CENTRAL MASSACHUSETTS TO THE TAURUS-LITTROW VALLEY. A CRYSTAL CHEMICAL EXAMINATION OF THE FORMATION AND EVOLUTION OF THE PRIMORDIAL LUNAR CRUST

The primordial crust of the Moon consists of ferroan anorthosites (FANs) and a suite of plutonic rocks (Mg-suite; dunites, troctolites, spinel troctolites, norites). Samples representing the ancient lunar crust were returned by the Apollo and Luna 20 missions and are represented in the meteorite collection. In most models for the formation and evolution of the lunar crust the FANs represent flotation cumulates formed during the initial stages of lunar differentiation through a large-scale melting event (e.g., Lunar Magma Ocean, LMO), whereas the Mg-suite represents post-LMO magmatism associated with LMO cumulate overturn and melting. Using these two rock suites, we are applying multiple petrologic and chronometer approaches to examine the timing of early lunar events (e.g., primordial differentiation, basin formation), the thermal and chemical evolution of the lunar crust, and the origin of planetary asymmetry. Our results indicate that many of these ancient crustal rocks experienced varying degrees of metamorphism in the upper crust to the crust-mantle boundary. Mineralogical responses to metamorphism and rapid excavation include exsolution in pyroxene, spinel-bearing assemblages, and pyroxene-chromite symplectites interstitial to plagioclase and olivine. We have demonstrated that combining mineralogical thermometers-barometers with multiple chronometers (e.g., Ar-Ar, Rb-Sr, Sm-Nd) provides a unique solution to reconstructing the thermal and excavation histories of these crustal rocks. We find that FANs and Mg-suite rock fragments collected by the Luna 20 mission in the Apollonius highlands (Terra Apollonius) adjacent to the Crisium basin have pyroxene with fine-scale exsolution, suggesting either rapid cooling in the shallow crust or rapid excavation. Several FANs and Mg-suite rocks from Apollo 16 and 17 are from the deep lunar crust and show a variety of distinctly different cooling histories. Comparisons between lithologies within (Apollo) and outside (Luna 20) the Procellarum KREEP terrane indicate the Mg-suite rocks from Luna 20 have a smaller KREEP component. This suggests that the generation of Mg-suite magmas is not entirely related to the distribution of heat-producing elements (e.g., U, Th, K). However, the volume of Mg-suite magmas may be linked to this relationship.