Implications of a highly convective lunar magma ocean: Insights from phase equilibria modeling

Abstract

The Moon's internal structure was largely defined within the first 200 million years following the initial Moon-forming impact. During this period, the lunar magma ocean (LMO) lost most of its heat through early vigorous convection, crystallizing and forming an initial cumulate stratigraphy through, potentially, robust equilibrium crystallization followed by fractional crystallization once the LMO became sufficiently viscous. This rheological transition is estimated to have occurred at 50 % to 60 % LMO solidification, and although the petrological effects of the regime switch have been frequently investigated at the lower value, such effects at the upper limit have not been formally examined until now. Given this scenario, we present two new internally consistent, high-resolution models that simulate the solidification of a deep LMO of Earth-like bulk silicate composition at both rheological transition values, focusing on the petrological characteristics of the evolving mantle and crust. The results suggest that increasing the volume of early suspended solids from the oft-examined 50 % to 60 % may lead to non-trivial differences. The appearance of minor mantle garnet without the need to invoke a refractory-element enriched bulk silicate Moon composition, a bulk mantle relatively richer in orthopyroxene than olivine, a lower density upper mantle, and a thinner crust are shown to change systematically between the two models, favoring prolonged early crystal suspension. In addition, we show that late-stage, silica-enriched melts may not have sufficient density to permit plagioclase to continue building a floatation crust and that plagioclase likely sinks or stagnates. As the ability of a lunar magma ocean to suspend crystals is directly tied to the Moon's early thermal state, the degree of early LMO convection – and the immediate Solar System environment that drives it – require as much consideration in LMO models as more well-investigated parameters such as bulk silicate Moon composition and initial magma ocean depth.