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Hydrothermal activity associated with the Ries impact event, Germany

Authors


G. R. Osinski, Canadian Space Agency, 6767 Route de l'Aeroport, Saint-Hubert, Quebec J3Y 8Y9, Canada.
Email: osinski@lycos.com. Tel: +1 450 926 4478, Fax: +1 450 926 4766.

Abstract

Combined field studies, optical and scanning electron microscopy, and electron microprobe studies of impactites from the Ries impact structure, Germany, have allowed a clearer picture of the hydrothermal system associated with the Ries impact event to be made. Hydrothermal alteration is concentrated within impact-generated suevites in the interior of the crater (crater suevites) and around the periphery (surficial suevites), with minor alteration in the overlying sedimentary crater-fill deposits. The major heat source for the Ries hydrothermal system was the suevite units themselves. Hydrothermal alteration of crater-fill suevites is pervasive in nature and comprises several distinct alteration phases that vary with depth. An early phase of K-metasomatism accompanied by minor albitization of crystalline basement clasts and minor chloritization, was followed by pervasive intermediate argillic alteration (predominantly montmorillonite, saponite, and illite) and zeolitization (predominantly analcite, erionite, and clinoptilolite). Hydrothermal fluids were typically weakly alkaline during the main stage of alteration. In contrast to the crater-fill suevites, alteration within surficial suevites was typically restricted to montmorillonite and phillipsite deposition within cavities and fractures. The pervasive nature of the alteration within the crater-fill suevites was likely due to the presence of an overlying crater lake; whereas alteration within surficial suevites typically occurred under undersaturated conditions with the main source of water being from precipitation. There are exceptional outcrops of more pervasively altered surficial suevites, which can be explained as locations where water pooled for longer periods of time. Hydrothermal fluids were likely a combination of meteoric waters that percolated down from the overlying crater lake and groundwaters that flowed in from the surrounding country rocks.

Introduction

Impact cratering is a ubiquitous geologic process that affects all planetary objects with a solid surface. The formation of an impact crater is a catastrophic, instantaneous geologic event that releases huge amounts of energy. Ever since the discovery of the Chicxulub impact structure, Mexico, and its link to the Cretaceous–Tertiary mass extinction event, the deleterious effects of impact events have received much attention (e.g., Kring 2000, 2003). However, in recent years, it has become apparent that there are several beneficial effects of impacts events, the most observable of which is post-impact hydrothermal activity. The interaction of shock-melted and/or heated rocks and minerals within an impact crater with H2O in the near-surface region will induce a hot rock-water circulatory system that can dissolve, transport, and precipitate various mineral species (Osinski et al. 2001).

It is becoming increasingly clear that hydrothermal activity will occur after the impact of an asteroid or comet into any water/ice-rich solid planetary surface, exceptions being with small craters (<1–2 km diameter on Earth) and in extreme arid environments (Naumov 2002). Recent studies have also shown that impact-induced hydrothermal activity can result in economically viable mineral deposits (e.g., Zn–Pb–Cu–Ag–Au deposits at the Sudbury impact structure, Canada; Ames et al. 1998). Impact-associated hydrothermal systems may also provide habitats for thermophilic (heat-loving) microorganisms, which may have been important for the origin and evolution of life on Earth, and possibly other planets such as Mars (e.g., Cockell & Lee 2002; Rathbun & Squyres 2002).

Evidence for hydrothermal activity has now been recognized at many of the approximately 170 impact structures on Earth (see Naumov 2002, for a review). However, there have been relatively few detailed studies published to date. Some notable exceptions include investigations of the Chicxulub (Ames et al. 2004; Hecht et al. 2004; Lüders & Rickers 2004; Zürcher & Kring 2004), Kara (Naumov 2002), Kärdla (Jõeleht et al. 2005; Versh et al. 2005), Haughton (Osinski et al. 2001, 2005), Popigai (Naumov 2002), Puchezh-Katunki (Naumov 1993, 1999, 2002), Siljan (Hode et al. 2003), and Sudbury (Ames et al. 1998, 2000, 2002, 2005) impact structures.

In this study, the results of combined field, optical and scanning electron microscopy (SEM) investigations of post-impact hydrothermal products at the Ries impact structure, Germany, are presented. Several previous workers have noted the occurrence of secondary minerals within the various types of Ries impactites (e.g., Förstner 1967; Engelhardt 1972; Stähle 1972; Jankowski 1977; Stöffler et al. 1977; Engelhardt & Graup 1984; Engelhardt et al. 1995; Graup 1999; Osinski 2003; Osinski et al. 2004), but only a few in-depth studies have been made (Salger 1977; Stähle & Ottemann 1977; Newsom et al. 1986). It should be noted that it was not until the early 1980s that the presence of secondary minerals within impact craters was linked with impact-induced hydrothermal activity (e.g., Newsom 1980). Furthermore, it is only in the past few years that hydrothermal activity has been recognized as a fundamental process associated with impact events. This study is the first to investigate the nature of the Ries hydrothermal system in its entirety. In addition, SEM and electron microprobe techniques, in particular back-scattered electron (BSE) imagery, have allowed a better understanding of the petrography and mineral chemistry of hydrothermal alteration products to be made. These new results, when combined with the results of previous studies at the Ries and other impact sites, allows a better understanding of the Ries hydrothermal system to be developed.

Geologic setting of the Ries impact structure

The approximately 24 km diameter Ries impact structure is one of the best preserved terrestrial complex impact structures (Pohl et al. 1977). This unique geologic site in Bavaria, southern Germany, formed between 14.3 and 14.5 Ma (Schwarz & Lippolt 2002; Buchner et al. 2003; Laurenzi et al. 2003) in an approximately 470–820 m thick flat-lying sequence of predominantly Mesozoic sedimentary rocks that unconformably overlay Hercynian crystalline basement (Fig. 1; (Pohl et al. 1977; Schmidt-Kaler 1978b).

Figure 1.

(A) Simplified geologic map of the Ries impact structure, after Schmidt-Kaler (1978a), with sample locations of surficial suevites investigated in this study. (B) Simplified stratigraphic column showing the target sequence at the Ries structure. Compiled with data from Schmidt-Kaler (1978b).

The Ries structure consists of a central crater cavity approximately 12 km in diameter (so-called inner ring) that is filled by a series of crater-fill impactites (rocks affected by impact metamorphism; Stöffler & Grieve 1994) (Fig. 1A; Table 1), overlain by an approximately 400 m thick series of post-impact lacustrine sedimentary rocks (Figs 1A and 2; Pohl et al. 1977). No surface exposures exist of the crater-fill or so-called ‘crater suevites’; however, this unit has been sampled by three drill holes (Deiningen, Nördlingen 1973, and Hole 1001; Fig. 1A). Only the 1206 m deep Nördlingen 1973 drillhole penetrated the entire suevite layer, with approximately 270 m of suevite core being recovered (Fig. 2). Magnetic field measurements reveal a negative anomaly (−300 nT) coincident with the central cavity, which has been attributed to the presence of reversely magnetized crater-fill suevites (Pohl & Angenheister 1969). The central basin is bounded by a prominent ‘inner ring’ composed of weakly shocked rocks from the uppermost part of the crystalline basement, together with sedimentary lithologies. No central peak is present at the surface; however, geophysical studies suggest the presence of an uplifted ring of basement rocks with a diameter of approximately 4–5 km (Pohl et al. 1977). The region between the inner ring and the tectonic rim (approximately 24 km diameter) is characterized by a chaotic mixture of large (m to 100 m scale) blocks of both crystalline and sedimentary rocks (megablock zone; Pohl et al. 1977), overlain by various different types of proximal impactites (Figs 1A and 3; Table 1): (1) Bunte Breccia and megablocks; (2) polymict crystalline breccias; (3) surficial suevites; and (4) coherent impact melt rocks.

Table 1.  Main attributes of proximal impactites of the Ries impact structure Germany.
 Bunte Breccia*Surficial suevite‡Crater suevite‡Impact melt rocks§
  1. *Data from Hörz et al. (1983).

  2. ‡Data from Pohl et al. (1977) and Osinski et al. (2004).

  3. ‡Reliable data only available for melt-rich suevite. Data from Stöffler et al. (1977).

  4. §Data from Osinski (2004).

Average thicknessApproximately 20 to >100 mApproximately 30 mApproximately 300 m<10 m
VolumeApproximately 200 km3Approximately 0.084 km3Approximately 15 km3Not known
Radial rangeApproximately 6–37 kmApproximately 6–22 km<6 km radiusApproximately 9–12 km
Distribution of depositsContinuousIsolated patchesContinuousIsolated patches
Nature of the groundmassClasticMeltNot knownMelt
Groundmass phasesN/ACalcite, glass, clays, crystallites (plag., gnt., pyx.), francolite, zeolitesZeolites, clays, K-feldsparK-feldspar, glass, plagioclase
Total clast content (vol.%)10030–40Approximately 30–35<15
Predominant clast typeSedimentaryCrystallineCrystallineCrystalline
Max. shock level of clasts (GPa)
 Crystalline<40Approximately 100Approximately 100Approximately 100
 Sedimentary<10Approximately 100<10 
Figure 2.

Simplified stratigraphy and alteration mineralogy of the Nördlingen 1973 drill core (see Fig. 1A for location of drill site). Arrowheads indicate sample depths investigated for this study. Data for the sedimentary crater-fill deposits and the brecciated crystalline basement are from Salger (1977).

Figure 3.

Modal composition (grain size fraction <1 mm) of selected samples of surficial suevite from the Ries impact structure. Calcite, fine-grained ‘clays’, glass, and crystallites represent primary impact melt phases. Platy montmorillonite and zeolites are hydrothermal alteration phases (see text for details).

Samples and analytic techniques

Over 100 samples of proximal impactites (suevites, impact melt rocks, Bunte Breccia, polymict crystalline breccias, and shocked target lithologies) were collected from outcrops around the Ries impact structure during August 2000 and June 2001. This study also included optical microscopy studies on samples from the impact collection of the Geological Survey of Canada, which now resides at the University of New Brunswick, Canada. Crater-fill suevites were sampled from the Nördlingen 1973 drill core, archived at the Center for Ries Crater and Impact Crater Research, Nördlingen, Germany (ZERIN). Optical and SEM microscopy and electron microprobe investigations were performed on polished thin sections of 61 samples of proximal impactites and 24 samples of crater-fill suevites. Quantitative compositions of hydrothermal mineral and impact melt phases were obtained using a combination of wavelength dispersive X-ray techniques on a JEOL JXA-8900 L electron microprobe (beam operating conditions of 15 kV and 20 nA), and energy dispersive spectrometry (EDS) on a JEOL 6400 digital scanning electron microscope (beam operating conditions of 15 kV and 2.5 nA at a working distance of 37 mm). SEM and electron microprobe data were reduced using ZAF procedures incorporated into the operating systems. BSE imagery was used to investigate the micro-textures of the various impactites. The clast content and modal composition of the various impactites were measured on representative digital BSE images using an image analysis program (Scion Image). X-ray diffraction (XRD) was performed on powdered samples using a Bruker D8 X-ray diffractometer.

Alteration within impact craters

Impact events generate shock pressures and temperatures that deform, metamorphose, and melt substantial volumes of the target rocks. Many of these materials, in particular impact glasses, are unstable at the surface of the Earth and are susceptible to alteration. Thus, high temperature devitrification of impact melt rocks and glasses, in addition to hydrothermal alteration, hydration, and diagenesis at lower temperatures, can greatly modify the chemistry and textural relationships of impactites to varying degrees. Subsequent metamorphic and tectonic events can also influence the textural and chemical evolution of impact-generated and impact-modified rocks and minerals, particularly in older impact craters.

Alteration of proximal impactites

Primary composition and devitrification of impact melt phases

No impact melt phases in the Bunte Breccia or polymict crystalline breccias have been documented to date. Coherent impact melt rocks occur as isolated bodies, with lateral extents of approximately 10–100 m, around the periphery of the Ries structure (Fig. 1). These lithologies comprise a reddish, highly vesicular microscopic groundmass containing variably shocked lithic and mineral clasts (Engelhardt et al. 1969; Pohl et al. 1977; Graup 1999). The groundmass comprises a series of primary impact melt phases: alkali feldspar (An0.0−14.6Ab16.4−31.4Or54.0−81.0), plagioclase (An1.5−44.2Ab23.0−57.8Or1.5−75.4), quartz (β-quartz and α-cristobalite), and ilmenite (decreasing order of abundance) with the interstices filled by either a fresh or devitrified glassy mesostasis (Osinski 2004).

The Ries surficial suevites comprise a fine-grained groundmass containing variably shocked mineral and lithic clasts from a wide range of target lithologies (e.g., Engelhardt & Graup 1984; Engelhardt 1990). Recent optical and SEM studies have revealed that the groundmass of the suevites consists of calcite, ‘clays’, silicate impact melt glass, crystallites (plagioclase [An35−62Ab35−60Or2−6], almandine garnet, and aluminian subsilicic pigeonite), francolite (carbonate–hydroxy–fluor–apatite), and Ba-phillipsite (Figs 2, 4 and 5; Osinski et al. 2004). Analytic data and micro-textures indicate that the calcite, silicate glass, francolite, and the majority of the fine-grained ‘clays’ represent a series of impact-generated melts that were molten at the time of, and after, deposition (Fig. 5; Osinski et al. 2004). The fine-grained ‘clay’ component of the suevites is either X-ray amorphous clay and/or an as yet unidentified hydrous phase(s) formed from the devitrification of hydrous impact melt glasses, and/or that it remains (at least in part) in its original glassy state (Osinski 2003; Osinski et al. 2004). Thus, it is apparent that minerals that are typically ascribed to post-impact secondary alteration (e.g., calcite and clays) can also represent impact-generated melt phases and/or devitrification products of primary melt phases.

Figure 4.

Boxplots showing the range of major oxide contents for glasses in the suevite groundmass. The composition of the four types of impact glass clast present in suevites is plotted for comparison (data from Osinski 2003). Boxes define the interquartile range with the median value shown as a horizontal line. Whiskers extend from the boxes to the highest value in the data set, or to a distance of 1.5 times the interquartile range, whichever is less. Outliers are plotted as solid diamonds. From Osinski et al. (2004).

Figure 5.

Back-scattered electron photomicrographs showing representative micro-textures displayed by fine-grained ‘clays’, calcite, and glass, indicating that these phases are primary impact melt phases (see discussion in Osinski et al. 2004). (A) Globules of clay and interstitial glass showing evidence for deformation and flow. A large vesicle is present in the right of the image. (B) Evidence for liquid immiscibility: globules and irregular patches of clays within calcite and silicate glass. (C) Globules of calcite within a silicate glass–calcite groundmass.

In addition to being present in the groundmass, impact-generated glasses are common as clasts in the Ries surficial suevites. The glasses are typically vesiculated, schlieren-rich mixtures, containing abundant mineral and lithic fragments (e.g., Engelhardt 1967, 1972; Stähle 1972; Engelhardt & Graup 1984; Engelhardt et al. 1995; See et al. 1998; Vennemann et al. 2001; Osinski 2003). The results of a recent analytic SEM study reveal that four main types of glass are present as clasts within the suevites (Fig. 4; Osinski 2003).

Petrography and mineral chemistry of alteration phases

Combined field, optical microscopy, and analytic SEM studies reveal that a series of alteration phases are present in the Ries surficial suevites and that these phases occur in three distinct 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. Secondary alteration products are rare in the other types of proximal impactites at the Ries structure. Therefore, except where otherwise indicated, the following description of alteration phases applies only to the suevites.

Clay minerals

Impact glass clasts are present in all stages of alteration in the Ries suevites (Fig. 6). However, it is notable that, on the whole, glasses are remarkably well preserved in the surficial suevites (cf., Engelhardt & Graup 1984; Engelhardt et al. 1995; Graup 1999; Osinski 2003). The first sign(s) of alteration within impact glass clasts is montmorillonite formation along perlitic and quench fractures (Fig. 6A,B). At the majority of the suevite outcrops studied here (35 of 44 suevite samples) and in others documented in the literature, this is the extent of the alteration of glass clasts. As the intensity of alteration increases, alteration fronts move out from the fractures toward the unfractured domains, which resulted in the formation of globular masses of montmorillonite (Fig. 6B–D). This montmorillonite alteration is associated with a systematic increase in the H2O and K2O content of the glasses, at the expense of all other oxides (Table 2). In three of the samples studied (00-001, 00-056, and 01-011), over half the glass clasts are completely replaced by montmorillonite (e.g., Fig. 6F) (cf., Stähle 1972) and zeolites (see below).

Figure 6.

Back-scattered electron photomicrographs showing the progressive alteration of impact glass clasts within Ries surficial suevites. (A) Montmorillonite formation along perlitic fractures in otherwise pristine glass. (B) Pronounced alteration of a glass clast, predominantly centered on perlitic fractures. The groundmass in this sample is predominantly Ba-rich phillipsite. (C, D) Altered glass clast in suevites from Steinbühe. The boxed letters in (D) refer to analyses of variably-altered glass in Table 2. (E) Glass clast completely replaced by Ba-rich phillipsite. (F) Glass clast completely replaced by montmorillonite.

Table 2.  EDS analyses displaying the progressive alteration of a type 1 glass clast (shown in Fig. 6D) (weight%)*. From Osinski (2003).
 Holohyaline glass ‘cores’†Altered glass ‘A’‡Altered glass ‘B’§Altered glass ‘C’¶
  1. *Cr, Mn, Ni, and S were below detection limit for all analyses. The following notes refer to the glass clast shown in Fig. 6D.

  2. †Holohyaline glass ‘cores’ are deemed to be the original fresh glass composition.

  3. ‡Glass ‘A’ contains sub-microscopic crystallites of pyroxene (<0.1–0.5 μm in diameter) and appears slightly darker than the holohyaline cores in BSE mode. This glass displays a slight decrease in FeO, MgO, and CaO contents, and a large increase in K2O content, compared with the holohyaline cores.

  4. §Glass ‘B’ is ‘mottled’ in appearance and significantly darker than the less altered glasses in BSE mode, indicating the presence of elements with a lower atomic number. Totals are approximately 90–92 wt.% indicating approximately 8–10 wt.% H2O. EDS analysis reveals that this glass is characterized by high K2O contents, with all other major oxides lower in concentration than less altered varieties.

  5. ¶Glass ‘C’ is very porous and very dark in BSE mode. This glass is thoroughly hydrated (totals approximately 87 wt.%) and bears no compositional resemblance to the original glass.

SiO265.9065.9565.7865.4765.8666.3360.8161.8861.8854.2853.1953.33
TiO20.880.880.860.920.910.910.721.080.530.030.470.69
Al2O315.7115.6315.7815.5615.7015.5014.9716.2415.4619.6618.2417.60
FeO5.335.295.284.554.083.380.991.180.979.6911.2410.39
MgO2.432.492.732.542.121.841.551.581.721.72
CaO4.584.544.413.023.132.622.292.092.071.451.431.38
Na2O4.454.644.763.273.473.402.293.562.90
K2O0.610.590.684.494.485.517.796.235.930.200.301.37
P2O50.260.270.290.270.280.320.290.32
Cl0.280.15
Total100.16100.02100.26100.09100.0399.7691.6892.2790.0587.1887.0186.79

In addition to replacing impact glass clasts, montmorillonite also occurs as vesicle linings within impact glass clasts and as cavity and fracture fillings in the groundmass of surficial suevites (Fig. 7A,B). These latter hydrothermal clays are typically present in amounts up to approximately 10–15 vol.% and are characterized by their platy habit and open-space filling textures (Fig. 7A,B), and by having a composition close to montmorillonite (Table 3). Importantly, this montmorillonite cross-cuts the fine-grained groundmass ‘clays’ (Fig. 7A). This distinguishes secondary hydrothermal clays from fine-grained groundmass ‘clays’, which display wide range in composition and which represent devitrified hydrous impact glasses (see above). This agrees with the XRD work of Newsom et al. (1986), which shows that montmorillonite is the only identifiable clay mineral present and that it comprises <10–15 vol.% the groundmass.

Figure 7.

Back-scattered electron photomicrographs showing typical textures of alteration phases within the groundmass of surficial suevites. (A) Fine-grained groundmass-forming clays formed via devitrification of hydrous glasses cross-cut by later platy montmorillonite, which displays open-space cavity fillings. (B) Platy montmorillonite in a highly altered suevite sample. (C) Ba-rich phillipsite has replaced all the primary groundmass phases in this sample.

Table 3.  Selected electron microprobe analyses of hydrothermal montmorillonite in surficial suevites from the Ries impact structure (weight%)*.
Analysis no.12345678910
Sample no.00-00100-00100-025b00-025b00-02900-052a00-052a01-01101-01101-028b
TextureVugPlaty vugPlaty vugPlaty vugVesicle liningVesicle liningVesicle liningPlaty vugPlaty vugPlaty vug
  1. *Cr, Mn, Na, Ni, P, and S were below detection limit for all analyses.

SiO252.8352.9756.2154.2155.3957.0354.6151.7949.1056.02
TiO2 0.34 0.31 0.14 0.73 0.55 0.15 0.36 0.52 0.26
Al2O316.9214.5018.9920.6916.6819.4117.7113.7811.8515.43
FeO 6.41 8.01 7.18 6.03 5.52 2.76 2.8213.3512.60 3.63
MgO 2.69 6.13 1.83 3.01 3.89 3.82 4.04 2.32 2.01 4.53
CaO 1.59 0.60 1.20 1.60 2.39 2.41 2.10 0.83 0.71 2.38
K2O 0.17 0.38 0.17 0.33 1.14 0.41 0.19 1.37 3.86 0.22
Cl 0.14 0.14 0.17 0.10 0.09 0.14 0.13 0.10 0.10
Total81.1183.0385.8986.7185.5685.9481.7783.9280.7482.57

Zeolites

Zeolites are present as vesicle fillings within impact glass clasts (cf., Engelhardt et al. 1995), and as a replacement mineral in the groundmass of Ries suevites. The only previous report of zeolites in the groundmass of surficial suevites is a reference to ‘tiny prismatic zeolite crystals (probably erionite)’ by Engelhardt & Graup (1984, p. 455). However, Osinski et al. (2004) recognized the presence of zeolite minerals in the groundmass of surficial suevites from four different locations (Fig. 3). Further studies have revealed the presence of zeolites in 9 of 44 suevite samples, from six locations (samples 00-001, -009, -049, -055, and -060, in addition to the four samples illustrated in Fig. 3).

Energy dispersive spectrometry analyses indicate that Ba-rich phillipsite is the dominant zeolite mineral present (Table 4); however, as noted by Osinski et al. (2004), the high Ba content in phillipsite may be due to a component of harmotome, which often occurs as an interpenetrating complex twin in this phase (Deer et al. 1963). Ba-rich phillipsite typically accounts for <1–2 vol.% of individual samples, with the exception of samples 00-050 and 00-056 which have 9.5 and 34.2 vol.% zeolites, respectively (Fig. 3). It is notable that at localities where zeolites are abundant in the groundmass, impact glass clasts are typically highly altered (e.g., Fig. 7C).

Table 4.  Average composition of Ba-phillipsite in the groundmass of surficial suevites. Modified from Osinski et al. (2004)*.
Series no.123456
Sample no.00-05600-05600-05600-05001-025a01-028b
Analysis no.955433
 wt.%SDwt.%SDwt.%SDwt.%SDwt.%SDwt.%SD
  1. wt.%, mean composition in weight%; SD, standard deviation (2σ); n.d., not determined.

  2. *Cl, Mn, S and Sr were below detection for all analyses.

SiO257.584.0158.031.4759.991.6255.621.7154.481.2852.772.61
Al2O318.801.4719.600.4518.300.5215.970.3317.220.6416.351.28
FeO0.060.24n.d.n.d.n.d.0.130.220.190.65
MgO0.040.22n.d.n.d.n.d.0.430.09n.d.
CaO3.911.743.831.353.360.724.430.676.850.125.140.62
Na2O0.600.940.770.940.830.940.160.65n.d.n.d.
K2O6.571.776.691.236.710.594.081.942.920.282.710.69
BaO1.220.362.700.452.590.642.900.170.050.183.310.38
Total88.793.5491.621.6091.792.5983.161.5182.081.3680.460.65

Quartz

Minor quartz (chalcedony or lussatite) is present as vesicle linings in impact glass clasts in the Ries suevites (cf., Engelhardt et al. 1995). A recent SEM study of Ries impact melt rocks has revealed that cristobalite cavity linings (Fig. 8) are not the result of low temperature hydrothermal alteration as previously thought (e.g., Engelhardt et al. 1969), but that they are products of high temperature vapor phase crystallization (Osinski 2004).

Figure 8.

Back-scattered electron photomicrograph of impact melt rocks from Polsingen in the northeast of the Ries impact structure. There is a consistent zonation within the cavities toward the interior from sanidine, to cristobalite, and finally hematite. Sanidine and cristobalite were deposited from the vapor phase. The origin of hematite is uncertain (see text).

Calcite

Calcite is present in many of the surficial suevite outcrops studied (Fig. 2). This mineral was originally ascribed to ‘weathering solutions’ (e.g., Engelhardt et al. 1995); however, recent work indicates that much of the calcite is a primary impact melt phase that originated via the shock melting of limestone target rocks (Graup 1999; Osinski 2003; Osinski et al. 2004). This impact melt calcite can be distinguished from calcite in the limestone target lithologies, both texturally (e.g., Fig. 5B,C) and chemically (high FeO, MnO, and SiO2 contents in the former) (Osinski et al. 2004).

Figure 9A shows a BSE image of a calcite melt clast from surficial suevites, which contains a series of silicate glass-coated calcite spherules embedded in microcrystalline calcite and an internal void, due to the presence of a coexisting vapor phase. Importantly, a second generation of large, euhedral calcite crystals project into the void (Fig. 9A). This calcite contains no spherules, is more Ca-rich, and contains only trace amounts of MnO or FeO, in contrast to the impact melt calcite (Fig. 9A,B). This coarse-grained vuggy calcite is, therefore, considered to be a late-stage secondary mineral phase. Similar late-stage vuggy calcite is present at many suevites localities. Calcite vugs and veins are also present in the polymict crystalline breccias and Bunte Breccia, where they are the dominant type of secondary alteration product.

Figure 9.

(A) Back-scattered electron photomicrograph showing a vesiculated calcite melt clast with embedded glass-coated calcite spherules. Note the vuggy calcite infilling the void at the center of the glass clast. (B) Electron microprobe element map of Mn, showing the difference in composition between the impact melt calcite and the later vuggy calcite. This late stage calcite is also poorer in Fe and Mg and richer in Ca than the impact melt calcite.

Hematite and goethite

The Ries impact melt rocks exhibit a characteristic red color, which is imparted by the presence of hematite, either lining or completely filling cavities (Fig. 8). It is unclear whether this mineral represents a low-temperature hydrothermal phase or if it was deposited from the vapor phase, or a combination of the two mechanisms (Osinski 2004). Goethite is a common minor alteration phase in many suevites. In only one sample (01-011) is this mineral present in any appreciable amount.

Alteration of crater-fill impactites

The allochthonous crater-fill within the ‘inner’ Ries crater (approximately 12 km diameter) comprises a thick series of so-called crater suevites. This work builds upon earlier studies of the crater suevites sampled in the 1206 m deep Nördlingen 1973 drillhole (Fig. 2).

Primary composition of impact melt phases

All primary impact-generated glasses in the crater suevite have been completely replaced by various secondary minerals (cf., Stähle & Ottemann 1977; Stöffler et al. 1977). In addition, the glasses contain only rare lithic and mineral clasts, in contrast to the surficial suevite glasses (Pohl et al. 1977). The only compositional data on glasses in the crater suevites is a single analysis published in Stähle & Ottemann (1977), which is similar to basement-derived glasses in the surficial suevites (type 1 glass of Osinski 2003).

Petrography and mineral chemistry of alteration phases

Post-impact hydrothermal alteration within the crater suevites is more pervasive than in the proximal impactites. Hand specimen observations, optical and SEM microscopy, and electron microprobe studies reveal the presence of several distinct alteration phases that vary distinctly with depth. Several of these phases have not been previously identified (e.g., K-feldspar and albite).

K-feldspar

Secondary K-feldspar occurs in two distinct associations: (1) irregular microscopic masses in the groundmass of crater-fill suevites (Fig. 10B); (2) completely or partly replacing plagioclase within clasts of crystalline basement material (Fig. 10F). K-feldspar forms up to approximately 5–10 vol.% in the upper parts of the melt-rich suevite unit and has not been recognized below approximately 530 m depth (Fig. 2). Electron microprobe analyses reveal that this mineral trends toward the orthoclase end-member with increasing depth (e.g., An5−8 Ab27−30 Or57−64 at 340.2 m and An0−1 Ab0−23 Or76−100 at 384.4 m).

Figure 10.

Back-scattered electron photomicrographs showing the variation in alteration of crater suevites (from the Nördlingen 1973 drill core) with depth. (A) A highly vesiculated impact glass clast completely replaced by analcite. Note the presence of clinoptilolite and calcite-filling vesicles. (B) Suevite groundmass replaced by early K-feldspar and later zeolites. (C) Glass clast altered to clay. Note the erionite crystals growing in from the margins of the altered clast. (D) Cavity in the suevite groundmass filled with analcite and minor calcite. (E) Impact glass clast replaced by analcite and saponite. Triangles indicate the outline of the clast. Analcite also occurs in the suevite groundmass. (F) Suevite groundmass replaced by K-feldspar, analcite, and calcite. (G) Suevite groundmass almost completely replaced by analcite. (H) Intergrowth of goethite and saponite with a cavity in the suevite groundmass, which is itself altered to analcite and saponite. (I) Plagioclase within a granitic clast altered to analcite and K-feldspar. (J) Pyrite–saponite vein cross-cutting a granitic clast. (K, L) Impact glass clast completely altered to montmorillonite and minor calcite and K-feldspar.

Albite

Minor albitization of primary plagioclase in crystalline clasts has been documented throughout the melt-rich suevite unit. At shallow depths, albite displays a wider range of compositions (e.g., An4−14 Ab66−80 Or15−19 at 340.2 m), than at deeper levels (e.g., An0−9 Ab89−99 Or0−2 at 528.0 m).

Clay minerals

Clay minerals are ubiquitous throughout the crater-fill suevites and have been documented previously by many workers, mainly using XRD techniques. Salger (1977) documented montmorillonite in the suevite groundmass, with illite and montmorillonite replacing crystalline basement clasts, in samples from the Nördlingen 1973 drill hole. Montmorillonite is the first alteration mineral in the impact glass clasts in the crater suevites (Stähle & Ottemann 1977). Mixed-layer montmorillonite–illite minerals have been observed in the Deiningen drill core (Förstner 1967).

The results of this study reveal that while montmorillonite is abundant throughout the crater-fill suevites, other clay minerals are also present (Figs 2 and 10). Saponite is particularly common, both replacing impact glass clasts (e.g., Fig. 10E) and in the groundmass (Figs 2 and 10H; Table 5, analyses 4–6). Illite and mixed-layer montmorillonite–illite minerals have also been documented in the Nördlingen 1973 drill core, predominantly in the upper parts of the melt-rich suevites unit (Fig. 2; Table 5, analyses 1, 2, and 7).

Table 5.  Selected electron microprobe analyses of hydrothermal phyllosilicate minerals in suevites from the Nördlingen 1973 drill core (weight%)*.
Analysis no.12345678910
Depth340.2340.2340.2384.4384.4435.2435.2476.0528.0528.0
PhaseIlliteIlliteMont.Sapon.Sapon.Sapon.IlliteChl.Mont.Mont.
TextureGr. ReplGr. Repl.Gr. Repl.Glass Repl.Glass Repl.Gr. Repl.Gr. Repl.Gr. Repl.Xst. Cl. Repl.Gr. Repl.
  1. Gr., groundmass; Repl., replacement; Sapon., saponite; Xst. Cl., crystalline clast; Chl., chlorite; mont., montmorillonite.

  2. *Cr, Mn, and Ni were below detection limit for all analyses.

SiO252.1148.9163.3232.4435.4727.1748.0627.6356.5956.69
TiO2 0.64 1.05 0.19
Al2O318.1317.7317.99 9.8413.7217.2626.9717.5218.6818.59
FeO 8.60 9.41 2.5221.3511.7122.46 2.2923.64 0.29-
MnO 0.42 0.46
MgO 4.49 5.54 1.96 8.3521.7415.50 1.6016.82 0.48
CaO 0.98 0.82 0.50 1.73 0.18 0.49 2.16 2.01
Na2O 1.92 0.97 8.40 7.82
K2O 5.36 5.74 0.38 0.23 0.12 0.37 8.21 0.23 0.17
SO3 0.41 0.21
Cl 0.10 0.11 0.09 0.08 0.20 0.10 0.09
Total90.4189.3288.6874.9882.7583.3787.9286.5787.1485.37

Chlorite

This mineral is present in minor amounts (typically 5 vol.%) throughout the suevites in the Nördlingen 1973 drill core (Fig. 2). Stähle & Ottemann (1977) noted that chlorite increases significantly in abundance at depths below approximately 525 m depth (i.e., in the melt-poor suevites) and into crystalline basement. Newsom et al. (1986) suggested that chlorite could represent weathered crystalline basement material. This would explain the increase in chlorite abundance in the crystalline megablocks in the Nördlingen 1973 drill hole. The results of this study are consistent with this theory; however, chlorite in the upper suevite layers, including the graded unit, occurs as a replacement mineral in the groundmass. Electron microprobe analysis reveals that the chlorite is an Fe-rich variety possibly with mixed corrensite layers (e.g., Table 5, analysis 8).

Zeolites

Several zeolites have been reported from samples of crater suevites from the Deiningen and Nördlingen 1973 drill holes: analcite, erionite, clinoptilolite, and wellsite, with minor phillipsite, harmotome, chabasite, and stilbite (Förstner 1967; Jankowski 1977; Salger 1977; Stähle & Ottemann 1977; Stöffler et al. 1977; Pfannschmidt 1985).

Analcite is the dominant zeolite mineral present and occurs in a variety of settings: (1) replacing impact glass clasts (e.g., Fig. 10A,E) (Table 6, analyses 5 and 6); (2) within cavities and fractures (e.g., Fig. 10D,G; Table 6, analysis 10); (3) replacing groundmass phases (e.g., Fig. 10E–G; Table 6, analyses 8 and 9); and (4) replacing clasts of crystalline basement material (e.g., Fig. 10F). It was previously believed that analcite dominates the lower half of the melt-rich suevite layer (438–525 m; Stähle & Ottemann 1977); however, this study reveals that this mineral is also abundant at shallower depths (e.g., Figs 2 and 10A,D–F). There is a notable decrease in the Si/Al ratio in analcite with depth.

Table 6.  Selected electron microprobe analyses of zeolite minerals in suevites from the Nördlingen 1973 drill core (weight%)*.
Analysis no.12345678910
Depth328.0340.2340.2340.2384.4384.4384.4528.0528.0528.0
PhaseClinop.ErioniteClinop.StilbiteAnalciteAnalciteErioniteAnalciteAnalciteAnalcite
TextureVesicle fillGr. Repl.Gr. Repl.VugGlass Repl.Glass Repl.Gr. Repl.Gr. Repl.Gr. Repl.Vein
  1. Clinop., clinoptilolite; Gr., groundmass; Repl., replacement.

  2. *Cl, Cr, Mn, Ni, P, S, and Ti were below detection limit for all analyses.

SiO265.9061.5765.4155.9461.8061.3158.1459.1557.9857.86
Al2O317.0319.4517.4116.7818.9918.8517.3020.8820.9020.17
FeO 0.19 0.50 0.28
MgO 0.53 0.47
CaO 3.54 5.63 0.67 8.19 1.64 0.15
Na2O 1.29 1.83 3.0310.21 9.43 4.08 8.34 8.1510.77
K2O 2.29 3.72 7.68 0.88 1.89
P2O5 0.10
Total90.7791.3692.9984.8291.0089.5984.1288.3787.0489.23

Of the various other zeolite minerals recognized in the crater-fill suevites, the most abundant are erionite, clinoptilolite, and stilbite (Figs 2 and 10A–C; Table 6, analyses 1–4 and 7). These minerals typically comprise <5 vol.% in total, although in a sample from 340.2 m depth, erionite and clinoptilolite were notably abundant (approximately 12 vol.% total). Furthermore, in the same sample, erionite and clinoptilolite also occur as a replacement mineral in the suevite groundmass, in contrast to their dominant setting as cavity and vesicle fills (Stöffler et al. 1977).

Calcite

Evidence for a primary impact melt origin for calcite in the crater-fill suevites has only been documented from the upper approximately 17 m thick graded unit in the Nördlingen 1973 drill core (Graup 1999). Stöffler et al. (1977) reported that finely dispersed calcite is a common constituent in the groundmass of the lower melt-poor suevite unit (525–602 m). High-resolution BSE imagery reveals that calcite is much more common than previously thought and occurs in the majority of the samples studied (Fig. 2). This mineral occurs as euhedral crystals, often associated with zeolites, in vesicles (e.g., Fig. 9A), cavities (e.g., Fig. 9D), replacing impact glass clasts (e.g., Fig. 9L), and in the groundmass (e.g., Fig. 9F) of crater-fill suevites. Electron microprobe analyses reveal that this secondary calcite is almost pure CaCO3, with <1 wt.% MgO and trace amounts of FeO and MnO (cf., secondary calcite in surficial suevites; Fig. 9).

Minor phases

Several other secondary minerals are present in minor amounts throughout the suevites of the Nördlingen 1973 drill core. Pyrite is present in the upper melt-rich suevites (e.g., Fig. 9J), while goethite occurs throughout (e.g., Fig. 9H). Stöffler et al. (1977) also recorded the presence of minor amounts of barite and siderite.

Mineralization within intra-crater sedimentary deposits

Crater suevites are overlain by an approximately 400 m thick series of post-impact lacustrine sedimentary rocks in the Nördlingen 1973 drill hole (Figs 1A and 2). Salger (1977) documented the presence of secondary illite, montmorillonite, kaolinite, and analcite in claystones and marls (0–256 m depth; Fig. 2), with the addition of chlorite in sandstones and conglomerates (256–324 m depth).

Mineralization around the crater rim region

The crater rim region is poorly exposed around the Ries structure. However, studies conducted in several quarries reveal the presence of goethite and calcite along faults and fractures. Several brecciated and mineralized pipe-like structures were observed in an abandoned quarry near Holheim, southwest of Nördlingen. These structures are continuous over several tens of meters and comprise a brecciated interior with extensive goethite and minor calcite alteration. These structures are similar in structure and extent to hydrothermal pipe structures seen around the rim region of the Haughton impact structure, Canada (Osinski et al. 2001).

Discussion

Alteration is an important part of the textural and chemical evolution of impact-generated and impact-modified rocks and minerals. By analogy with volcanic rocks, several processes may play a role in modifying impactites: devitrification, hydrothermal alteration, hydration, diagenesis, metamorphism, and tectonism (e.g., McPhie et al. 1993). Distinguishing between the effects of these various processes is critical in determining the origin and significance of different types of impactites, as well as for assessing the potential for hydrothermal mineral deposits and biologic habitats within a particular impact site.

Primary impact melt phases versus alteration products

The presence of clay minerals in the groundmass of Ries suevites has been recognized for several decades. It was generally accepted that these clays formed by post-impact hydrothermal alteration of impact-generated glasses and/or finely comminuted crystalline basement material (Engelhardt 1972; Stähle 1972; Engelhardt & Graup 1984; Newsom et al. 1986, 1990). This was also the case for the origin of calcite in the surficial suevites. However, recent studies reveal that calcite and a substantial proportion of the fine-grained ‘clays’ in the groundmass of Ries surficial suevites represent a combination of primary impact-generated melt phases and high temperature devitrification products (Graup 1999; Osinski 2003; Osinski et al. 2004). As a result, the amount of secondary alteration in the surficial suevites is lower than previously thought. Similar observations involving ‘montmorillonite globules’ were made in impact glasses from the West Clearwater Lake impact structure by Dence et al. (1974). The same can also be said for the Ries impact melt rocks, where cristobalite and K-feldspar cavity linings have been attributed to vapor phase crystallization (Osinski 2004), and not low temperature hydrothermal alteration as previously thought (e.g., Engelhardt et al. 1969). Thus, detailed textural and geochemical observations are required before certain minerals, such as clays and carbonates, can be attributed to hydrothermal alteration.

Heat source(s)

The localization and concentration of post-impact hydrothermal alteration within suevites suggests that these impactites were the dominant heat source that drove the Ries hydrothermal system (cf., the post-impact hydrothermal system at the similarly sized 23 km diameter Haughton impact structure, Canada; Osinski et al. 2001, 2005). Given a structural uplift of approximately 1–2 km, a minor amount of heat may have been contributed by the elevated geothermal gradient within central uplift lithologies (approximately 40–70°C assuming a geothermal gradient of 30°C km−1 and a surface temperature of approximately 10°C). An additional, as yet unquantified, heat component from the passage of the shock wave is also likely.

Thus, the post-shock temperatures of the suevite layer controlled the development of the Ries hydrothermal system. In terms of the initial temperatures of these impactites, surficial suevites show a reversed remanent magnetization (Pohl & Angenheister 1969), which, together with the complete loss of fission tracks in sphene from suevites from Otting (Miller & Wagner 1979), requires temperatures throughout these impactites of >580°C. Higher temperatures of >750–900°C are suggested by the presence of impact glasses, which show evidence for flow after deposition (Osinski et al. 2004), and the presence of decarbonated rims on some limestone clasts (Baranyi 1980). Temperatures in excess of approximately 600°C are also indicated for the approximately 194 m thick melt-rich crater suevite unit, with lower temperatures for the underlying melt-poor layer (Miller & Wagner 1979; Engelhardt 1990).

Post-impact alteration sequence

Based on the petrographic observations and mineral chemistry of alteration phases, a paragenetic sequence for hydrothermal activity associated with the Ries impact event is proposed (Fig. 11). Early, high temperature alteration is restricted to the crater suevites and is characterized by K-metasomatism throughout the upper parts of the suevite layer, with minor Ca–Na alteration (albitization) of crystalline basement clasts. Minor chloritization also probably occurred during this early stage. Geochemical and textural data suggest that the composition of these early hydrothermal fluids within the crater-fill suevites was largely controlled by the interaction of groundwaters with shock-metamorphosed and/or melted feldspars and glasses derived from the crystalline basement. Fluid inclusion studies necessary to constrain the fluid temperatures during this early stage have not been performed to date; however, given the mineral assemblage and by comparison with other craters where the same associations have been observed (e.g., Zürcher & Kring 2004; Versh et al. 2005), temperatures of approximately 200–300°C are estimated for this early stage. These temperatures are substantially lower than the original temperature of the suevites, which is consistent with rapid initial cooling of this unit (Osinski et al. 2004). Indeed, hydrothermal mineral assemblages indicate that the bulk of post-impact hydrothermal alteration within all impact craters studied to date occurred at temperatures <350°C (Naumov 2002). This suggests that the initial high temperature alteration stages (>300°C) are typically too short for alteration to achieve equilibrium phases (cf., Versh et al. 2005).

Figure 11.

Paragenetic sequence for hydrothermal alteration within the Ries impact structure (see text for details). CFS, crater-fill suevite; SS, surficial suevite; BB, Bunte Breccia; PCB, polymict crystalline breccias; SCF, sedimentary crater-fill deposits. The difference in relative temperatures for the deposition of illite and analcite within the crater-fill suevites and sedimentary crater fill lithologies is due to the time required for the deposition of the latter.

The main stage of hydrothermal alteration within the Ries structure was characterized by intermediate argillic alteration and zeolitization of the various impactites and overlying sedimentary crater-fill deposits, associated with a progressive cooling of the crater-fill and surficial suevite deposits (Fig. 11). During this stage, pervasive alteration of the crater-fill suevites occurred, with complete replacement of the unstable components of the groundmass (impact glass and comminuted target material), complete replacement of all impact glass clasts, and alteration of crystalline basement clasts. The main alteration phases throughout the bulk of the crater-fill suevite layer are saponite, montmorillonite, and analcite. Substantial illite, mixed-layer clays, erionite, and clinoptilolite formation also occurred in the upper levels of the crater suevites. The predominance of alkali and calcic zeolites is indicative of weakly alkaline hydrothermal solutions, which is typical for impact-induced hydrothermal systems in general (Naumov 2002). The source of the fluids for the Ries hydrothermal system were likely a combination of surface (meteoric) waters that percolated down from the overlying crater lake and groundwaters that flowed in from the surrounding country rocks into the hydrostatic void created by the impact event. There is no evidence for the input of magmatic or metamorphic fluids into the Ries hydrothermal system.

Alteration in surficial suevites is characterized by montmorillonite and Ba-phillipsite formation, typically within cavities, fractures, and vesicles, although more pervasive alteration occurred at a few locations. It is notable that in the zeolitized suevites, none of the clasts of pre-impact target rocks or impact-derived phases contain more than trace amounts of Ba2+. Therefore, hydrothermal fluids must have dissolved additional components from a relatively Ba-rich source in the underlying or surrounding sedimentary lithologies. The lack of illite interlayers in the montmorillonite constrains the main alteration event in the surficial suevites to <100–130°C (Newsom et al. 1986). Conversely, the presence of these minerals in the crater suevites indicates a higher temperature regime. The circulation of hydrothermal fluids and meteoric waters resulted in an increase in the K2O and H2O of impact glass clasts and a change in color from pale yellow-colorless to brown-red (Osinski 2003). The accommodation of strain following volume increases associated with the diffusion of meteoric water into the solid glass resulted in the formation of perlitic fractures in many glass clasts (cf., Marshall 1961). For example, impact glasses that display perlitic fractures typically have analytic totals <95–97 wt.%, compared to those glasses without fractures (totals typically >99 wt.%; Osinski 2003). The final stages of hydrothermal circulation within the suevites resulted in the deposition of calcite, and probably clays, within cavities and fractures. Vuggy calcite is also the dominant alteration phase within the surficial Bunte Breccia and polymict crystalline breccias. This indicates that the late stage hydrothermal fluids were saturated in Ca2+ ions, which is consistent with the predominance of Malm limestones underlying the various types of proximal impactites.

Intensity of hydrothermal alteration

It is evident that crater-fill suevites in the Nördlingen 1973 drill hole are pervasively altered with complete replacement of all primary impact-generated glasses. In contrast, alteration in surficial suevites is typically restricted to cavity and fracture fillings and the majority of the glasses are unaltered. These observations are consistent with the findings of Förstner (1967) who reported that surficial suevites in the Wörnitzostheim drill hole are typically unaltered; whereas the majority of glasses in crater-fill suevites in the Deiningen drillhole have been replaced by a series of alteration phases (montmorillonite, mixed-layer montmorillonite–illite, calcite, and analcite). There are, however, several surficial suevite outcrops that are intensely altered and weathered. There is no correlation between the intensity of alteration and the glass/carbonate content of the surficial suevites. Importantly, these suevites are always overlain, or were overlain, by sedimentary crater-fill deposits (cf., Graup 1999). The pervasively altered crater-fill suevites are similarly overlain by approximately 400 m of lacustrine crater-fill sediments. This suggests that the presence/absence of an overlying crater lake played a critical role in determining the level of hydrothermal alteration of impactites at the Ries impact structure. The alteration in the bulk of the surficial impactites, therefore, likely occurred under undersaturated conditions with the main source of water being from precipitation (cf., Newsom et al. 1986). The more pervasively altered surficial suevites outcrops may have been locations where water pooled for longer periods of time. Given that the suevites themselves provided the heat source for the hydrothermal system, it may also be that the higher intensity of alteration of the crater-fill suevites is due, in part, to their greater thickness, thus allowing hydrothermal circulation to continue for a longer period of time than in the surficial suevites.

These observations are consistent with a growing body of evidence, which suggests that the paleogeographic setting plays a critical role in determining the intensity of hydrothermal alteration within impact craters. For example, impact-associated hydrothermal alteration within the crater-fill impactites at the Haughton impact structure is discrete in nature and restricted to vugs and veins in the lower levels of the crater-fill layer (cf., the same unit at the Ries structure, which is pervasively altered; Osinski et al. 2005). It has been suggested that the low overall intensity of alteration at Haughton is consistent with the continental setting of the impact site so that the limited supply of fluids may have been a constraining factor (Osinski et al. 2005). Naumov (2002) cited differences between the level of alteration within the Kara, Popigai, and Puchezh-Katunki impact structures, Russia, as evidence that the most intensive impact-induced hydrothermal alteration takes place in craters that form in shallow continental shelf or intra-continent shallow basins (Kara and Puchezh-Katunki). However, differences between the level of alteration at the similar-sized Haughton and Ries structures, suggest that subtleties also exist for craters in continental settings. The simplest explanation is that the Ries crater filled up more rapidly with a lake, and possibly to a greater depth, than Haughton [cf., the greater thickness of sedimentary crater-fill rocks at Ries (approximately 400 m), compared with Haughton (<50 m)], either because of differences in groundwater levels and/or climate.

Conclusions

Hydrothermal systems will develop anywhere in the Earth's crust where water coexists with a heat source. The majority of active hydrothermal systems on Earth are associated with magmatic heat sources (e.g., Farmer 2005). However, studies of the geologic record also reveal that hydrothermal activity is a fundamental process associated with meteorite impact events (e.g., Newsom 1980; Naumov 2002; Ames et al. 2005; Osinski et al. 2005). A hydrothermal system was generated at the approximately 14.5 Ma Ries impact structure by the interaction of groundwaters with hot impact-generated suevite deposits. Hydrothermal alteration is recognized in two main settings: within crater-fill suevites in the central portion of the Ries structure, and within surficial suevites in the near-surface crater rim region. Minor hydrothermal alteration also occurs within sedimentary crater-fill deposits, in the Bunte Breccia and polymict impact breccias, and along impact-generated concentric fault systems in the crater periphery. The source of the hydrothermal fluids was likely a combination of meteoric waters that percolated down from the overlying crater lake and groundwaters that flowed in from the surrounding country rocks.

Hydrothermal alteration of the crater-fill suevites was pervasive in nature with complete replacement of all primary impact-generated glasses. These impactites record an early phase of K-metasomatism accompanied by minor albitization and chloritization at temperatures of approximately 200–300°C. This was followed by pervasive intermediate argillic alteration (predominantly montmorillonite, saponite, and illite) and zeolitization (predominantly analcite, erionite, and clinoptilolite). These alteration zones also affected the overlying sedimentary crater-fill deposits that were being deposited contemporaneously. In contrast, hydrothermal alteration in surficial suevites is typically restricted to cavity and fracture fillings and the majority of the glasses are unaltered. Alteration products are typically limited to montmorillonite and Ba-phillipsite and the lack of illite interlayers in the montmorillonite constrains the main alteration event in this unit to <100–130°C (cf., Newsom et al. 1986). The final stages of hydrothermal alteration within the Ries impact structure resulted in calcite and minor montmorillonite precipitation within cavities and fractures in the various types of proximal impactites (i.e., Bunte Breccia, polymict crystalline breccia, and suevite) and crater-fill suevites. It is clear that the development of a crater lake immediately following the Ries impact event was an important factor in the development of the hydrothermal system within this structure.

Acknowledgements

Financial support for fieldwork at the Ries impact structure came from the Eugene Shoemaker Impact Cratering Award presented to the author. Gisela Pösges and Michael Schieber of the Rieskratermuseum, Nördlingen, are thanked for their hospitality and help during fieldwork and sampling of the Nördlingen 1973 drill core. Preliminary studies for this work were carried out while the author was a graduate student at the University of New Brunswick, Fredericton, Canada, and for this, thanks go to John Spray (supervisor) and Douglas Hall (SEM technician). Lang Shi of the Microanalysis Laboratory at McGill University provided valuable help during electron microprobe studies. The author is supported by the Canadian Space Agency (CSA) and the Natural Sciences and Engineering Research Council of Canada (NSERC), through the Visiting Fellowships in Canadian Government Laboratories program.

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