The nature of groundmass of surficial suevite from the Ries impact structure, Germany, and constraints on its origin

Abstract

Surficial suevites from the Ries impact structure have been investigated in the field and using optical and analytical scanning electron microscopy. The groundmass of these suevites comprises calcite, clay minerals, impact melt glass, crystallites (plagioclase, garnet, and pyroxene), francolite, and Ba-phillipsite. The latter zeolite is a secondary phase. Abundant textures have been observed: intricate flow textures between the various groundmass phases, globules of each phase in the other phases, spheroids of pyrrhotite in calcite, the "budding-off" of clay globules into silicate glass and/or calcite, euhedral overgrowths of francolite on apatite clasts, and quench-textured crystallites in the groundmass. Groundmass-forming calcite displays higher FeO, MnO, and SiO2 contents than limestone target material. The composition of suevite "clay minerals" is highly variable and not always consistent with montmorillonite. Three types of glasses are distinguished in the groundmass. Type 1 glasses are SiO2-rich and are clearly derived from sandstones in the sedimentary cover, while the protoliths of the other two glass types remains unclear. Analytical data and micro-textures indicate that the calcite, silicate glass, francolite, and clay minerals of the groundmass of the Ries suevites represent a series of impact-generated melts that were molten at the time of, and after, deposition. On cooling, plagioclase, pyroxene, and garnet crystallized from the groundmass. These results are at variance with the current, traditional descriptive definition of suevite. Given that Ries is the original type occurrence of "suevite," some modification to the traditional definition may be in order. As the results of this study are most consistent with the groundmass of Ries surficial suevites representing a mix of several types of impact-generated melts, we suggest that a possible origin for these suevites is as some form of impact melt flow(s) that emanated from different regions of the evolving crater.