The materials of the lunar Procellarum KREEP Terrane: A synthesis of data from geomorphological mapping, remote sensing, and sample analyses

Major features of the Moon's Procellarum KREEP Terrane include subdued reli ef and extensive resurfacing with mare basalt, consistent with high concent rations of Th and other heat-producing elements at depth. We relate the che mistry of sampled materials to the geomorphology, Th surface concentrations determined by the Lunar Prospector (2 degrees pixels), and FeO and TiO2 co ncentrations derived from Clementine ultraviolet-visible spectral data, On the basis of geologic maps, each pixel was classified as mare, terra, or mi xed. Near the periphery of the terrane, terra pixel compositions are relati vely feldspathic; in the interior they mainly represent Imbrium basin rim o r ejecta deposits and are mainly incompatible trace element rich norites an d presumably represent materials from a thick section (tens of kilometers) of the pre-Imbrium crust of the terrane excavated by the Imbrium event. (Al though Imbrium ejecta are the principal source of surface terra materials, the Imbrium event did not create the Th-rich Procellarum KREEP Terrane.) Br oad, continuous expanses of mare pixels are observed, with little interrupt ion from protruding terra or terra-penetrating craters. The mare-basalt-dom inated regoliths of these areas have a wide range of TiO2 concentrations (< 1 - 15%) and higher Th concentrations (2 to 6+ ppm) than most sampled mare basalts. Traverse profiles show high Th over broad regions of highest FeO ( >18%), leading to the conclusion that the high Th concentrations are in the mare basalts and are not present in the regoliths as terra-derived materia ls. Volcanic glasses and impact glasses of mare basalt composition collecte d from the Procellarum KREEP Terrane support this conclusion.