Remelting mechanisms for shallow source regions of mare basalts

Abstract

Two mechanisms are studied which could induce remelting at shallow depths within the Moon: a radioactive heating model in which remelting is the result of lateral variations of the heat source concentration emplaced by the end of the initial differentiation event, and a thermal insulation model in which remelting is induced by lateral variations of near-surface conductivity as a result of a high-porosity ejecta blanket created by basin-forming impacts. The effects of these perturbations are studied by evaluating the thermal evolution of an otherwise radially symmetric lunar model. The radioactive heating models predict extensive pre-basin volcanism, which would probably remove from the source region many of the heat-producing elements potentially responsible for melting. Therefore, localized high concentrations of heat-producing elements emplaced early would most probably not be heat sources for the basin-filling mare basalts; however, they are plausible heat sources for pre-basin mare basalts. The thermal insulation model predicts a delay of about 200 m.y. between impact and volcanism. Thermal insulation may also allow for the persistence of source regions for young basalts at shallow depths. If greater basin ages are correct, from 4.2 to 3.9 Ga, and most volcanism occurred between 3.8 and 3.6 Ga, then the thermal insulation model may explain the delay between impact and basin filling. If the basins formed during a terminal cataclysmic event at about 3.9 Ga, then the earliest stages of basin filling at about 3.8 Ga may have been a result of melting beneath the basin as a result of mantle uplift. Basin filling may have been characterized by a migration of the source region from beneath the basin to beneath the adjacent highlands. Cooling of the top 200 km beneath the basin after 200 m.y. because of its high surface conductivity is consistent with the mascon support requirement.