Models for the crystallization of the magma ocean, phase equilibria constraints, and geochemical criteria constrain the mare basalt source region to lie deep within the Moon and within olivine-orthopyroxene cumulate layers of the magma ocean. The Mg* compositions of the mare basalt source region, the ferroan anorthosite crust and the mg-suite troctolites, require that most if not all the cumulates were produced by a magma ocean undergoing fractional rather than equilibrium crystallization. Estimates of the depth of the elastic lithosphere, the existence of mascons, and the need to sequester some heat-producing elements in the lunar interior are consistent with the lunar overturn model which transports the refractory cumulates, the more fertile ilmenite-diopside cumulates and some of the KREEP components into the lunar interior. ¿ 2000 American Geophysical Union