Reassessment of pre-eruptive water content of lunar volcanic glass based on new data of water diffusivity

Abstract

Lunar volcanic glasses associated with mare basalt magmatism experienced significant degree of degassing, and to retrieve their initial water contents requires data of water diffusivity. We carried out diffusion experiments at 0.5 GPa and 1703–1903 K in a piston cylinder apparatus for two synthesized lunar basaltic melts with compositions corresponding to Apollo green glass and yellow glass. The water diffusion profiles measured by FTIR spectroscopy yield water diffusivities 0.5–1 order of magnitude greater than those of terrestrial basaltic melts, which is attributed to the difference in melt polymerization and modest contribution from H2 diffusion. However, hydroxyl (OH) is not only the dominant H species but is also inferred to be the major carrier of H in our experiments at oxygen fugacity estimated IW ± 1 (IW: iron-wüstite buffer). Modeling of previously reported profiles of volatile loss in an Apollo green glass bead using the new water diffusivity indicates an average cooling rate of 1–2°C/s and an initial water content of 120–260 μg/g. With the assumption of limited degassing before magma fragmentation, the lunar mantle source is inferred to contain 6–22 μg/g H2O. The lunar interior appears to be less hydrous the Earth's interior but still contains a considerable amount of water.