Lunar Feldspathic Meteorites: Constraints on the Geology of the Lunar Farside Highlands, and the Origin of the Lunar Crust

Introduction: The Lunar Magma Ocean (LMO) hypothesis holds that, early in its history the Moon was wholly or mostly molten [1,2]. Mafic minerals (olivine and pyroxene) crystallized first from the magma and sank to form the mantle, enriching the remaining magma in Fe and incompatible elements. Later, plagioclase floated in the dense Fe-rich magma [3,4], and concentrated at the Moon’s surface to form a global crust of ferroan anorthosite. The LMO residue became strongly enriched in incompatible elements and produced abundant ilmenite and residual melt enriched in KREEP to form new basaltic magmas that then intruded the global ferroan anorthosite, now represented by Mgsuite plutonic rocks and mare basalts. This global LMO hypothesis is consistent with most petrologic and geochemical data from the Apollo returned samples. Most Apollo anorthosites are ferroan. Crystallization ages of the anorthosites are ancient [2,5], consistent with formation early in the Moon’s history. Orbital remote sensing shows that all of the Moon’s highland crust is anorthositic [6-8], as required by the LMO model. Mare basalts show a strong Eu depletion, complementary to the strong Eu enrichment of ferroan anorthosites[9], and consistent with flotation and thus removal of plagioclase from the mare basalts’ source regions. Lunar meteorites, ejected from the lunar surface by impact events, provide additional tests of the global distribution of LMO products. The meeorites come from random sites across the lunar surface [10]. About 2/3 of the meteorites are feldspathic, 1/3 are basaltic, consistent with the proportion of the lunar surface covered by highland material and mare basalts [10]. Feldspathic lunar meteorites, such as ALHA81005 and NWA 2996, represent these highland materials. Samples and Method Meteorites ALHA81005 and NWA2996 are feldspathic, polymict, regolith breccias composed of rock and mineral fragments from the lunar highlands [11,12] with very little (NWA2996) to no (ALHA81005) KREEP component [13,14]. In thin section both meteorites contain fragments of anorthosites, 300μm to 3.5mm in diameter (Fig. 1), with 90-98 vol% plagioclase and 2-10 vol% olivine, lowand high-Ca pyroxenes, and/or accessory ilmenite. Quantitative mineral analyses were obtained with a Cameca SX100 at NASA Johnson Space Center. Operating conditions were: 15kV accelerating voltage, 20nA beam current, 1μm beam diameter for olivine and pyroxene, and 5μm for plagioclase. Geochemistry: Mineral compositions are uniform within each clast in these meteorites, but vary widely