Suevite breccia from the Ries crater, Germany: Origin, cooling history and devitrification of impact glasses

Abstract

Abstract-The distribution and petrography of surficial suevite breccias of the Ries impact crater in Southern Germany are reviewed, and the morphology, petrography and chemical composition of impact glasses in suevite breccias and their postdepositional devitrification is synthesized. Origin and thermal history of suevite breccia and suevite glasses are inferred from these data and from recent results of cooling and crystallization experiments with suevite glass melts under controlled conditions. In a montmorillonitic groundmass, the suevite breccia contains pieces of glass, up to some decimeters in size, and crystalline rock clasts of all stages of shock metamorphism. The glass particles originated in impact melt of basement gneisses and cooled by adiabatic pressure release from -80 GPa to atmospheric pressure during ejection from the crater. They were deposited on the ground together with the other suevite components at a temperature of~750 DC. Fractured glass pieces in the breccia show that during deposition of the suevite the temperature was below the temperature at which undercooled melt transforms to rigid glass. The suevite cooled after deposition mainly by convection of heat by emanating gases and vapors. In chilled layers at the base and at the top of suevite deposits, the glasses are preserved in vitreous state. Between these zones, the glasses were devitrified, yet crystallization of pyroxene, plagioclase and magnetite took place below the glass-transformation temperature. Annealing experiments show that this unusual devitrification below the transformation temperature can be explained by the impact origin of suevite glasses. Due to rapid adiabatic cooling on decompression, the glasses were oversaturated with water and internally strained. Under these conditions, devitrification, especially the formation of plagioclase, was possible at temperatures below the transformation range. The origin from adiabatically cooled impact melt of deep-seated rocks distinguishes water-bearing suevite glasses from the Ries-derived, water-free moldavite tektites, which are interpreted as condensates of vaporized, surficial sediments (Engelhardt et ai., 1987).

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

“…The surficial suevite is lithologically different from the so-called crater-fill suevite, which is only observed in drill cores. The crater-fill suevites form a massive (up to 400 m thick) layer in the crater depression below the post-impact crater lake sediments (Engelhardt et al 1995). The crater suevite is a polymict breccia with lesser amount of glassy material as compared to surficial suevites (Engelhardt et al 1995;Engelhardt 1997).…”

“…Surfical suevites contain components from the crystalline basement at all stages of shock metamorphism and shockmelted glassy inclusions, specifically vitreous and devitrified impact glass (Engelhardt et al 1995;Engelhardt 1997). Surficial suevites are found in a large number of small isolated patches inside the morphological rim and up to radial distances of ∼14 km beyond the rim to the south-southwest and east-northeast (Fig.…”

“…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).…”

Ancient Impact and Aqueous Processes at Endeavour Crater, Mars

“…At the Ries, thin deposits of surficial suevite (19), often just a few m thick, overlie the Bunte Breccia that dominates the impactites of the rim (20). The Bunte Breccia is a poorly sorted polymict lithic breccia.…”

Incipient melt formation and devitrification at the Wanapitei impact structure, Ontario, Canada

“…The rock is a strongly shocked metasedimentary rock (i.e.. a pebbly, arkosic wacke) that originally and prior to the shock event contained various metasedimentary and crystalline rocks as clasts The rock is not an inclusionbearing impact melt derived through ineking and homogenization of a large volume of target rocks. In comparison with some recent investigations on glasses of the Ries structure impact (Engelhardt et al, 1995), we attempt to estimate the postdeposition annealing temperature of the investigated sample.…”