Employing magma ocean crystallization models to constrain structure and composition of the lunar interior

Abstract

The process of lunar magma ocean solidification provides crucial constraints on the composition and extent of distinct chemical reservoirs in the lunar mantle that formed during the early evolution of the Moon. We use a combination of phase equilibria models consistent with recent experimental results on fractional crystallization of the lunar magma ocean to study the effect of bulk silicate Moon composition on the properties of lunar mantle reservoirs.

We find that the densities and relative proportions of these mantle reservoirs, in particular of the late forming ilmenite bearing cumulates (IBC), strongly depend on the FeO content of the bulk silicate Moon. This relation has implications for post-magma ocean mantle dynamics and the resulting mass distribution in the lunar interior, because the IBC form at shallow depths but tend to sink towards the core mantle boundary due to their high density. We quantify the relations between bulk silicate Moon FeO content, IBC thickness and bulk Moon density as well as mantle stratigraphy and bulk silicate Moon moment of inertia in order to constrain the bulk silicate Moon FeO content and the efficiency of IBC sinking.

In combination with seismic and selenodetic constraints on mantle stratigraphy, core radius and extent of the possibly IBC bearing low velocity zone at the core mantle boundary as well as considerations about the present day selenotherm and the effects of reservoir mixing by convection our model indicates that the bulk silicate Moon is only moderately enriched in FeO compared to the Earth's mantle and contains most likely about 9.4–10.9 wt% FeO (with a lowermost limit of 8.3 wt% and an uppermost limit of 11.9 wt%). We further conclude that the observed bulk silicate Moon moment of inertia requires that sinking of the IBC layer by mantle convection was incomplete: only ~20–60% of the IBC material might have reached the core mantle boundary, while the rest either remained at the depth of its formation right beneath the crust or was mixed into the middle mantle.