Water in the Moon : a geochemical approach

Barnes, Jessica Jane (2015). Water in the Moon : a geochemical approach. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f875


The last decade of lunar volatiles research has revolutionised the way we view the amount and potential sources of water (and other volatiles) in the lunar interior. In particular, studies of the most commonly occurring hydrous mineral in lunar samples, apatite, have been in the spotlight, especially with regards to mare basalt samples. In contrast, there are very few studies which investigate the hydrogen isotopic composition of water in samples representing the lunar highlands. The aim of this work was to determine the amount and source(s) of water in the lunar interior and this was achieved through studying a range of lunar samples of various ages and lithologies, with a particular focus on some of the rocks of the lunar highlands.

A method was developed to accurately and precisely determine the amount and hydrogen isotopic composition of water in lunar apatite, using a Cameca NanoSIMS 50L ion microprobe. A total of ten Apollo samples and one basaltic lunar meteorite were analysed by this method, including four mare basalts, four highlands samples, and three impact-related rocks. Complementary analyses of volatile abundances in lunar apatite were accomplished using both NanoSIMS and electron microscopy. In addition, a pilot study was undertaken to characterise the water content of mineral standards, the use of which will make it possible to extend the analysis of water by NanoSIMS to nominally anhydrous minerals in lunar samples.

Of the lunar samples studied, two samples were shown to have experienced post-magmatic metasomatic alteration, which is believed to have affected the volatile abundances of the apatite in these rocks. Similarly, apatites in two impact melt breccias were studied and are also thought to record secondary processes, in this case, relating to cratering processes. Thus, none of these samples retain information about the sources of indigenous lunar water. Apatite in mare basalts record water contents between ~ 700 and 7500 ppm H2O, with associated δD values from ~ +90 to + 1010 ‰. This work has confirmed that apatites in mare basalts likely record the process of magmatic degassing of H2, which resulted in the elevated δD signatures of the water trapped in these apatites from their initial δD values between ~ -200 and 0 ‰. The results of this study confirm the hypothesis that water is heterogeneously distributed in the lunar interior.

Additionally, apatites were studied in three ancient (ca. 4.3 to 4.4 Gyr old) lunar highlands samples and were found to contain between ~ 70 and 1640 ppm H2O, with δD values between ~ -390 and +200 ‰. This work considerably expands the existing dataset for the δD-H2O systematics for apatites in lunar highlands samples, and is consistent with a petrological link between the Mg-suite and alkali-suite rocks. After careful consideration of the potential secondary processes that may have altered the indigenous δD signature of lunar apatites, we conclude that these apatites do indeed preserve their magmatic H-isotopic compositions. By extension, they also record the δD signature of the purported lunar magma ocean stage in lunar history.

This research has important implications for the types and proportions of materials that were likely to have been bombarding the Earth-Moon system ca. 4.5 billion years ago. It is suggested that most lunar interior water was likely derived from CI-type carbonaceous chondrites, together with some CM and/or CO chondrite material, invoking a common origin for water in the Earth-Moon system (and perhaps also Mars).

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