Evidence of extensive lunar crust formation in impact melt sheets 4,330 Myr ago

White, L.F.; Cernok, A.; Darling, J.R.; Whitehouse, M.J.; Joy, K.H.; Cayron, C.; Dunlop, J.; Tait, K.T. and Anand, M. (2020). Evidence of extensive lunar crust formation in impact melt sheets 4,330 Myr ago. Nature Astronomy, 4(10) pp. 974–978.

DOI: https://doi.org/10.1038/s41550-020-1092-5

Abstract

Accurately constraining the formation and evolution of the lunar magnesian suite is key to understanding the earliest periods of magmatic crustal building that followed accretion and primordial differentiation of the Moon. However, the origin and evolution of these unique rocks is highly debated. Here, we report on the microstructural characterization of a large (~250-μm) baddeleyite (monoclinic-ZrO₂) grain in Apollo troctolite 76535 that preserves quantifiable crystallographic relationships indicative of reversion from a precursor cubic-ZrO₂ phase. This observation places important constraints on the formation temperature of the grain (>2,300 °C), which endogenic processes alone fail to reconcile. We conclude that the troctolite crystallized directly from a large, differentiated impact melt sheet 4,328 ± 8 Myr ago. These results suggest that impact bombardment would have played a critical role in the evolution of the earliest planetary crusts.

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