Assessing the volatile inventory and history of the Moon using lunar meteorites

Hayden, Tara Siobhan (2023). Assessing the volatile inventory and history of the Moon using lunar meteorites. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0001569f

Abstract

Many in situ measurements of volatile elements (F, Cl, H, and S) have been collected on lunar samples. Whilst this has significantly broadened our understanding of the volatile inventories and evolution of the Moon, these studies have been focused mainly on crystalline samples from the lunar nearside which restricts our understanding of lunar geology, geochemistry, and volatiles of the regions sampled. Lunar meteorites may have been ejected from anywhere on the lunar surface, hence detailed analysis of a diverse set of lunar meteorites may expand our knowledge and challenge existing models of lunar evolution. A large proportion of lunar meteorites (and the Apollo samples) are brecciated, however. The processes which give rise to brecciation on the Moon may have led to the loss of geological context and key phases for the study of volatiles. This study primarily focuses on investigating volatiles (H, F, Cl) in apatite from five brecciated lunar meteorites to gain new insights into the history of volatiles in the Moon.

This study primarily focuses on Cl and H, using the NanoSIMS 50L, Cl and H₂O abundances and Cl and H isotopic compositions measured on 34 apatite grains. CeO₂ abundances of apatite are used as a proxy for rare earth element contents, which are compared to Cl and H results to investigate correlations and to attempt to reconstruct parental melt H₂O contents. Apatite Cl abundances range from ~ 10 to 36500 ppm and are associated with a δ³⁷Cl range from – 1 to + 53 ‰. Apatite H₂O abundances range from ~ 130 to 4500 ppm and are associated with a δD range from – 940 to + 1030 ‰. The higher δ³⁷Cl values are associated with negative δD in some apatite grains located in highlands-type clasts as well as in isolated grains occurring within the matrix of these breccias. No obvious correlation between δD and δ³⁷Cl is apparent, however. The heavier δ³⁷Cl values appear to bridge the gap between existing data on Apollo and basaltic lunar meteorites and extreme δ³⁷Cl values observed in one lunar meteorite, which supports greater heterogeneity of the lunar Cl record. Light H isotopes observed in some quartz monzodiorite clasts may indicate a significant solar wind/nebular input into the lunar precursor material and/or interaction with solar-wind derived H in the lunar regolith.

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