported by Ohtake et al. [1] are stunning and potentially have great petrologic significance. Using the Kaguya multiband imager and spectral mixing procedures, the team investigated " crater central peaks, walls, ejecta and basin rings, " that is, outcrops of rock where there is little regolith. They observed numerous locations in the highlands where " anorthosite composed of nearly 100% anorthite is found in large exposures. " They also noted a strong correlation between PAN (" purest-anorthosite, " >98% plagioclase) rocks and crater size. They conclude that " PAN rocks are ubiquitously present within the depth range from 3 km to at least 30 km " and that " a global layer of PAN rock may exist within the upper crust. " The results are important because the generation of pure anorthosite requires very efficient separation of plagioclase from coexisting (ma-fic) melt. At issue is whether anorthosite forms a global layer or it occurs as discrete intrusive bodies, similar to terrestrial Proterozoic massif anorthosites. Here we consider these observations within the context of the feld-spathic lunar meteorites, which are a sample suite dominated by components of the Moon's upper crust. The observation of anorthosite with >95% plagio-clase has been made before. Hawke et al. [2] report numerous regions of anorthosite with <5% pyroxene exposed at the surface on the basis of Earth-based telescopic , Galileo, and Clementine spectral reflectance data. They conclude " The distribution and modes of occurrence of anorthosites clearly indicate that a thick, global layer of anorthosite is present at various depths beneath most portions of the lunar surface. " Then there are the Apollo samples. The 269-g rock sample 15415 from Apollo 15 is pure plagioclase. Several equivalently large rocks from the Apollo 16 mission are nearly pure plagioclase. The question is how large and how pure are the bodies from which these anorthosites derive? To address this question from a sample perspective, we consider the ~37 meteorites that originate from randomly distributed locations in the feldspathic highlands [3–6]. All of the feldspathic lunar meteorites are brec-cias and most consist of lithified regolith and fragmental near-surface material (meters to tens of meters). They are mechanical mixtures of many rock types but mainly represent an integrated accumulation of rocks ejected by craters of all sizes. In 2003, on the basis of the average composition of 8 feldspathic lunar meteorites (28.2% Al 2 O 3), we estimated …