¹J. F. Pernet-Fisher,¹K. H. Joy,¹M. E. Hartley
Journal of Geophysical Research (Planets)(in Press) Link to Article [https://doi.org/10.1029/2022JE007570]
¹Department of Earth and Environmental Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL UK
Published by arrangement with John Wiley & Sons

Rocks of the lunar granulite suite are the product of high-temperature metamorphism within the Moon’s crust. However, to date, their formation conditions have few constraints. Here we combine Ti-in-pyroxene element diffusion modelling and two-pyroxene thermometry with thermal modelling of the lunar crust in order to assess potential heat sources that could generate granulite metamorphism within the lunar highland crust. For the samples investigated in this study, the pyroxene crystals experienced peak metamorphic temperatures between ∼1027 and 1091 ºC over timescales ranging from ∼153 years to ∼15.1 kyrs. To best satisfy these temperature and timescale constraints, hot (∼2300 ºC) impact melt sheets with thickness ranging from 350 m to 3.35 km – equating to impact crater diameters between ∼60 and 280 km – have the potential to heat the underlying anorthositic crust. Deep (>20 km) igneous bodies, such as the large (>10 km thick) sills observed by the GRAIL mission near the base of the lunar crust, also have the potential to generate the required peak metamorphic temperatures; however, the thermal equilibration timescales in this scenario are modelled to be much larger (> 100 kyrs) than was witnessed by the granulites investigated. Our modelling highlights that, while lunar granulites are only a minor component within the Apollo and meteorite collection, they are likely an important and ubiquitous lithology within the lunar highland crust.