In this new paper, group members John Pernet-Fisher and Katherine Joy review high temperature (> ~1000 oC) thermal metamorphism on the Moon.
Thermally metamorphosed rocks on the Moon are an important, yet under-studied group of rock types that have been identified within the Apollo and lunar meteorite collections. These rocks, with their characteristic granoblastic and poikilitic textures, are generally referred to as granulites. These distinctive textures are the result of recrystallization. That is to say, the temperature of the original rock (protolith) was sufficiently hot (~1000 oC) over sufficiently long timescales (hundreds to thousands of years) that the rock completed recrystallised.
The textural and chemical information published in the literature for the granulites have enabled the thermal conditions that resulted in metamorphism to be constrained. Currently, the leading hypothesis states that granulites formed in proximity to hot (~ 2000 oC) impact melt sheets. In these areas, underling rocks were able to be heated over the time scales needed to fully recrystallize the original crustal rock. Additionally, the chemical data reported gives important insights into the protoliths (original rock type) of the granulites. Indeed, some granulites have chemistry that indicate they were recrystallised from crustal rocks not found in the Apollo collection. Thus, the granulite suite offers a unique insight into the range of rock types that make up the lunar crust.
Understanding the range of rock types in the lunar crust is important as this enables us to test how primary crusts were formed on inner solar system bodies, a record of which is no present on geologically active bodies such as the Earth (see our previous post exploring lunar crustal formation models).
The full citation for our new paper is below and it can be accessed here:
Pernet-Fisher, J. F., & Joy, K. H., (2021). Thermal Metamorphism on the Moon as Recorded by the Granulite Suite. Journal of the Geological Society. doi.org/10.1144/jgs2021-044 or green open access here
Further Lunar Sample Resources:
NASA Apollo sample curation office https://curator.jsc.nasa.gov/lunar/index.cfm
LPI Lunar Sample resources http://www.lpi.usra.edu/lunar/samples/
Virtual Microscope http://www.virtualmicroscope.org/content/moon-rocks
Lunar Meteorite List http://meteorites.wustl.edu/lunar/moon_meteorites_list_alumina.htm
Why we should go back and explore the Moon in the future https://earthandsolarsystem.wordpress.com/2017/05/12/the-moon-putting-an-end-to-been-there-done-that/