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

Abstract

The nature of the 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 SiO 2 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 SiO 2 -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.

Post‐impact alteration of surficial suevites in Ries crater, Germany: Hydrothermal modification or weathering processes?

“…Zeolites have been recognized in the groundmass and impact glass clasts of surficial suevites by several authors (e.g., Engelhardt and Graup 1984;Osinski et al 2004;Osinski 2005). In this study only harmotome was recognized in one Altenbürg sample (degassing pipe) with amounts of ∼2.7 wt%.…”

“…The "main suevites" comprise consolidated impact glass clast-rich blue-grey suevite, which constitutes the bulk of all suevite outcrops. The underlying "basal suevites" are poorly consolidated finegrained suevite with minor content of glass and rock fragments (Bringemeier 1994;Osinski et al 2004). The surficial suevite is lithologically different from the so-called crater-fill suevite, which is only observed in drill cores.…”

“…IHT is a well known phenomenon that has been found at several terrestrial craters varying in size and target compositions (e.g., Koeberl et al 1989;McCarville and Crossey 1996;Ames et al 1998;Sturkell et al 1998;Naumov 2002;Osinski et al 2001Osinski et al , 2005Kirsimäe et al 2002;Hagerty and Newsom 2003;Hode et al 2003;Versh et al 2005); similar features have also been suggested for impacts on extraterrestrial planetary bodies (e.g., Allen et al 1982). At the Ries crater, IHT has been addressed in detail by Newsom et al (1986) and Osinski (2003Osinski ( , 2005, and Osinski et al (2004). These studies have shown that hydrothermal alteration of the crater-fill suevites (polymict impact breccias-crater suevites, Engelhardt et al 1995) is pervasive and characterized by argillic-type hydrothermal alteration with zeolitization; an early phase of K-metasomatism accompanied by minor albitization and chloritization at temperatures of approximately 200-300 °C also occurred (Osinski 2005).…”

“…Newsom et al 1986;Osinski et al 2004;Osinski 2005). It has been generally accepted that these clays formed by postimpact hydrothermal alteration of impact-generated glasses and/or finely comminuted crystalline basement material (Engelhardt 1972;Stähle 1972;Engelhardt and Graup 1984; Newsom et al 1986Newsom et al , 1990.…”

“…Newsom et al (1986) suggested that ∼10-15 vol% of the montmorillonite is of hydrothermal origin in the groundmass of surficial suevites. However, Engelhardt (1997) and Osinski et al (2004) reported that the suevite groundmass contains up to ∼50 vol% of "clayey" material that is evidently a hydrous silicate (montmorillonite, according to Engelhardt 1997); however, the exact nature and origin of this clayey groundmass remains poorly constrained. Similarly, Ames et al (2004) concluded from mineral assemblages of impactites in core Yaxcopoil-1, Chicxulub impact crater, that the alteration of impact glasses has resulted from low temperature seawater interaction rather than typical high temperature hydrothermal alteration.…”