Cooling History and Evolution Dynamics of Green Glass Beads During Lunar Fire-Fountain Eruptions: Insights From Na, K and Cu Distributions

Volcanic glass beads on the Moon have traditionally been thought to only record volatile loss during pyroclastic eruptions. However, recent discoveries have shown that lunar orange glass beads, representing primitive high-Ti basalts, experienced both outgassing and in-gassing of volatile elements such as Na, K, Cu, and S. In this work, we examine lunar green glass beads from samples 15421 and 15366, representing primitive very-low-Ti basalts, for the distribution of Na, K and Cu using EMP analyses and LA-ICP-MS mapping. It is found that all studied lunar green beads show increased Na, K and Cu concentrations near the bead surfaces, indicative of in-gassing. A quantitative model was developed to simulate the concentration evolution of Na and Cu in individual green glass beads during eruption and cooling. The presence of similar in-gassing diffusion profiles of volatile elements in beads from different eruptions indicates a common behavior of lunar volcanic gas. In addition to volatile in-gassing, LA-ICP-MS mapping of Na and K in one green bead from sample 15366 shows features suggesting collision of melt droplets during the fire-fountain eruption, revealing more details in the dynamic aspects of lunar fire-fountain eruptions. Compared to orange glass beads, the varying boundary conditions of green glass beads during formation may suggest that their eruption plume evolved and dissipated more rapidly, potentially linked to changes in the global lunar atmosphere.