Effects of Mantle Melting by a Giant Impact on the Lunar Nearside

Abstract

A giant impact on the Procellarum region on the lunar nearside has been proposed as a possible mechanism to explain different aspects of the lunar dichotomy, including crust thickness, topography and composition. In this work we study the first steps of the post-impact evolution of the lunar mantle and crust by modeling impact melting, melt pool formation and differentiation and their contribution to the development of the lunar dichotomy.

Using the iSALE code, we modeled the impact of a 800km sized projectile onto the Procellarum region on the lunar nearside. Based on the pressure and temperature conditions predicted by the impact model, solidus and liquidus curves and compositional partial melting models for target and projectile materials we calculated the volumes and compositions of local partial melts, that ascend to form a melt pool in the impact region.

We used alphaMELTS to model fractional crystallization of the melt pool, the formation of a secondary crust by plagioclase floatation and the subsidence of the Procellarum region resulting from melt pool solidification. In addition we determined the duration of melt pool crystallization, considering melt pool convection and heat conduction through a growing crust.

We found that partial melting and melt pool formation lead to a local enrichment of the upper mantle in incompatible elements (e.g. K, REE, P, Fe, Ti, Th), potentially explaining the compositional anomaly in the Procellarum region. The thickness of the secondary crust forming in the Procellarum region is consistent with GRAIL observations and the duration of magma pool crystallization is consistent with the ages of crustal rocks and the model age of urKREEP.