WHY DO LUNAR MAGMA OCEAN COOLING MODELS STRUGGLE TO REPRODUCE A GRAIL-ERA CRUSTAL THICKNESS?

Data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission have been used to constrain the Moon’s mean crustal thickness to 34–43 km, about half of that based on earlier seismic estimations. The assumed composition and thickness of a plagioclase flotation crust that formed from a crystallizing lunar magma ocean (LMO) are non-trivial parameters for estimating bulk silicate Moon (BSM) compositions and for validating aspects of the Giant Impact hypothesis, as they would reveal to what extent the Earth and Moon shared the same ancient geochemical reservoir. However, crystallization models have historically had difficulty in reproducing GRAIL-based crustal thicknesses, often predicting crustal thicknesses well in excess of 43 km.

A frequently modeled BSM composition is the Lunar Primitive Upper Mantle (LPUM; Longhi, 2003, 2006), which uses a BSM Al₂O₃ wt. % of ~3.9; however, LMO solidification models using this composition do not produce a GRAIL-based crustal thickness. Longhi also showed how much Al₂O₃ mantle source regions require to produce Apollo 15 pyroclastic green glasses, reasoning that the remaining BSM Al₂O₃ was bound to a plagioclase crust 50-70 km thick, while both Al₂O₃ sources needed to account for the LPUM Al₂O₃ concentration. Consequently, it is unsurprising that crystallization models using LPUM produce crustal thicknesses greater than 43 km. Despite this tendency to overpredict crustal thickness, LPUM cooling models may still hold value for investigating mechanisms that alter the initial crust, such that the original thickness may be obscured. We briefly highlight some of the results of two new models of LPUM crystallization from a 1400-km deep LMO. Both models reflect an initial period of equilibrium crystallization before fractional crystallization dominates but with different amounts of trapped melt. We present an overview of the consequences for crustal thickness, what minerals – aside from anorthite – would enter the resultant crust, the mechanisms that may have altered the original thickness, and the potential model-predicted Al₂O₃ concentration for various source regions.

This work was partially supported by an NSF/GSA Graduate Student Geoscience Grant # 13410-22, which is funded by NSF Award # 1949901; and an NSF-supported IIE-GIRE fellowship under Grant # 1829436.