The "Procellarum KREEP Terrane": Implications for mare volcanism and lunarevolution

Geophysical, remote-sensing, and sample data demonstrate that the Procellar um and Imbrium regions of the Moon make up a unique geochemical crustal pro vince (here dubbed the Procellarum KREEP Terrane). Geochemical studies of I mbrium's ejecta and the crustal structure of the Imbrium and Serenitatis ba sins both suggest that a large portion of the lunar crust in this locale is composed of a material similar in composition to Apollo 15 KREEP basalt. K REEP basalt has about 300 times more uranium and thorium than chondrites, s o this implies that a large portion of Moon's heat-producing elements is lo cated within this single crustal province. The spatial distribution of mare volcanism closely parallels the confines of the Procellarum KREEP Terrane and this suggests a causal relationship between the two phenomena. We have modeled the Moon's thermal evolution using a simple thermal conduction mode l and show that as a result of the high abundance of heat-producing element s that are found in the Procellarum KREEP Terrane, partial melting of the u nderlying mantle is an inevitable outcome. Specifically, by placing a 10-km KREEP basalt layer at the base of the crust there, our model predicts that mare volcanism should span most of the Moon's history and that the depth o f melting should increase with time to a maximum depth of about 600 km. We suggest that the 500-km seismic discontinuity that is observed in the Apoll o seismic data may represent this maximum depth of melting. Our model also predicts that the KREEP basalt layer should remain partially molten for a f ew billion years. Thus the Imbrium impact event most likely excavated into a partially molten KREEP basalt magma chamber. We postulate that the KREEP basalt composition is a by-product of mixing urKREEP with shallow partial m elts of the underlying mantle. Since Mg-suite rocks are likely derived from crystallizing KREEP basalt, the provenance of these plutonic rocks is like ly to be unique to this region of the Moon.