The “Procellarum KREEP Terrane”: Implications for mare volcanism and lunar evolution

Geophysical, remote-sensing, and sample data demonstrate that the Procellarum and Imbrium regions of the Moon make up a unique geochemical crustal province (here dubbed the Procellarum KREEP Terrane). Geochemical studies of Imbrium's ejecta and the crustal structure of the Imbrium and Serenitatis basins 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. KREEP basalt has about 300 times more uranium and thorium than chondrites, so this implies that a large portion of Moon's heat-producing elements is located 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 model and show that as a result of the high abundance of heat-producing elements that are found in the Procellarum KREEP Terrane, partial melting of the underlying 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 of 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 Apollo seismic data may represent this maximum depth of melting. Our model also predicts that the KREEP basalt layer should remain partially molten for a few 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 melts of the underlying mantle. Since Mg-suite rocks are likely derived from crystallizing KREEP basalt, the provenance of these plutonic rocks is likely to be unique to this region of the Moon.

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