Differential Alteration of Glass Clasts in the Surficial Suevites of the Ries Crater, Germany

The mid-Miocene (14.3 + 0.2 Ma; [1]) Ries impact crater located in southern Germany was formed in a two-layer target comprised of dominantly Mesozoic flat lying siliciclastic and carbonate sedimentary rocks that unconformably overlie crystalline Hercynian basement [2]. Ries is a complex crater with a total diameter of ~24 km [2]. Impactite and ejecta deposits are exceptionally well preserved. The surficial suevites comprise one of four main proximal ejecta deposits [3] and are divided into two distinct lithological units: 1) the dominant main suevite that represents a clast-rich particulate impact melt rock or impact melt-bearing breccia [3, 4]; and 2) subordinate basal suevite [5]. Four main glass types occur within the main suevite both as ground-mass phases and as discrete glass clasts [6]. Glass clasts are typically vesiculated, schlieren-rich mixtures containing abundant mineral and lithic fragments [3]. Here we present a detailed mi-cro-textural and chemical study of the surficial suevites focusing on alteration of the glass clasts. Evidence of a post-impact hydrothermal system at the Ries crater has been documented [e.g., 7]. Despite the identification of hydrothermal alteration phases within the surficial suevites [7], it has been suggested that the Ries post-impact hydrothermal system was restricted to the crater-fill units [8]. Our study suggests that alteration of glass clasts within the surficial suevite followed a progression from high-to low-temperature alteration with textures consistent with hydrothermal alteration, sensu stricto, between the two temperature end members. Alteration textures indicative of hydrothermal alteration are spatially restricted and include colliform/rhythmic banding, amygdaloidal infilling, pervasive alteration to complete replacement of glass clasts by clay minerals and the occurrence of course-grained, platy clays. This hydrothermal alteration was preceded by high-temperature devit-rification or autometamorphism and followed by low-temperature weathering. The latter two processes represent the dominant form of alteration of the surficial suevites, which is consistent with a limited hydrothermal system within the surficial suevites. Ongoing work seeks to understand the relative importance and timing of the various alteration processes.