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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The authors declare that data supporting the findings in this study are available within the paper and its Supplementary Information files. All other data are available from the corresponding author upon request.
The code that was used for the U–Pb analysis is available from the corresponding author upon request.
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L.F.W. and A.C. are funded by a Hatch postdoctoral fellowship. A.C. has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 704696 RESOLVE, and M.A.’s contribution to this work was partly funded by the UK Science and Technology Facilities Council (ST/L000776/1 and ST/P000657/1). The NordSIMS facility is supported by Swedish Research Council infrastructure grant 2017-00671 and the Swedish Museum of Natural History; this is NordSIMS publication 609. K.H.J. acknowledges STFC grant ST/M001253 and Royal Society grant UF140190. J.R.D. acknowledges STFC grant ST/S000291/1 and Royal Society Research Grant RG160237. We thank G. Long for conducting colloidal silica polishing of the thin section to facilitate EBSD analysis of the target grain. M.A. thanks NASA CAPTEM for the allocation of polished thin section 76535, 51.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Marc Norman and Jennifer Whitten for their contribution to the peer review of this work.
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White, L.F., Černok, A., Darling, J.R. et al. Evidence of extensive lunar crust formation in impact melt sheets 4,330 Myr ago. Nat Astron (2020). https://doi.org/10.1038/s41550-020-1092-5
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