Thermal metamorphism on the Moon as recorded by the granulite suite

Abstract

Thermally metamorphosed rocks on the Moon are an important, yet under-studied, suite of lithologies that have been identified within the Apollo and lunar meteorite collections. These rocks, with granoblastic and poikilitic textures, are generally referred to as granulites. However, unlike their terrestrial counterparts, which are the metamorphic products of both high temperatures and high pressures, lunar granulites are thought to be the products of only high-temperature (>1000°C) thermal metamorphism, which completely recrystallized their protoliths. We summarize the range of textures and chemical systematics reported from lunar granulites. These data enable constraints to be placed on the thermal conditions in the lunar crust required for high-temperature metamorphism to have taken place. Most studies indicate that impact melt sheets have the relevant thermal properties to sustain high temperatures over the timescales required to fully recrystallize the surrounding crustal lithologies. However, the roles of alternative heat sources, such as magmatic intrusions into the crust, have not been extensively investigated and, as such, cannot be ruled out. In addition, the chemical data yield important insights into the protoliths of the granulite suite. By identifying the protoliths, we greatly enhance our understanding of the range of lithologies that make up the primary lunar crust. In turn, this enables crustal formation models to be better constrained.Supplementary material: All the geochemical data discussed within this review are available at: https://doi.org/10.6084/m9.figshare.c.5623326