Magmatic Processes That Produced Lunar Fire Fountains: Evidence from Vesicular Rims on Picritic Glass Beads

Introduction: The standing hypothesis for the formation of the lunar picritic glass beads is eruption in a volatile-driven fire fountain followed by degassing while suspended in a hot, turbulent vapor plume. Reanalysis of the Apollo 15 A, B, and C green glass beads from slide 15426,72, led to the discovery of beads with small patches of highly vesicular green glass adhering to their rims (fig. 1). There are now three related compositional data sets for the lunar pi-critic glasses: the glass bead compositions themselves, the vesicular glass rims, and thin surface coatings on the beads, which may represent the gases that drove the fire fountain eruptions (e.g., [1,2], c.f. [3]). Methods: The beads and their rims were analyzed with a JEOL-JXA 733 Superprobe electron microscope at MIT. The interior of each glass bead was analyzed five times with a 10-micron beam spot size, using a 10 nA beam current and an accelerating voltage of 15 kV, for major oxide components. Rims were also analyzed five times when space permitted. During a separate analysis session we measured the trace elements Ni, S, Cl, Cu, Zn, and F, with five separate analyses of each sample, using a 10 micron spot size but a 200 nA beam current and counting times up to 300 seconds. Results: We found four beads with partial vesicu-lar rims: beads 80A, 84A, 155B, and 190B (all bead numbers are from Delano's notation). All the vesicular rims are extremely high in S (1000 to 2300 ppm) and contain as much as 420 ppm Ni. The S content in particular makes the rim compositions distinctive from the glass beads they adhere to. There is no detectable F, P2O5, Zn, or Cu in the vesicular rims, and Cl is near or below the detection limit (fig. 2). The beads themselves contain no F or Zn and low S, but one A subgroup contains notably high Cl. The rim glasses have different major and minor element compositions than the beads they are attached to, and they show no diffusion profiles with the interior bead. In contrast to the ve-sicular rims, the thin surface films on many beads are enriched in S, Cl, F, and Zn (e.g., [4]). Discussion: The vesicular rim glasses must have erupted simultaneously with the glass beads. Bead 80, in particular shows vesicles from the glass rim pressing into the glass of the bead itself (fig. 1). This process …