Post-impact alteration of surficial suevites in Ries crater, Germany: Hydrothermal modification or weathering processes?
Item Type Article; text
Authors Muttik, N.; Kirsimäe, K.; Somelar, P.; Osinski, G. R.
Citation Muttik, N., Kirsimäe, K., Somelar, P., & Osinski, G. R. (2008). Postimpact alteration of surficial suevites in Ries crater, Germany: Hydrothermal modification or weathering processes?. Meteoritics & Planetary Science, 43(11), 1827-1840.
DOI 10.1111/j.1945-5100.2008.tb00646.x
Publisher The Meteoritical Society
Journal Meteoritics & Planetary Science
Rights Copyright © The Meteoritical Society
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Link to Item http://hdl.handle.net/10150/656493 Meteoritics & Planetary Science 43, Nr 11, 1827–1840 (2008) Abstract available online at http://meteoritics.org
Post-impact alteration of surficial suevites in Ries crater, Germany: Hydrothermal modification or weathering processes?
Nele MUTTIK1*, Kalle KIRSIMÄE1, Peeter SOMELAR1, and Gordon R. OSINSKI2
1Department of Geology, University of Tartu, 46 Vanemuise St., Tartu, 51014, Estonia 2Departments of Earth Sciences/Physics and Astronomy, University of Western Ontario, 1151 Richmond Street, London, ON, N6A 5B7, Canada *Corresponding author. E-mail: [email protected] (Received 12 February 2008; revision accepted 29 June 2008)
Abstract–Alteration of surficial suevites at Ries crater, Germany was studied by means of X-ray diffraction and scanning electron microscopy. Here, we discuss the origin of hydrous silicate (clay) phases in these suevites that have been previously interpreted as resulting from post-impact hydrothermal processes. The results of this study indicate that the dominant alteration phases are dioctahedral Al-Fe montmorillonite and halloysite, which are typical low temperature clay minerals. We suggest that the surficial suevites are not altered by hydrothermal processes and that alteration occurred by low temperature subsurface weathering processes. If the surficial suevites were indeed hydrothermally modified during the early stages of post-impact cooling, then the alteration was of limited character and is completely masked by later weathering.
INTRODUCTION (polymict impact breccias—crater suevites, Engelhardt et al. 1995) is pervasive and characterized by argillic-type The Ries impact crater (Fig. 1) is one of the most studied hydrothermal alteration with zeolitization; an early phase of impact structures on Earth. Its impact origin was first K-metasomatism accompanied by minor albitization and recognized by Shoemaker and Chao in 1961 and since that chloritization at temperatures of approximately 200–300 °C time the geology and formation of the Ries crater has been also occurred (Osinski 2005). addressed in numerous studies (see Pohl et al. 1977 and In contrast, the IHT mineralization in surficial suevites Engelhardt 1990, for reviews of the Ries geology and (fallout suevites, Engelhardt et al. 1995) at the Ries is not so impactites). The Ries crater was also one of the first impact well defined with the typical alteration product being craters where possible impact induced hydrothermal montmorillonite-type clay mineralization at the expense of alteration of impactites was suggested and described impact glasses (Newsom et al. 1986). Newsom et al. (1986) (Engelhardt 1972; Salger 1977; Stähle and Ottemann 1977). suggested that ∼10–15 vol% of the montmorillonite is of Impact-induced hydrothermal activity (IHT) occurs hydrothermal origin in the groundmass of surficial suevites. following meteorite impacts in water- or ice-rich targets due However, Engelhardt (1997) and Osinski et al. (2004) to differential temperatures in the impact heated and/or reported that the suevite groundmass contains up to ∼50 vol% melted crater rocks (see Naumov [2005] for a review). IHT is of “clayey” material that is evidently a hydrous silicate a well known phenomenon that has been found at several (montmorillonite, according to Engelhardt 1997); however, terrestrial craters varying in size and target compositions (e.g., the exact nature and origin of this clayey groundmass Koeberl et al. 1989; McCarville and Crossey 1996; Ames et al. remains poorly constrained. Similarly, Ames et al. (2004) 1998; Sturkell et al. 1998; Naumov 2002; Osinski et al. 2001, concluded from mineral assemblages of impactites in core 2005; Kirsimäe et al. 2002; Hagerty and Newsom 2003; Hode Yaxcopoil-1, Chicxulub impact crater, that the alteration of et al. 2003; Versh et al. 2005); similar features have also been impact glasses has resulted from low temperature seawater suggested for impacts on extraterrestrial planetary bodies interaction rather than typical high temperature (e.g., Allen et al. 1982). At the Ries crater, IHT has been hydrothermal alteration. addressed in detail by Newsom et al. (1986) and Osinski In this contribution we study the composition of the (2003, 2005), and Osinski et al. (2004). These studies have secondary phases within surficial suevite deposits from the shown that hydrothermal alteration of the crater-fill suevites Ries impact crater in order to understand their origin and
1827 © The Meteoritical Society, 2008. Printed in USA. 1828 N. Muttik et al.
Fig. 1. Simplified geological map of the Ries impact structure showing the sample locations investigated in this study. The map shows the distribution of impactites, where surfical suevites are marked with black. Modified after Hüttner and Schmidt-Kaler (1999). alteration mechanism(s), using a detailed structural- temperatures last for exponentially longer times, results in a mineralogical and geochemical characterization of alteration small number of mineral phases (mineral associations), mineralogies. Our aim is to determine the origin of these typically characterized by clay mineral-zeolite-calcite- phases—i.e., low-temperature hydrothermal mineralization (pyrite) association (Naumov 2005). This alteration is rock- versus diagenetic devitrification and aerial-to-subaerial dominated and dependent on target lithologies (Osinski et al. weathering of surficial suevites. 2005). It is apparent, however, that clays and especially zeolites IHT ALTERATION ASSEMBLAGES are thermodynamically metastable minerals, which are easily transformed by post-impact weathering and/or diagenetic and The composition, structure and morphology of clay and metamorphic alteration of impactites. Moreover, secondary zeolite phases depend on a number of environmental clays and zeolites, as well as feldspars, can form directly parameters, such as temperature, fluid composition/amount, under ambient conditions or diagenetically by devitrification pH, Eh etc. This makes them useful and important indicators of silicate glasses (e.g., Kastner and Siever 1979; Sandler for reconstructing any IHT environments. The cooling of hot et al. 2004; Meunier 2005). and/or moderately heated lithologies (e.g., impact melt rocks, melt glass-bearing breccias) in an impact crater is GEOLOGICAL SETTING characterized by exponential temperature evolution with a fast temperature drop in the early stages of the cooling, The Ries impact crater is a 24 km diameter complex followed by long and slow temperature decreases down to impact structure in southern Germany (Pohl et al. 1977) (Fig. 1). ambient conditions (Jõeleht et al. 2005). The range of The age of the Ries event has been constrained by dating of suevite temperature variations, in space and time, results in specific glasses at 15.1 ± 0.1 Ma using the 40Ar/39Ar step-heating method secondary paragenetic mineral associations. The latest stages (Staudacher et al. 1982), and more recently, 14.3 ± 0.2 Ma by of the cooling occurs at temperatures below 300 °C, when the 40Ar/39Ar laser-probe technique (Buchner et al. 2003). Post-impact alteration of surficial suevites in Ries crater, Germany 1829
The crater is characterized by an almost circular and relatively sites of intense alteration by Chao et al. (1978), and later flat inner basin of 12 km in diameter. The diameter of this ring reinterpreted as degassing pipes by Newsom et al. (1986). coincides approximately with the maximum extent of the Within the pipes, the fine-grained matrix of the suevite has transient crater cavity (Wünnemann et al. 2005). The inner been removed, leaving the filling of coarser fragments that are basin is surrounded by a crystalline inner ring of uplifted coated with a light brown layer of undetermined Fe-rich oxide basement and an outer tectonic ridge representing a system of material (Newsom et al. 1986). concentric normal faults (megablock zone) with a maximum Engelhardt (1997) concluded that there is no coherent extension of approximately 24 km in diameter (Pohl et al. impact melt sheet at the Ries crater. Moreover, Stöffler (1977) 1977). The Ries impact occurred in a two-layer target suggested if there exists such a sheet or lens, deep-seated in consisting of flat-lying sequence of sedimentary limestones, the crater centre, it would be of minor size. However, Graup sandstones, and shales of Triassic to Tertiary age that overlay (1999) and Osinski (2004) suggest that isolated patches of the Hercynian crystalline basement composed of gneisses, so-called “red suevite” (e.g., at the Polsingen locality) can be granites and amphibolites (Pohl et al 1977; Graup 1978). considered as impact melt rock. This impactite overlies Bunte The inner basin is entirely formed in crystalline basement Breccia or megablocks within the crater and occurs as isolated and filled by crater-fill suevitesoverlain by ∼400 m of post- bodies, with a lateral extent of approximately 10–100 m. impact lacustrine sedimentary rocks (Pohl et al. 1977). The These impact melt rocks comprise a reddish microscopic outer zone of the structure is overlain by different types of groundmass that contains lithic and mineral clasts proximal impactites (Engelhardt 1990): (1) Bunte Breccia; (2) (predominantly quartz) shocked to variable extent polymict crystalline breccias; (3) surficial suevites; and (4) (Engelhardt 1997; Pohl et al. 1977; Graup 1999). The coherent impact melt rocks. Surficial suevites, up to 25–30 m microcrystalline groundmass of impact melt rocks comprises thick can be found in a distance of 22 km from the crater alkali feldspar, plagioclase, quartz, and ilmenite, with centre (Newsom et al. 1990; Engelhardt 1990). interstices filled by either a fresh or devitrified glassy Surfical suevites contain components from the crystalline mesostasis (Osinski 2004). basement at all stages of shock metamorphism and shock- melted glassy inclusions, specifically vitreous and devitrified MATERIAL AND METHODS impact glass (Engelhardt et al. 1995; Engelhardt 1997). Surficial suevites are found in a large number of small The surficial suevites studied here were collected from 4 isolated patches inside the morphological rim and up to radial outcrops within and around the Ries impact structure (Fig. 1). distances of ∼14 km beyond the rim to the south-southwest Altogether 28 samples were studied. Samples include and east-northeast (Fig. 1). The thickness of the layer is suevites that varies from fresh (with no or limited noticeable typically from few meters up to 25–30 m (Engelhardt 1990, alteration) to highly altered. The samples include also wall 1997); although substantial local variations are likely, given rocks of degassing pipes from locations at Altenbürg, the presence of ∼80 m of surficial suevite in the Aumühle, and Oettingen. In addition, 2 samples of coherent Wörnitzostheim drillhole (Engelhardt 1990). impact melt rock (“red suevite”) were analyzed from The suevites in the crater rim area have been divided Polsingen outcrop. During sampling rock clasts were avoided into two units by Bringemeier (1994): (1) “main suevites” and only glass rich parts and fine-grained groundmass were and (2) “basal suevites”. The “main suevites” comprise hand-picked under the binocular for mineralogical and consolidated impact glass clast-rich blue-grey suevite, chemical analyses. “Glass shards” are here defined as dark which constitutes the bulk of all suevite outcrops. The violet-grey glassy inclusions in suevite mass. The surface of underlying “basal suevites” are poorly consolidated fine- glassy particles was cleaned but no attempt was made to grained suevite with minor content of glass and rock separate the glass and crushed rock particles inside the fragments (Bringemeier 1994; Osinski et al. 2004). The inclusion. “Suevite groundmass” is defined here as a fine- surficial suevite is lithologically different from the so-called grained mixture of glass and crushed rock particles. A crater-fill suevite, which is only observed in drill cores. The complete list and description of the studied samples is given crater-fill suevites form a massive (up to 400 m thick) layer in Table 1. in the crater depression below the post-impact crater lake The mineralogical composition of samples was studied sediments (Engelhardt et al. 1995). The crater suevite is a by means of X-ray powder diffractometry (XRD). The XRD polymict breccia with lesser amount of glassy material as patterns of whole-rock samples were measured in grinded and compared to surficial suevites (Engelhardt et al. 1995; homogenized unoriented powder samples with DRON-3M Engelhardt 1997). diffractometer (University of Tartu) using Ni-filtered Cu-Kα The massive main surficial suevites are dissected by radiation in the 2θ range between 2 to 50 or 60°, with a step numerous vertical pipe like structures, up to some centimeters size of 0.03 or 0.05 °2θ and 5 seconds counting time. in diameter (Newsom et al. 1986). The pipe structures were Clay minerals were separated by standard sedimentation considered as pathways for hydrothermal fluids and as the procedures. The <2 µm size clay fraction was flocculated and 1830 N. Muttik et al.
Table 1. List of the Ries surfical suevite samples used in study. Lab no Location Description RS1-1 Aumühle Groundmass of grey fresh suevite RS1-2 Aumühle Glass/melt of grey fresh suevite RS2 Aumühle Clayey material in degassing pipe RS3-1 Aumühle Groundmass of grey fresh suevite RS3-2 Aumühle Glass/melt of grey fresh suevite RS4-1 Aumühle Interior surface of contact between degassing pipe and surrounding suevite RS4-2 Aumühle Grey suevite RS5-1 Lehberg White clasts/fragments of highly altered yellowish suevite RS5-2 Lehberg Main mass of highly altered yellowish suevite RS6-1 Lehberg White clasts/fragments of soft highly altered yellowish suevite RS6-2 Lehberg Groundmass of soft highly altered yellowish suevite RS7 Polsingen Groundmass of “red suevite”—hard coherent impact-melt rock RS8 Altenbürg Groundmass of suevite from upper part RS9 Altenbürg Groundmass of suevite from lower part RS10 Altenbürg Interior surface of degassing pipe in suevite RS11 Altenbürg Groundmass of suevite RS12 Altenbürg (Purple-)grey melt phase/fragment RS16-1 Lehberg White clasts/fragments of yellowish suevitic dike in monomict breccia blocks RS16-2 Lehberg Groundmass of yellowish suevitic dike in monomict breccia blocks RS17-1 Polsingen Greenish fragments of “red suevite”—hard coherent impact-melt rock RS17-2 Polsingen Reddish main mass of “red suevite”—hard coherent impact-melt rock RS18-1 Aumühle Groundmass of grey (fresh) suevite RS18-2 Aumühle Glass/melt of grey (fresh) suevite RS19-1 Oettingen White fragment of impact glass around gneiss core RS19-2 Oettingen Glass/melt of impact glass around gneiss core RS20 Oettingen Groundmass of altered suevite RS21-1 Aumühle Interior surface of degassing pipe in suevite RS21-2 Aumühle Suevitic groundmass of degassing pipe Coordinates for localities: Altenbürg—48.81392° N, 10.43110° E; Aumühle—48.97178° N, 10.62933° E; Lehberg—48.91535° N, 10.44555° E; Oettingen—48.978793° N, 10.58499° E; Polsingen—48.89552° N, 10.74578° E.
2+ Sr -saturated with 0.1 M SrCl2. Excess salt was removed by RESULTS repeated rinsing with distilled water and ethanol. Oriented clay aggregates were made by smearing the clay pastes onto XRD—Whole-Rock Mineralogy glass slides. XRD data of oriented air-dry and ethylene glycol-solvated (EG) preparations were obtained using a The mineralogical composition of the studied whole- DRON-3M diffractometer with Ni-filtered CuKα radiation. rock samples is summarized in Table 2 and representative Scanning steps of 0.03 °2θ from 2 to 30 or 50 °2θ and a XRD patterns of unoriented samples are shown in Figs. 2a counting time of 4 s per step were used for clay slides. The and 2b. The mineralogical composition of the suevites is ambient relative humidity varied from 60% to 65%. The characterized by an assemblage of montmorillonite-type Green-Kelly Li-test according to Muravyov (1970) was smectite, plagioclase, quartz, and K-feldspar. Both the applied to distinguish the tetrahedrally substituted beidellite qualitative and quantitative composition of different suevite and octahedrally substituted montmorillonite. types is rather persistent and does not show any variation The XRD patterns were analyzed and modeled by using between different locations. However, highly altered SIROQUANT-2.5TM (Taylor 1991) for unoriented powders suevites, particularly those at Lehberg were found to contain and MLM2C code (Plançon and Drits 2000) for oriented clay halloysite, in remarkable amounts (up to 81 wt% of aggregates, respectively. The micromorphology of freshly crystalline phases). broken interior surfaces of the most characteristic representative samples of suevite were examined with Smectite scanning electron microscopy (SEM) using Zeiss DSM 940 The most prevalent mineral phase in the suevites is equipped with a SAMx SDD EDX detector, accelerating smectite, which is evidently a dioctahedral montmorillonite voltage of 25–30 kV, a beam current of 5 nA (University of phase (Fig. 2c). The average content of montmorillonite is Tartu). The SEM specimens were coated with gold. The ∼34 wt%, but its content can vary from 4.8 to 64 wt% (Table 2). interpretation of the SEM images was checked against the In the groundmass of fresh-suevites, the content of EDS spectra. montmorillonite ranges from 23 to 45 wt%. Its abundance in Post-impact alteration of surficial suevites in Ries crater, Germany 1831 percentages are shown deviations of the weight Halloysite Calcite Dolomite HematiteZeolite Amphibole Garnet HematiteZeoliteAmphibole Halloysite Calcite Dolomite , based on XRD analyses. The estimated standard % 28.5 (0.83) 26.8 (0.84) 3.5 (0.30) 4.4 (0.27) 17.2 (0.48) 28.2 (0.68) 1.1 (0.27)17.3 (0.42) 31.4 (0.60) 41.7 (0.55) 1.8 (0.29) 2.7 (0.39) 2.3 (0.23) .45) 24.3 (0.55) 39.7 (0.72) 0.9 (0.30) 15.7 (0.59) 1.4 (0.31) (0.87) 22.7 (0.52) 44.1 (0.63) 1.2 (0.26) 1.6 (0.20) (0.32) 15.7 (0.45)(0.52) 18.8 (1.35)(0.39) 7.6 (0.51) 50.8 (1.32) 24.0 (0.46) 59.0 (0.93) 43.8 (0.65) 1.1 (0.14) (1.13) 13.6 (0.78) 17.1 (0.66) 28.1 (0.55) 2.4 (0.27) 44.9 (0.67) 0.5 (0.15) 0.9 (0.28) 3.3 (0.19) (0.55) 46.2 (0.81) 27.8 (0.95) 1.6 (0.57) 1.4 (0.31) 5.8 (0.97) 33.4 (0.81) 45.9 (0.81) 0.8 (0.27) tr tr 2.9 (0.53) 17.7 (0.53) tr? 74.8 (0.73) 0.7 (1.69) 43.3 (0.90) 22.7 (0.67) 1.4 (0.20) 1.1 (0.20) 0.0 (1.70) 37.1 (1.12) 38.7 (1.04) 0.7 (0.39) 1.5 (0.32) 1.8 (0.82) 32.1 (0.50) 34.9 (0.52)1.8 (0.99) 2.1 (0.20) 22.3 (0.62) 1.8 (0.15) 41.2 (0.62) 3.0 (0.21) 0.9 (0.16) 12.1 (2.07) 19.4 (0.89) (1.13)11.5 33.4 (0.72) 37.3 (1.33) 43.1 (0.73) 3.0 (0.55) 1.5 (0.27) tr ). % position of bulk suevite in wt RS10 Altenbürg 3.6 (0.17) RS1-1RS1-2 AumühleRS2 AumühleRS3-1 15.3 (0.21) 2.1 (0.17)RS3-2 16.8 (0.47) Aumühle 7.9 (0.23) Aumühle 1 RS4-1 10.5 (0.26) AumühleRS4-2 17.7 (0.04) 5.7 3.3 (0.22) 3.0 (0.20) Aumühle 25.2 (0.73) 3.0 (0.47) Aumühle 1 RS5-2 7.6 (0.28)RS6-1 28.1 (0.51) 3.0 (0.25) LehbergRS6-2 LehbergRS8 6.6 (0.30) LehbergRS9 2.9 (0.29) 9.2 (0.21) 12.4 (0.27) Altenbürg 2.0 (0.20) 7.9 RS11 1.5 (0.24) 8.7 (1.18) 16.8 (0.28) Altenbürg 3.4 (1.23) 11.8 1.7 (0.22) 12.0 (0.26) 16.5 (0.42) Altenbürg 1.8 (0.24) 4.4 14.4 (0.26) 56.6 (0.73) 4.3 (0 1.0 (0.29) RS20 5.0 (0.45) RS21-1 Oettingen Aumühle 18.6 (0.50) 10.6 (0.24) 1.4 (0.31) 1 4.6 (0.97) 8.6 (0.99) 58.1 (1.33) 3.9 (0.64) 6.2 (0.67) Lab no. Location Quartz Coesite K-feldspar Plagioclase Smectite RS5-1 Lehberg 7.9 (0.22) 2.4 (0.21) 3.3 RS12RS16-1RS16-2 Altenbürg LehbergRS18-1 14.5 (0.42) Lehberg 14.1 (0.27)RS18-2 Aumühle 33.7 (0.38)RS19-1 Aumühle 18.9 (0.39)RS19-2 1.8 (0.21) Oettingen (0.30) 11.3 Oettingen 1 7.9 (0.66) 16.5 (0.39) 4.3 (0.66)RS21-2 2.5 (0.30) (1.12) 11.6 3.1 (0.43) Aumühle 12.3 (1.18) 4.0 63.8 (1.56) 10.6 (0.90) 16.6 (0.29) 67.3 (1.00) 43.6 (1.90) 10.1 (0.77) 37.5 (0.95) 0.7 (0.28) 8.7 (1.13) 38.5 (0.71) 4.8 (0.44) 34.4 (0.65) 2.1 (0.31) 81.1 (1.15) 1.7 (0.24) 2.3 (0.24) tr? in brackets, tr—trace amount (<0.5 wt Table 2. Mineralogical com Table 1832 N. Muttik et al.
Fig. 2. Representative XRD patterns of the Ries samples. A—groundmass of surfical suevites; B—degassing pipe materials; C—surfical suevite, Aumühle (sample RS 1-1): the position of smectite d060 reflection at 1.50 Å confirms dioctahedral nature of Al-Fe montmorillonite; D—fresh glass in surfical suevite (sample RS 3-2). Sm—smectite (montmorillonite), Hal—halloysite, Q—quartz, Kf—K-feldspar, Pl— plagioclase, Ca —calcite. the glass rich surfical suevite groundmass is about the same significant differences in quartz content between the varying from 10 to 44 wt%, but these samples show on XRD groundmass and the glass clasts in suevite. The content of patterns also a remarkable bulge of amorphous maximum (see microcristalline quartz varies from 12 to 19 wt% in the Fig. 2d) indicating the presence of still a large amount of groundmass, and from 11 to 25 wt% in the glass phases. In vitrous glass in the groundmass. Highly altered suevites show the highly altered suevite the amount of quartz is about 14 wt% a somewhat higher smectite content (18.8–59 wt%), except in or slightly less, except in sample RS 16-2 of a suevitic dike sample of a suevitic dike where the smectite content was with a quartz content of up to 34 wt%. A variation in the significantly lower, i.e., 4.8 wt%. quartz content occurs in degassing pipes at interior surfaces of pipes rich in calcite, the quartz content ranges from 3.6 wt% Plagioclase to 11 wt%, whereas in wall rock of the pipes the quartz The groundmass of the suevite and impact glasses of Ries content was as high as 28 wt%. surficial suevites is characterized by the presence of Coesite, a high-pressure polymorph of quartz was plagioclase. Structural refinement of the XRD data indicate identified in several suevite samples (Fig. 3a). The highest that the plagioclase is oligoclase-andesine with 25–45 mol% coesite content (8 wt%) was found in a fresh glass of a suevite anorthite (An) component in good agreement with from the Aumühle location. In most cases, coesite occurs only microprobe data for plagioclase in Ries suevites (Engelhardt in trace amounts (<2 wt%). In addition, in highly altered 1997; Osinski 2004). suevites, opal-CT and alpha-cristobalite phases may be The large XRD reflection widths indicate that the present, yet quantification by XRD is limited due to their plagioclase is evidently structurally disordered and/or poorly poorly crystallized nature. crystallized with extremely small coherent stacking domain sizes. The highest content of plagioclase was found in isolated K-Feldspar glass shards from the suevite, where it composes up to 67 wt% XRD analysis indicates variable amounts of K-feldspar of crystalline mineral matter. The lowest amounts of in all samples, ranging from trace amounts up to an plagioclase are found in the highly altered suevites, where its exceptional 61 wt% in sample RS17-1, a “red suevite.” content is on average less than 24 wt%. Though, the highest values are found in impact melt rocks (samples RS17-1, RS17-2 and RS7). There the K-feldspar is a Quartz high-temperature sanidine in keeping with the melt rock The quartz content of the studied samples is rather interpretation of red suevites by Osinski (2004). The average constant, with an average of 14 wt%. There are no value of K-feldspar in suevitic samples is about 7 wt%. Post-impact alteration of surficial suevites in Ries crater, Germany 1833
Fig. 3. Representative XRD patterns of the Ries samples. A—coesite in the groundmass of surfical suevite, Altenbürg, (sample RS 8); B— halloysite in highly altered suevite dike, Lehberg (sample RS 16-1): note that both halloysite varieties are present (7 Å, and 10 Å); C—XRD patterns of the <2 µm clay fraction (in ethylene glycol-solvated state) from surfical suevites; D—XRD patterns of the <2 µm clay fraction (in ethylene glycol-solvated state) from the degassing pipe material. Sm—smectite (montmorillonite), Hal—halloysite, Q—quartz, Kf—K- feldspar, Pl—plagioclase.
Halloysite Amphibole and pyroxene were recognized in samples RS9 Halloysite is a kaolinite group mineral and originate by and RS20, with contents of 1.4 and 6.2 wt%, respectively. The intensive alteration (leaching) of aluminosilicate minerals, amount of hematite ranges from 2.3 to 3.8 wt% in samples such as feldspars (Meunier 2005). In the Ries samples, RS12 and RS7, respectively. Garnet (andradite) was halloysite occurs in the highly altered suevites, particularly identified in few samples in small amounts, <2 wt%. those from Lehberg. In the groundmass of highly altered Zeolites have been recognized in the groundmass and suevite samples the content of halloysite is <4 wt%, except for impact glass clasts of surficial suevites by several authors (e.g., sample RS5-1 with a halloysite content in groundmass of up Engelhardt and Graup 1984; Osinski et al. 2004; Osinski 2005). to 14 wt%. The halloysite content is remarkably high in In this study only harmotome was recognized in one Altenbürg altered gneiss fragments within the highly altered suevites sample (degassing pipe) with amounts of ∼2.7 wt%. Osinski ranging from 17 up to 81 wt%. Halloysite is recognized in two (2005) recognized various zeolite minerals at six locations; structural forms: (i) 10 Å halloysite, and (ii) 7 Å halloysite however, none of these sites were studied here. (Fig. 3b). Traces of halloysite have been identified also in other samples, but its content is generally less than 2 wt% XRD—Clay Mineralogy
Calcite The clay fraction of the suevites (<2 µm) is characterized Calcite was recognized in samples of surficial suevites by montmorillonite-type smectite as indicated by strong 001 from Aumühle, Altenberg and Lehberg, comprising <2–4 wt%. reflection at 14–15 Å-s in air-dry state that expands to 17 Å in The highest calcite content was found in degassing pipes at the ethylene-glycolated state (Figs. 3c,d). In some samples, Altenbürg quarry, reaching peaks in samples RS10 and RS11 halloysite along with smectite was identified. Halloysite was 75 wt% and 32 wt%, respectively. found in the same samples where it was identified in whole- rock XRD analysis. Minor Phases The smectite nature of the clays was confirmed by applying The content of other mineral phases in the Ries samples the Green-Kelly Li-test (Muravyov 1970) and by numerical was usually less than 3%. Typically, trace amounts of modeling of montmorillonite structures by MLM2C code pyroxene, amphibole, hematite, and garnet were identified. (Plançon and Drits 2000), which indicates montmorillonite 1834 N. Muttik et al. composition of smectitic clay in all samples. However, in some highly altered suevite samples either as delicate vermiform samples the Li-test points to the presence of some beidellitic dendritic aggregates growing into the vesicular cavities and layer-type clays as indicated by weak reversible expansion by pore space or massive aggregates of dendritic crystallites after Li-treatment. Structure refinement indicates that the filling the space between glass vesicle polymorphs of montmorillonite is of Al-Fe type. Low-charge octahedrally smectite aggregates (Fig. 6). substituted dioctahedral Al-montmorillonite is a typical low temperature smectite, whereas the high temperature smectite is DISCUSSION usually tetrahedrally substituted Al-beidellite (e.g., Haymon and Kastner 1986; Meunier 2005). Hydrothermal Alteration versus Weathering The structural composition of smectite does not show any variance regardless of the location within the structure. Smectite with a composition corresponding to Moreover, no signs of compositional or structural variation dioctahedral Al-Fe montmorillonite is the principal product (e.g., illitization) were found between the possible fluid of alteration in the surficial suevites of Ries crater. Its conduits and surrounding rocks that would have been induced composition and structural state does not show any significant by temperature gradients. variation between the different sampling localities at the Ries crater. This finding is not consistent with a hydrothermal Scanning Electron Microscopy origin of the suevite clay by post impact “alteration”. Hydrothermal alteration in active endogenic hydrothermal The SEM study was undertaken to study the occurrence systems is typically characterized by a zonal distribution of and spatial distribution of clay minerals as a major secondary alteration and directional distribution of altered minerals that phase. SEM images illustrating characteristic morphologies is principally related to mass transfer between minerals and and distinctive textures of minerals or rock of the Ries hydrothermal solutions as temperature decreases when the suevites are shown in Figs. 4 and 5. hydrothermal solutions pass through rock (Helgeson 1979). Freshly broken interior surfaces of a suevite sample show The distance between the high-temperature and low- the typical vesicular structure of groundmass and melt phases. temperature zones is a factor of the vertical thermal gradient The glass phase has frequently a flow-texture (Fig. 4a) and at a given place (Inoue 1995; Meunier 2005). Importantly, as contains irregularly shaped vesicles. All samples show the the number of variables in a hydrothermal system increases, a ubiquitous clay coatings and vesicles within the glasses are greater variety of clay minerals can be expected to form under lined by clay aggregates (Fig. 5), which is consistent with the different hydrothermal environments (Inoue 1995). The same high percentage of clay in the suevite identified by XRD spatial structure and specific alteration gradients of the analysis. In addition, other authigenic minerals such as hydrothermal alteration are described for IHT systems. There calcite, plagioclase(?), halloysite(?) and opal-CT were the most intense alteration with thermal gradients in order of identified by EDS spectra and characteristic morphology 100 °C km−1 occurs within the hottest central parts of an replacing the vitric matrix and glass fragments in studied impact structure (Masaitis and Naumov 1993; Naumov 2005; suevites. Versh et al. 2005). Petrographic relations indicate that smectite was the first In contrast to hydrothermal alteration, weathering of and the major mineral to form. The lepispheres of opal-CT rocks occurs under virtually constant temperature-pressure that rarely coexist with smectite (Fig. 4) appear to postdate conditions at the Earth’s surface. As a consequence, the most the major smectite formation. The initial devitrification of important factor of weathering is the composition of the glasses proceeds most probably by slow dissolution indicated acting fluid; whereas temperature and especially pressure by micrometer scale dissolution pits on the glass surfaces in variation are practically negligible, at least in short term fresh suevites. These etched surfaces are apparently related to (Meunier 2005). The variability of secondary phases (clays) glass dissolution (Figs. 4c and 4d). generated during weathering is a function of the minerals The SEM images indicate that the smectite precipitation is present in the rock, and the number of coexisting secondary initiated at the glass surfaces in two different arrangements — phases in a system is much less (frequently monovariant) than as spherical aggregates of sub-radial orientation of smectite in hydrothermal environments. This general understanding crystallites and as flaky smectite crystallite aggregates that fully agrees with the alteration to clay mineral observed in form loose pseudo-polygonal webs known also as “honey surficial suevites at Ries crater. The dominant smectitic comb” or “corn-flake” microstructures (Meunier 2005) on alteration of these suevites with halloysite in highly altered the fresh glass surface (Fig. 5). Smectite coatings are suevites is best explained by a low temperature hydrous abundant in all samples and have the same distinct devitrification of the impact glasses and not by hydrothermal morphology. The edges of flakes do not show any alteration alteration. or overgrowths, which would suggest the illitization of Alteration of Ries suevites has been noted since the early smectite (Fuente et al. 2002). Halloysite was identified in 1970s (e.g., Engelhardt 1972; Chao et al. 1978; Newsom Post-impact alteration of surficial suevites in Ries crater, Germany 1835
Fig. 4. Micrographs (SE images) of surfical suevite. A—fluidal texture of glass phase in surfical suevite; B—opal-CT lepispheres on smectite lined vesicles; C, D—dissolution of glass. Note the dissolution pits at the glass surface. et al. 1986; Osinski 2005). Indeed, the IHT alteration in systems in general (Naumov 2002, 2005; Osinski 2005). The crater-fill suevites is well defined. Osinski (2005) suggests source of the fluids for the Ries hydrothermal system was that the major heat source for the Ries hydrothermal system likely a combination of surface (meteoric) waters that was the suevite unit themselves and that the hydrothermal percolated down from the overlying crater lake and alteration of crater-fill suevites is pervasive in nature and groundwater that flowed in from the surrounding country comprises several distinct alteration phases that vary with rocks into the hydrostatic space created by the impact event depth. All impact glasses in the crater-fill suevite are (Osinski 2005). completely replaced by various secondary minerals (Stöffler In contrast to the crater-fill suevites, as this work shows, et al. 1977). In these rocks argillic alteration (predominantly the IHT alteration within surficial suevites is typically montmorillonite, saponite, and illite) and zeolitization restricted to smectite with a composition corresponding to (predominantly analcite, erionite, and clinoptilolite) can be dioctahedral Al-Fe montmorillonite with minor zeolite observed (Pfannschmidt 1985; Osinski 2005). There is an (mainly phillipsite) deposition within cavities and fractures additional early phase of K-metasomatism accompanied by (cf. Newsom et al. 1986; Osinski et al. 2004; Osinski 2005). It minor albitization and chloritization at temperatures of has been generally accepted that these clays formed by post- approximately 200–300 °C. The predominance of alkali and impact hydrothermal alteration of impact-generated glasses calcic zeolites is indicative of weakly alkaline hydrothermal and/or finely comminuted crystalline basement material solutions, which is typical for impact-induced hydrothermal (Engelhardt 1972; Stähle 1972; Engelhardt and Graup 1984; 1836 N. Muttik et al.
Fig. 5. Micrographs (SE images) of smectite “corn-flake” aggregates. A—initial globular/spheroidal aggregates on the glass surface; B— cross section of a layer of box-work smectite aggregates on the glass surface; C, D, E, F—smectite aggregates lining the inner surfaces of vesicles. Post-impact alteration of surficial suevites in Ries crater, Germany 1837
Fig. 6. Micrographs (SE images) of halloysite in strongly altered suevite. A—vermiform crystal aggregates of halloysite; B—massive aggregate of halloysite embedding a polymorph of vesicle lined by smectite aggregate. Sample RS1-1.
Newsom et al. 1986, 1990). The same was assumed for the improbable (Meunier 2005) and we consider this type of origin of calcite in the Ries surficial suevites. However, recent alteration as irrelevant for the Ries surfical suevites. studies have suggested that calcite and a substantial Engelhardt (1972) noted that pipe like structures proportion of the fine-grained “clays” in the groundmass of (“vertical chimneys”), which intersect the suevite in several surficial suevites represent a combination of primary impact- suevite outcrops, indicate the degassing of the breccia. Chao generated melt phases and high temperature devitrification et al. (1978) described these as pathways for hydrothermal products (Graup 1999; Osinski 2003, 2005; Osinski et al. fluids and as the sites of intense alteration. He suggested that 2004), which are altered to smectite type clays on various the interior surfaces of the degassing pipes are coated with scales. As a result, secondary hydrothermal alteration of the (hydrothermal) clays. Newsom et al. (1986) studied the surficial suevites is definitely less important than previously degassing pipes in detail and found that the surfaces of thought, if it occurred at all. degassing pipes are not coated with clay minerals, but a thin The absence of significant hydrothermal alteration in (<2 µm) layer of iron oxide or hydroxide. They suggested that surficial suevites is further confirmed by textural the absence of clay in the pipes indicates that degassing pipes relationships. Graup (1999) pointed out that majority of the have probably remained mostly dry since the original glasses are unaltered and all extremely delicate textures of degassing event occurred, indicating that the suevite was in primary silicate impact-melt glass have well preserved, ruling the unsaturated or vadose zone during the main period of the out at least large-scale replacement processes. Moreover, only hydrothermal alteration. Moreover, results of this study show a limited number of alteration products (montmorillonite, that there is no evidence of hydrothermal clay mineral halloysite, zeolite) have been found. Newsom et al. (1986) alteration within the degassing pipes. There is no variation in previously noted that the lack of illite-smectite type mixed- clay mineral composition between internal surface layers of layering in the montmorillonite constrains the main alteration pipes and at a distance from the pipe. In all cases the dominant event in this unit to temperatures <100–130 °C. It is secondary phase is the same Al-Fe type montmorillonite. noteworthy that similar secondary calcite and However, the internal surfaces of degassing pipes are montmorillonite precipitation also occurs within cavities and frequently over-/intergrown with calcite, which fractures in the all types of the proximal impactites—Bunte paragenetically postdates the clay phases. Breccia, polymict crystalline breccia—that were not heated enough or cooled too rapidly for IHT initiation. Alternatively, Smectite Formation Kieffer and Simonds (1980) predicted the formation of clays by penecontemporaneous alteration of ejecta by hot volatiles Smectite is one of the major alteration products of during the actual formation of the crater. Similarly, it has been volcanic glass (e.g., Tomita et al. 1993; Fuente et al. 2002) suggested that during bentonite formation, the volcanic glass and it is recognized as an important secondary phase in is altered very early in the volcanic context and sedimentation impact glasses (e.g., Naumov 2005). Usually, the formation of of bentonite involves already transformed ash, or the smectite from a siliceous glass proceeds through removal of alteration may take place during ash fall (e.g., Grim 1968). excess of Si and alkali compounds. As the consequence the However, this kind of transformation is considered to date as smectite is a typical alteration product in open to half-open 1838 N. Muttik et al. systems. However, if sufficient Si and alkalis are retained in fluid source. IHT fluids are typically weakly alkaline to near half-open or nearly closed systems then zeolites and/or neutral and supersaturated in silica during the entire authigenic quartz polymorphs (opal, cristobalite) form. hydrothermal process (Naumov 2005). The abundant presence of smectite in surficial suevites at Relatively low pH of interstitial waters is also indicated by the Ries crater indicates that the alteration in these deposits limited occurrence of authigenic quartz and zeolite minerals. proceeded at relatively low temperatures. Smectite is Zeolites have been reported in surficial suevites by earlier thermally a metastable phase and its conversion to illite authors (e.g., Pfannschmidt 1985; Osinski 2005), but their through mixed-layer illite-smectite takes place over a broad occurrence is scarce in the samples studied here, only one temperature interval of 70–150 °C. The temperature interval sample RS 11 contain traces amount of zeolite. The pH is the varies as function of kinetically controlled processes of the most important control on zeolitization, which requires neutral reaction progress (Inoue et al. 1992; Essene and Peacor to alkaline conditions and sufficient Si and alkali activity in the 1995). In addition, a low temperature (<100 °C) origin of this fluid (Hey 1978). These conditions were most probably not met mineral is indicated by its composition—octahedrally in most part of the surficial suevites in the crater interior. substituted Al-Fe montmorillonite—whereas, the high Indeed, most of the earlier zeolite findings are reported in more temperature smectites are typically tetrahedrally substituted fine-grained suevites outside of crater rim (e.g., Osinski 2005), beidellites (Inoue et al. 1992). which probably were characterized by lower hydraulic Our results suggest that in the surficial suevites the glass conductivity and could have provided sufficiently high Al/Si appears to have transformed to smectite through dissolution saturation of pore water for zeolite precipitation. However, of glass without forming amorphous precursor or primitive cristobalite and opal-CT, which were identified in some suevite clays (see Fig. 4). Randomly oriented montmorillonite-type samples from crater interior, indicate that an excess of silica smectite flakes fill or coat the walls of existing round cavities was probably present in the solution and at least opal-CT and pores or that forming during the dissolution of the glass. formed paragenetically later than smectite. The similar appearance of the smectite in different parts of the rock and at different localities indicates that precipitation of CONCLUSIONS smectite has not been controlled by hydrothermal fluids, but rather by slowly percolating meteoritic waters. One question The mineral composition of altered surficial suevites at that arises is why not all the glasses have been transformed. the Ries impact crater shows that the dominant alteration Phases that are now clay are seen to be intimately intergrown phase is smectite with a composition corresponding to with unaltered silicate glasses (e.g., see Fig. 8 in Osinski et al. dioctahedral Al-Fe montmorillonite. This is a typical low 2004). One possible explanation is that the unstable hydrous temperature smectite pointing to weathering as the major impact melt glasses were transformed into clays, leaving the reason for alteration without evidence of hydrothermal more stable volatile-free impact glasses fresh. modification. According to microscopic evidence the impact In addition to smectite, alteration of the suevite glass was glasses have been transformed directly to smectite through accompanied by formation of minor halloysite. Both secondary dissolution-precipitation mechanisms without intermediate phases appear to have formed broadly contemporaneously amorphous precursor and/or primitive clay masses. from the glass, although the halloysite is found in the most The low temperature hydrous devitrification of the heavily altered suevites and is likely a later alteration phase as impact glasses is verified by the occurrence of halloysite in denoted by the most intensively weathered rocks as at the highly altered suevites. The composition and the structural Lehberg locality. There, most fractured, and therefore, well state of the clay phases do not show any significant variation permeable monomict (gneiss) breccias are host rocks to within the structure. Smectite coating is abundant in different suevite dykes. The brecciated nature of these gneisses has parts of the surfical suevites and there is no compositional allowed strong leaching of suevite dykes by percolating variation between internal surface layers of pipes and at surface waters. The formation of halloysite at conditions distance from the pipes. generally promoting the conversion of glass to smectite is These results indicate that the surficial suevites are not interpreted to result from more intense percolation of slightly altered by hydrothermal processes as suggested earlier and acidic meteoritic waters through highly porous suevites. The that the alteration has occurred due to low temperature formation of halloysite further confirms the importance of subsurface weathering processes. If the surficial suevites low temperature hydrous weathering. The chemical were indeed hydrothermally modified at the early stages of composition of solutions in weathering is determined by the cooling after the cratering event, then the alteration was at a properties of meteoric fluid that essentially contain CO2 and limited scale and is now completely masked by later accordingly are acidic and also undersaturated. Composition weathering. This is consistent with the suevites themselves of IHT fluids, however, depends upon the composition of the being the main heat source for an impact-induced target rocks (e.g., crystalline versus sedimentary, maffic hydrothermal system at the Ries crater; therefore, impact- versus acidic rocks) present at the impact site as well as on the induced hydrothermal activity has only played a role in the Post-impact alteration of surficial suevites in Ries crater, Germany 1839
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