Kreep glass and the exotic provenance and formation of polymict breccia 66055

Abstract

Polymict breccia 66055 from the Apollo 16 site is unique in that it contains abundant brown KREEP glass (0.3–0.4% K2O) and related brown glassy breccias which have fluid forms. During assembly it consisted of hot and cold debris which flowed together. Most of the rock consists of clasts of poikiloblastic breccia (granulitic impactite) and mesostasis‐rich impact melt (VHA basalt), both of which predate the assembly of 66055. In contrast, the KREEP glass was created in the impact which produced the rock. KREEP compositions are sparse among glasses in soils and other breccias at the Apollo 16 site, and the rock would not have formed locally; it must be exotic. Terrains in which 66055 might have formed exist to the west of the landing site. Suitable igneous target materials for the KREEP glass have not positively been identified in 66055, so the petrogenesis of Apollo 16 KREEP remains enigmatic, although the presence of a ‘granitic’ glass fragment embedded in KREEP glass suggests that it is igneous. A local volcanic origin is more consistent with the spatial relationships, as deduced from orbital geochemical data, than is a basin ejecta deposit.