A Study of Apollo 16 Feldspathic Glasses: Locally Produced or Ballistically Deposited?

Introduction: Continuous bombardment by aster-oidal and cometary debris over the last 4.5 Ga has churned the lunar surface into a fine-grained regolith. The violent nature of these impacts has largely destroyed the primordial lunar crust, reworking it into a variety of breccia and glass fragments that make up the majority of most Apollo soils. Nowhere is this more true than at the Apollo 16 site, located in the ancient, heavily cratered (even for the Moon) lunar highlands. Numerous investigators have analyzed glass fragments in Apollo 16 soil samples [1-7]. Two recent studies make different assumptions about the provenance of impact glass in the Apollo 16 regolith. The study of Taylor et al. [4] targets " agglutinitic glass. " They assume that most (" >90% " [5]) of impact-produced glass in the regolith derives from agglutinates, that is, that most glass fragments in lunar soils are produced locally by micrometeorite impacts. Delano et al., while acknowledging that a significant proportion of feldspathic glasses appeared to be locally produced (although not necessarily by micrometeorite impacts), suggest that a non-trivial proportion of the glasses (including feld-spathic glasses) were produced by large post-basin (<3.9 Ga) impacts at significant distances from the Apollo 16 site and ballistically transported to the Apollo 16 [6]. In this study we have begun a coordinated major-, minor-, and trace-element study of Apollo 16 glasses (concentrating on the feldspathic glasses) to ascertain the relative abundances of glasses that have a " local " composition (essentially the same as the local Apollo 16 soil) or a composition exotic to the Apollo 16 site, suggesting a provenance at a significant distance from the Apollo 16 site. Methodology: The glasses presented in this study are " clean " glasses found in two grain mounts of the 64-105 μm and 105-210 μm size fractions of sample 61241, a submature Apollo 16 soil (I s /FeO of 47, [8]). In each grain mount, an area encompassing >1500 particles was imaged in transmitted, cross-polarized, and reflected light as well as with high-resolution back-scattered electron image mosaics and elemental x-ray maps. These images were used to identify which particles were glasses, and what their morphologies and textures were. These glasses were then analyzed for major-and minor-elements by EPMA [7]. Petrography: A variety of glass types were identified , including clean (N=379), clast-laden (102), cryptocrystalline (53), vitrophyric (69), and agglutinitic (614) glasses. Only data for the " clean …