NEW ROCK TYPES OF THE LOWER LUNAR CRUST

A new spinel-rich rock type has been discovered by the M³ Team on the farside of the Moon. High-resolution compositional data for the Moscoviense region on the lunar farside acquired by the Moon Mineralogy Mapper (M³) on Chandrayaan-1 reveal three unusual, but distinctive, rock types along the inner basin ring. These are designated OOS since they are dominated by high concentrations of orthopyroxene, olivine, and Mg-rich spinel, respectively. The OOS occur as small areas, each a few km in size, but widely separated within the highly feldspathic setting of the basin. Although the abundance of plagioclase is not well constrained within the OOS, the mafic mineral content is exceptionally high and the first two could approach pyroxenite and harzburgite in composition. The third is a new rock type identified on the Moon that is dominated by Mg-rich spinel with no detectible other mafic mineral present (<5% pyroxene, olivine). All OOS surfaces are old and undisturbed since basin formation. They are effectively invisible in image data and are only recognized by their distinctive composition identified spectroscopically. The origin of these unusual lithologies appears to result from one or more magmatic intrusions into the lower crust, perhaps near the crust/mantle interface, that resulted in clean concentration of the mafic components within zones several km in dimension. The OOS are embedded within highly anorthositic material from the lunar crust; they may thus be near cotemporaneous with crustal products from the cooling Magma Ocean.