A Mechanism For the Generation of the Lunar-Mare Basalts

Abstract

According to the magma ocean, or magmasphere model of lunar formation, rapid accretion would result in the formation of a solid, primitive core of undifferentiated material, surrounded by a spherical shell of molten silicate. As the outer layers of the Moon cool by radiation of heat to space, the deep interior will warm due to the decay of trace radionuclides. This will lead to compression of the remnant magmasphere, producing excess pressure in the fluid magma. It is proposed that relief of this excess pressure through vents opened by the excavation of the mascon basins was sufficient to drive the major episodes of mare volcanism between 3.9–3.2 Ga. Thermal models of the Moon are constructed, and a range of initial conditions, based on the magmasphere model, are identified which satisfy known constraints on lunar evolution. the rate at which radial thermoelastic stress accumulates within the Moon is calculated, and from this estimates of the maximum volume of basaltic magma which could be produced by this mechanism are obtained. By comparing these calculated magma-generation rates with estimates of the volume of mare basalts residing on the lunar surface we are able to identify a set of initial temperature profiles which both satisfy the known constraints on lunar evolution and are able to produce the observed volume of mare basalts. These models are characterised by deep magmaspheres and/or initially warm central temperatures.