Ries impact-generated hydrothermal system 14

The Ries crater is one of the first impact sites where an impact-generated hydrothermal system has been proposed (Engelhardt 1972; Salger 1977; Stähle & Ottemann 1977; Osinski 2005). The occurrence of secondary mineralization and hydrothermal alteration

of the impact suites has been noted and described (e.g., Förstner 1967; Engelhardt 1972; Stähle 1972; Jankowski 1977; Stöffler et al. 1977; von Engelhardt & Graup 1984; Newsom et al. 1986; Engelhardt et al. 1995; Graup 1999; Osinski 2003, 2004; Osinski et al. 2004; see Osinski, 2005 for a detailed study of hydrothermal alteration of the Ries impactites).

Using a combination of petrographic and analytical SEM techniques, Osinski (2005) has identified a number of hydrothermal alteration phases within the surficial suevites including clays (dominantly montmorillonite), zeolites, quartz, calcite, hematite and goethite. Alteration phases of the crater suevite include: potassium-feldspar, albite, clays, chlorite, zeolites, calcite, and minor phases including pyrite, goethite, barite and siderite. Alteration assemblages occur in three main settings: 1) open-space cavity and fracture fillings within the groundmass; 2) vesicle linings/fillings within impact glass clasts; and 3) pervasive alteration of groundmass phases and glass clasts (Osinski 2005). Overall the glass clasts are well preserved in the surficial suevites (Engelhardt & Graup, 1984; Engelhardt et al. 1995; Graup 1999; Osinski 2003, 2005). The hydrothermal fluids of the Ries impact-generated hydrothermal system were likely derived from a combination of meteoric water from the overlying crater lake and ground waters from nearby country rocks. There is no evidence of a magmatic or metamorphic source (Osinski 2005).

Due to the focused hydrothermal alteration of the suevite units, these impactites were likely the main heat source driving hydrothermal circulation (Osinski 2005; Fig. 2.2). Emplacement temperatures >750o_{C – 900}o_{C have been suggested for the suevites based}

on evidence of ductile deformation in glasses following deposition (Osinski et al. 2004). Intense pervasive hydrothermal alteration is limited to the crater suevites indicating that early, high temperature (200oC – 300oC) hydrothermal activity was restricted to the crater fill units (Osinski 2005). The surficial suevites were affected by the main and late stages of hydrothermal activity that are characterized by lower peak temperatures (<100oC – 130oC constrained by the lack of illite) and intermediate argillic alteration and zeolitization (Osinski 2005). With the exception of rare instances of pervasive alteration noted in glasses at some localities, the dominant alteration of the surficial suevites is montmorillonite and Ba-phillipsite within cavities, fractures and vesicles. It is significant

to note that neither clasts of pre-impact target rocks nor impactite phases were enriched in Ba. Therefore the Ba must have been dissolved by the hydrothermal fluids, transported and precipitated during zeolitization of the surficial suevites (Osinski 2005).

Recent work by Muttik et al. (2008) suggests that the Ries impact-generated hydrothermal system was limited to the intensely altered crater suevites and that the alteration of the surficial suevites can be entirely attributed to ambient weathering processes. It is argued that the main alteration phase of the surficial suevites identified as montmorillonite by whole rock powder XRD is chemically homogenous throughout the surficial suevites consistent with low temperature hydrous devitrification of impact glasses. However, Osinski (2005) noted that hydrothermal alteration in the surficial suevites was limited to localized zones including fractures and vugs. Bulk XRD alone is not a suitable technique to identify trace assemblages in spatially restricted zones. It is likely that alteration assemblages formed by post-impact weathering processes are the predominant assemblages of the surficial suevites considering the limited extent of hydrothermal activity in these units. Furthermore no explanation is offered regarding the Ba-phillipsite phase within the surficial suevites.

Figure 2.2: Idealized cross section of the Ries crater schematically illustrating the post-impact hydrothermal system.

Schematic cross section showing the heterogeneous distribution of hydrothermal cells relative to the crater centre. The main, high temperature hydrothermal activity is concentrated in the crater fill material beneath a transient crater lake with isolated, patchy systems distal to the crater rim. The primary heat source driving hydrothermal circulation is from impact-heated materials. Note the discontinuous nature of isolated outcrops of glass-bearing breccia (suevite) overlying the Bunte breccia outside the crater rim. Modified from Osinski (2004).

2.2.3.1

Alteration of Crater Suevite

Alteration phases of the crater suevite include: K-feldspar, albite, clays, chlorite, zeolites, calcite, and minor phases including pyrite, goethite, barite and siderite. The alteration assemblages as recorded in the Nördlingen core, are consistent with an early, high- temperature (200 – 300oC) phase of K-metasomatism coinciding with albitization and chloritization followed by pervasive intermediate argillic alteration and zeolitization (Osinski 2005).

2.2.3.2

Alteration of Surficial Suevite

A number of hydrothermal alteration phases consistent with low-temperature (<100 – 200oC) hydrothermal activity including clays, zeolites, quartz, calcite, hematite and goethite have been identified in glass bearing breccia located beyond the crater rim (Newsom et al. 1986). The main alteration phase is montmorillonite and Ba-phillipsite. It is significant to note that neither clasts of pre-impact target rocks nor impactite phases were enriched in Ba. Therefore the Ba was likely dissolved by the hydrothermal fluids, transported and precipitated during zeolitization of the surficial suevites (Osinski 2005).

2.2.3.3

Alteration of Surficial Suevite at Depth

Study of the Wörnitzostheim core has shown alteration assemblages consistent with the surficial suevites described above defined by vesicle filling montmorillonite, Ba-rich phillipsite forming within vesicles, and groundmass montmorillonite. At depth (>78 m) montmorillonite and illite become major components and zeolitization occurs. Mineralogical and petrographic evidence of the hydrothermal alteration assemblages present in glass-bearing breccias at the Ries impact structure are presented blow.

2.2.3.4

Evidence for Hydrothermal Activity Outside the Crater Rim

Alteration textures are spatially restricted at the metre scale and include coliform/rhythmic banding, vesicle infilling, pervasive alteration to complete replacement of glass clasts by clay minerals and the occurrence of platy clays. If a limited extent of hydrothermal activity is assumed in these units, then alteration assemblages within the ejected glass-bearing breccia are predominantly formed by post-impact weathering

processes. However, an extremely spatially limited hydrothermal system outside the crater rim does not offer an explanation for the Ba-phillipsite phase within the glass- bearing breccias. Furthermore, the similarity of the alteration assemblages between the surficial suevite and the suevite in the Wörnitzostheim core(s) suggests these phases are not due to weathering processes as the Wörnitzostheim core suevite was protected by ~20 m of overburden.

2.2.3.5

Hydrothermal Alteration Summary

Studies of the alteration textures of glassy and formerly glassy clasts within both the ejected and crater-fill glass-bearing breccias has shown a consistent progression from fresh glass through various states of alteration. The phases of alteration inferred from such textures include incipient, low temperature alteration (perlitic fracturing, devitrification and decomposition textures) to evidence of fluid circulation (alteration zones surrounding perlitic fractures and vesicles, banding and zonation) resulting in progressive alteration (globular replacement textures, platy clays) and finally pervasive alteration and complete replacement including the formation of Ba-phillipsite (harmatone) and montmorillonite in both the crater-fill and ejected glass-bearing breccias. Alteration of the surficial suevite followed a progression from high- to low- temperature with textures consistent with hydrothermal alteration, sensu stricto over a wide temperature spectrum. Hydrothermal systems were likely spatially extensive in the surficial suevites with localized, higher intensity systems sporadically distributed (Fig. 2.2). Hydrothermal alteration was likely preceded by high-temperature devitrification or autometamorphism and followed by low-temperature weathering.

Similar textural and mineralogical evidence of hydrothermal alteration in both the crater- fill and ejected glass-bearing breccias suggests a similar progression of alteration processes in both units consistent with hydrothermal alteration. It is suggested that the impact-generated hydrothermal system at the Ries impact structure was much more extensive and pervasive outside the crater rim area than previously reported.