In our new paper, we explore the conditions within the Moon’s crust that result in the formation of granulites. Granulites are a rock type that formed from high temperature recrystallisation of existing highland rocks (see our previous post reviewing the granulite suite). The chemistry of the minerals within these rocks reflect their formation. Therefore, we can use mineral chemistry to constrain the formation conditions of granulites. In this work, we investigated 10 granulites from Apollo 16 and 17.
The minerals orthopyroxene and clinopyroxene can be used to estimate the formation temperature of the granulites (i.e., peak metamorphic temperature). The samples investigated in our study formed at temperatures between 1027 and 1091 oC. At these temperatures some elements will migrate through the crystal structure of a mineral (a process known as diffusion). This results in what is known as ‘chemical zoning profiles’, where the concentration of an element will change between the centre and the rim of a mineral. The speed at which elements ‘diffuse’ within a mineral (i.e., diffusion rate) is related to the peak metamorphic temperature. Therefore, as we know the metamorphic temperature and if we know the diffusion rate of a given element, then we can estimate how long a sample was held at high temperatures during metamorphism. The granulites investigated in our study were held at high temperatures over time periods ranging from ~153 years to 15.1 Kyrs.
Using these temperature and time constraints we are able to place constraints on the type of processes that could act as the heat source within the Moon’s crust. Hot (> 2000 oC) impact melt sheets are able to heat the surrounding crust to the timescales and temperatures indicated by the samples in our study. The thicker the melt sheet, the longer the crust can be kept at high (> 1000 oC) temperatures. For the samples investigated here, impact melt sheets with thicknesses that range from 350 m to 3.35 km are needed. This is equivalent to impact craters ranging in size from 60 to 280 km. Impact craters of these sizes represent ∼13% of all craters that are >1 km in diameter. Consequently, this work indicates that granulites are an important rock type associated with impact craters.
The full citation for our new paper is below and it can be accessed here:
Pernet-Fisher, J. F., Joy, K. H., & Hartley, M. E. (2023). Using Lunar Granulites to Constrain Re-Equilibration Timescales of Contact Thermal Metamorphism on the Moon. Journal of Geophysical Research: Planets. doi.org/10.1029/2022JE007570.
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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/
Earth & Solar System 