Modeling the Thermochemical Evolution of the Early Moon

Kurzfassung

While on Earth the processes of early differentiation and crust formation have been largely overprinted by ongoing tectonic activity, the Moon has retained a well-preserved record of its early evolution. This provides a unique opportunity to reconstruct the details of the processes governing early lunar evolution, including lunar magma ocean (LMO) solidification, mantle convection, magmatism and impacts. Thereby the initial LMO composition sets the stage for all later steps of lunar evolution, since it determines the compositions and physical properties of the primary lunar crust, the magma ocean cumulates comprising the primary lunar mantle, and the magmatic rocks formed by lunar mantle melting. The FeO content of the LMO plays an important role for the gravitational overturn of the early lunar mantle, because it controls the density and thickness of the dense ilmenite bearing cumulates (IBC) that formed at shallow depths. The total amount of IBC and the fraction of IBC that sank to the core mantle boundary affect the bulk Moon density and the moment of inertia of the bulk silicate Moon, which links the bulk silicate Moon FeO content to directly observable physical properties of the Moon [1]. The LMO water content is another important factor controlling a) the time scales of LMO solidification [2], since the presence of water hinders plagioclase crystallization and hence the formation of the primary lunar crust, and b) the efficiency of mantle melting, since melting temperatures are lowered by the presence of water. Though the water contents of plagioclase from the primary lunar crust allow some insight into LMO water contents at the time of their formation, more experimental data on water partitioning between minerals and melt are required for a precise quantification [3]. During LMO evolution, its composition was not only changed by progressive solidification, but also altered continuously by impacting debris from the Moon forming disk that might either have melted upon impact or sunken through the LMO liquid and contribute to the solid cumulate. Evaluating how much of the impacting material was melted and how much survived sinking through the magma ocean, requires modeling of projectile fragmentation and melting, both upon impact and during sinking through the magma ocean [4]. Together with models of LMO crystallization, such models provide insights into the possible compositions of the mantle reservoirs after LMO solidification and hence the source compositions of lunar volcanic rocks. References: [1] Schwinger, S., & Breuer, D. (2022). Employing magma ocean crystallization models to constrain structure and composition of the lunar interior. Physics of the Earth and Planetary Interiors, 322, 106831. [2] Maurice, M., Tosi, N., Schwinger, S., Breuer, D., & Kleine, T. (2020). A long-lived magma ocean on a young Moon. Science advances, 6(28), eaba8949. [3] Mallik, A., Schwinger, S., Roy, A., & Moitra, P. (2022). Controls on determining the bulk water content of the Moon. Meteoritics & Planetary Science, 57(12), 2143-2157. [4] Schwinger, S., Röhlen, R. & Wiesner, N. (2023). The Fate of Projectile Material Impacting the Lunar Magma Ocean. European Lunar Symposium 2023, Padua, Italy.