A MODEL FOR THE THERMAL AND CHEMICAL EVOLUTION OF THE MOONS INTERIOR - IMPLICATIONS FOR THE ONSET OF MARE VOLCANISM

Crystallization of the lunar magma ocean creates a chemically stratifi ed Moon consisting of an anorthositic crust and magma ocean cumulates overlying the primitive lunar interior. Within the magma ocean cumulat es the last liquids to crystallize form dense, ilmenite-rich cumulates that contain high concentrations of incompatible radioactive elements . The underlying olivine-orthopyroxene cumulates are also stratified w ith later crystallized, denser, more Fe-rich compositions at the top. This paper explores the chemical and thermal consequences of an intern al evolution model accounting for the possible role of these sources o f chemical buoyancy. Rayleigh-Taylor instability causes the dense ilme nite-rich cumulate layer and underlying Fe-rich cumulates to sink towa rd the center of the Moon, forming a dense lunar core. After this over turn, radioactive heating within the ilmenite-rich cumulate core heats the overlying mantle, causing it to melt. In this model, the source r egion for high-TiO2 mare basalts is a convectively mixed layer above t he core-mantle boundary which would contain small and variable amounts of admired ilmenite and KREEP. This deep high-pressure melting, as re quired for the generation of mare basalts, occurs after a reasonable t ime interval to explain the onset of mare basalt volcanism if the cont ent of radioactive elements in the core and the chemical density gradi ents above the core are sufficiently high but within a range of values that might have been present in the Moon. Regardless of details impli ed by particular model parameters, gravitational overturn driven by th e high density of magma ocean Fe-rich cumulates should concentrate hig h-TiO, mare basalt sources, and probably a significant fraction of rad ioactive heating, toward the center of the Moon. This will have import ant implications for both the thermal evolution of the Moon and for ma re basalt genesis.