In-gassing of Na, K and Cu in Lunar 15366 Green Glass Beads

It is commonly thought that volcanic glass beads only record volatile loss during the pyroclastic eruptions in the Moon. We recently reported the discovery of outgassing and in-gassing of moderately volatile element Na, K and Cu in lunar orange glass beads, the high-Ti basalts (Su et al., 2022). The observed "U-shaped" Na, K and Cu concentration profiles of the orange glass beads are caused by initial outgassing and subsequent in-gassing of these elements as a result of the cooling of the surrounding gas plume (Su et al., 2022). Here we report similar findings in green beads in Apollo sample 15366, part of the green glass clods collected with lunar sample 15425/6. These glass beads represent another important composition type of lunar volcanic beads, with ultramafic composition but very low Ti contents (<1 wt% TiO2). These beads also show an in-gassing feature of Na, K and Cu, as verified by EMP analyses and LA-ICP-MS mapping (Fig. 1). The in-gassing diffusion distances in the green beads are similar to or slightly longer than those in the orange beads.

A quantitative model is being developed to simulate the concentration evolution of these moderately volatile elements in the green glass beads, in order to constrain the cooling history of the volcanic gas in the Apollo 15 green bead eruption. The presence of similar in-gassing diffusion profiles of Na, K and Cu in different pyroclastic beads from different eruptions indicates a common behavior of lunar volcanic gas, such as a transient volcanic atmosphere. However, there are differences between the green and orange beads. Compared to the orange glass beads, a larger number of lunar 15366 green beads show a glassy texture and hence are inferred to have quenched more rapidly. Moreover, green beads have much smaller surface mineral condensates than the orange beads (Liu et al., 2021). The difference provides key information in the fire-fountain eruptions. For example, it may imply a higher initial temperature for the lunar green glass beads and an early dissipation of the surrounding transient volcanic gas plume.