Outgassing and Ingassing of Na in Lunar 74220 Orange Glass Beads

Volcanic glass beads are known to have undergone the degassing process during eruptions. During degassing, volatile elements within a glass bead volatilize and are lost to the ambient environment. However, when we measured Na concentration profiles in lunar 74220 orange glass beads using EMPA and SIMS, we found that the Na concentrations within many beads display unexpected increase towards the bead surface. This observation is consistent with the discovery of high Na signals in the ~20-nm thick surface layer of 74220 orange glass beads from depth profiles (Meyer et al., 1975). In contrast to Na enrichment towards edge, the centers show roughly uniform Na concentrations that are lower than the pre-eruptive Na concentration estimated from 74220 OH-MIs. This discovery means that, in addition to outgassing, there is also ingassing of Na into the glass beads during their flight in a fire-fountain eruption. The occurrence of both outgassing and ingassing of Na in orange glass beads might sound unintuitive. A diffusion model was developed, which successfully reproduces the observed Na profiles. The model assumes simple monotonic cooling, and a surface condition that depends on temperature with free fitting parameters. The key to understand the occurrence of both outgassing and ingassing during the flight and cooling of a glass bead is that the equilibrium Na vapor pressure has a positive dependence on temperature (De Maria et al., 1971). At the eruption temperature, the equilibrium Na pressure for a given glass bead is higher than the ambient Na partial pressure. Hence, Na diffuses out of the bead, leading to outgassing. At sufficiently low temperatures, the equilibrium Na pressure becomes lower than the ambient Na partial pressure. Hence, Na diffuses back into the bead, leading to ingassing. Therefore, the occurrence of both outgassing and ingassing is a natural consequence of cooling of a flying glass bead in an atmosphere containing some Na. In addition, the different shapes of Na concentration profiles in various glass beads can be explained by different cooling rates, which may be related to their flight trajectories through the eruption plume. The discovery of outgassing and ingassing of single glass beads has the potential to reconstruct volcanic vapor pressures of volatile elements during eruptions.