Lunar crustal formation by melt migration and differentiation within a stagnant lid.

The lunar anorthosite highlands represent the Moon’s primary crust, which formed during the solidification of a magma ocean following the Moon-forming giant impact. However, the canonical model of anorthite flotation in the crystallising magma ocean often struggles to reproduce the long > 200 Myr solidification timescale required by the ages of the anorthosite suite, as well as their diverse magmatic histories and the overlap in ages with the Mg-suite. If instead the anorthite crystals remain entrained in magma ocean due to vigorous convection, the magma ocean behaves as a liquid-crystal slurry with a rigid immobile lid at the surface, which reduces the cooling rate of the magma ocean and results in long solidification times. Here, we investigate whether sufficient melt differentiation and migration can occur via viscous compaction of the solid within the stagnant lid to produce an anorthite-rich crust. Our model consists of a simplified two-component anorthite-olivine melt system, which becomes progressively enriched in anorthite as the melt solidifies. We calculate the thermochemical evolution of this melt as it rises through a stagnant lid, cools and solidifies. We find that large-scale anorthite enrichment and shallow emplacement of anorthite-rich melt can take place if the partially molten zone is thick enough to allow the formation of porosity waves, and if the rate of melt advection is greater than the rate of lid cooling. We also demonstrate that crystal flotation is likely to be able to occur in these anorthite-rich melt regions, resulting in fractional crystallisation which could produce anorthosite massifs.