Using Lunar Granulites to Constrain Re-Equilibration Timescales of Contact Thermal Metamorphism on the Moon

Abstract

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 modeling and two-pyroxene thermometry with thermal modeling 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 ∼1,027 and 1,091°C over timescales ranging from ∼153 years to ∼15.1 Kyrs. To best satisfy these temperature and timescale constraints, hot (∼2,300°C) 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 modeled to be much larger (>100 Kyrs) than was witnessed by the granulites investigated. Our modeling 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.