The hydrogen isotopic composition of lunar melt inclusions: An interplay of complex magmatic and secondary processes

A Stephant,M Anand,R Tartèse,X Zhao,G Degli-Alessandrini,I.A Franchi

doi:10.1016/j.gca.2020.06.017

A Stephant, M Anand + Show 4 more

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https://doi.org/10.1016/j.gca.2020.06.017

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Abstract

Since the discovery of water (a term collectively used for the total H, OH and H2O) in samples derived from the lunar interior, heterogeneity in both water concentration and its hydrogen isotopic ratio has been documented for various lunar phases. However, most previous studies have focused on measurements of hydrogen in apatite, which typically forms during the final stages of melt crystallisation. To better constrain the abundance and isotopic composition of water in the lunar interior, we have targeted melt inclusions (MIs), in mare basalts, that are trapped during the earliest stages of melt crystallisation. Melt inclusions are expected to have suffered minimal syn- or post-eruption modification processes, and, therefore, should provide more accurate information about the history of H in the lunar interior. Here, we report H−/18O− measurements as calibrated water concentrations, and hydrogen isotope ratios obtained by secondary ion mass spectrometry (SIMS) in a large set of basaltic MIs from Apollo mare basalts 10020, 10058, 12002, 12004, 12008, 12020, 12040, 14072 and 15016. Our results demonstrate that partially crystallised MIs from lunar basalts and their parental melts were influenced by a variety of processes such as hydrogen diffusion, degassing and assimilation of material affected by solar-wind implantation. Deconvolution of these processes show that lunar basaltic parental magmas were heterogeneous and had a broadly chondritic hydrogen isotopic composition with δD values varying between −200 and +200‰.

Highlights

Since the discovery of indigenous water in lunar volcanic glasses (Saal et al, 2008), the abundance and origin of water in the lunar mantle has been vigorously debated (it is important to note that the term ‘‘water” used here collectively refers to all major forms of hydrogen (i.e., H, OH, H2O), being measured in the target lunar mineral/phase)
Homogenised melt inclusions (MIs) plot within the range defined by bulk-rock composition, while crystallised MIs display a large range in composition, revealing the evolution of the parent melt at their time of trapping
Major-element compositions suggest that the studied MIs could have potentially recorded a larger span of melt crystallisation, and represent a snapshot of melt composition at the time of their trapping

Summary

Introduction

Since the discovery of indigenous water in lunar volcanic glasses (Saal et al, 2008), the abundance and origin of water in the lunar mantle has been vigorously debated (it is important to note that the term ‘‘water” used here collectively refers to all major forms of hydrogen (i.e., H, OH, H2O), being measured in the target lunar mineral/phase). Stephant et al / Geochimica et Cosmochimica Acta 284 (2020) 196–221 g H2O for at least some portions of the lunar mantle (McCubbin et al, 2010b; Hauri et al, 2011, 2015; Hui et al, 2013, 2017; Tartese et al, 2013; Furi et al, 2014; Chen et al, 2015; Ni et al, 2019) This range of water content for the lunar mantle is still an order of magnitude lower than some estimates for the water content of the bulk silicate Earth (ca. 1000–3000 mg/g H2O (Marty, 2012); 1100

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