Solidification and Differentiation of a Mushy Lunar Magma Ocean: 3D Numerical Modeling

The lunar magma ocean (LMO) was formerly proposed to explain the anorthositic nature of the lunar crust as constrained by returned samples. The LMO was conventionally thought to experience a sequence of fractional crystallization, with the crust formed through plagioclase floatation. Such a conventional thinking, however, suffers from being unable to account for the $\sim$200 Myr lunar crustal formation timescale and from contradicting the measured overlapping ages between the lunar Mg-suite and ferroan anorthosites. Coming to the rescue is the slushy/mushy lunar magma ocean scenario that can sustain lunar crustal magmatism over $\sim$200 Myr. In this study, we develop a 3D spherical numerical model to quantify the solidification and differentiation of the Moon over its history. The model includes thermal and compositional mantle convection, a parameterized phase diagram for melting involving the two components of anorthite and olivine (representative of fertile and refractory components, respectively), porous melt segregation, and parameterized melt extraction via near-surface dikes. We find that the thermal effect of melt migration is so strong that it leads to a negative correlation between the duration of the crustal magmatism and the reference permeability of the mushy interior. Our results also affirm the previous scaling analysis that points out the possibility of $\sim$200-Myr lunar crustal growth from the slushy mantle. By considering compositional buoyancy, our model also identifies a possible overturn mechanism during the Moon's mushy stage, potentially reconciling the magma ocean theory with the observed age overlapping between the lunar Mg-suite and ferroan anorthosites.