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

Abstract

Accurately constraining the formation and evolution of the lunar magnesian (Mg) 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 characterisation 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 (> 2300 °C) which endogenic processes alone fail to reconcile. We conclude that the troctolite crystallized directly from a large, differentiated impact melt sheet 4326 ± 14 million years (Myr) ago. These results suggest that impact bombardment would have played a critical role in the evolution of the earliest planetary crusts. Insights into the formation, differentiation and impact bombardment of planetary bodies have been derived through geodynamic modelling, remote sensing observations, and isotopic analysis of planetary meteorites and returned samples. The resulting models are hindered by the paucity of mineralogical evidence that can place direct constraints on these very high temperature and pressure processes. Although many mineral thermobarometers can record geological temperatures ranging up to a maximum of ~1500 °C, empirical mineralogical and geochemical evidence of higher temperatures is often lost due to extensive

What EBSD and TKD Tell Us about the Crystallography of the Martensitic B2-B19′ Transformation in NiTi Shape Memory Alloys

“…There is another well-known cubic-monoclinic phase transformation in which this special feature is visible; it is that in zirconia. Even if the OR is not the same (the correspondence is just the identity in zirconia), we observed in previous studies the "dice-5" feature in monoclinic zirconia that comes from industrial refractory walls of glass furnaces [68], from Earth crustal rocks extracted under a meteoric impact [69], and from the Moon [70]. It was clear to us that this "dice-5" feature marks the coexistence of two ORs, but we could not explain it at that time, even by considering the tetragonal phase of zirconia as an intermediate state.…”

The response of zircon to the extreme pressures and temperatures of a lightning strike

“…Context is, therefore, critical to interpreting whether this high-temperature phase formed in response to a lightning strike, impact event, or other extreme pressure–temperature excursion. For example, petrographic context enabled White et al 39 to interpret evidence for precursor cubic ZrO 2 in a lunar sample as indicative of crystallization from a superheated impact melt 39 . Conversely, documenting precursor cubic ZrO 2 in glasses of unknown origin would be insufficient to infer an impact origin for such samples.…”

Effects of early intense bombardment on megaregolith evolution and on lunar (and planetary) surface samples

“…However, they also refer to three ferroan anorthosites dated by other authors at 4.43 AE 0.03, 4.47 AE 0.07, and 4.53 AE 0.12 Ga, which could record earlier impact and recrystallization events. Numbers of impact melt clasts, dated at~4.35 Ga, offer vestiges of one or more large impact structures during that period (Nemchin et al 2008;Grange et al 2013;and White et al 2020; see also Hartmann [2019] for review). Figures 1, 2a, and 3, of our present paper imply very intense bombardment, including multiple impacts of basin-forming magnitude, along with extreme crater oversaturation, during the suggested periods of magma ocean presence.…”

“…2013; and White et al. 2020; see also Hartmann [2019] for review). Figures 1, 2a, and 3, of our present paper imply very intense bombardment, including multiple impacts of basin‐forming magnitude, along with extreme crater oversaturation, during the suggested periods of magma ocean presence.…”