It is commonly thought that volcanic glass beads only record volatile loss during pyroclastic eruptions in the Moon. During outgassing, volatile elements within a glass bead volatilize and are lost to the ambient environment, resulting in decreasing volatile contents in the glass beads towards the surface. However, our recent work shows that many volatile elements (such as Na, K and Cu) of lunar volcanic beads are enriched near the bead surfaces and depleted in the bead interiors, forming an overall “U-shaped” profile. The “U-shaped” profile means that rather than being “lost” into space, those elements were “gained” into volcanic glass during eruptions.

We propose that the observed “U-shaped” profile of volatile elements was formed by initial outgassing and subsequent in-gassing of these elements in the volcanic glass beads during their formation with the presence of a local atmosphere. A quantitative model has been developed to simulate the concentration evolution in the beads and constrain the cooling history. This is the first time that in-gassing in lunar volcanic eruptions is discovered and explained.