Understanding Hydrogen Variations in Silicate Glasses as a Result of Degassing: Fire-Fountaining on the Moon and Earth

Volatiles, particularly hydrogen, play a key role in volcanic eruptions, especially explosive eruptions like fire-fountaining [e.g., Saal et al. 2002; Dixon 1997; Arndt & von Engelhardt 1987; Yoder 1976]. Discerning volatile abundance and behavior during ascent and eruption can aid in understanding the source melt and primary volatile content of planetary interiors. Volcanic glasses are samples closest to the primary melt, as they quench quickly enough to limit fractionation or crystallization. This is paramount for volatile studies, especially pertaining to water as its constituents are oxygen and hydrogen. Hydrogen is the most volatile element and one of the first to vacate the system via degassing, therefore glasses quenched within minutes can preserve traces of water [Ustunisik et al. 2015; Saal et al. 2008]. There are several instances of terrestrial fire-fountain eruption styles such as Hawaiian glass beads. The products of these fire-fountain events are glass beads have been likened to the glass beads returned by the Apollo missions. The picritic glasses from Apollo 15 and 17 samples have been evaluated to determine the amount of volatiles lost during degassing during ascent and subsequent eruption [Hauri et al. 2015; Saal et al. 2008]. High-resolution measurements of water in lunar glasses are needed to constrain lunar volatile evolution.

This work evaluates the abundance and distribution of water in glass beads from the Kilauea volcanic complex of Hawaii as well as lunar glass beads from Apollo 15 and 17 via FTIR analyses. The lunar glasses are from samples 15427,83 and 74220,226 as well as fresh beads from the Apollo Next Generation Sample Analysis (ANGSA) program core 73002. Results of this work indicate that degassing is most effective via fractures and vesicles, though core-to-rim degassing is also present in larger samples. The findings that the ANGSA Apollo 17 samples have lower water abundance than 15427,83 suggests that samples with modification from previous analyses can impact the accuracy of the data. These data indicate that degassing via fractures and vesicles disrupts preservation of diffusive core-to-rim degassing and that the smaller lunar glasses can show hydration distributions that suggest complete degassing and some in-gassing.