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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Outline
  5. Samples and Analytical Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgments
  10. Editorial Handling
  11. References
  12. Appendix

The El'gygytgyn impact structure, 18 km in diameter and 3.6 Ma old, in Arctic Siberia, Russia, is the only impact structure on Earth mostly excavated in acidic volcanic rocks. The Late Cretaceous volcanic target includes lavas, tuffs, and ignimbrites of rhyolitic, dacitic, and andesitic composition, and local occurrence of basalt. Although the ejecta blanket around the crater is nearly completely eroded, bomb-shaped impact glasses, redeposited after the impact event, occur in lacustrine terraces within the crater. Here we present detailed petrographic descriptions of newly collected impact glass-bearing samples. The observed features contribute to constrain the formation of the melt and its cooling history within the framework of the impact process. The collected samples can be grouped into two types, characterized by specific features: (1) “pure” glasses, containing very few clasts or new crystals and which were likely formed during the early stages of cratering and (2) a second type, which represents composite samples with impact melt breccia lenses embedded in silicate glass. These mixed samples probably resulted from inclusion of unmelted impact debris during ejection and deposition. After deposition the glassy portions continued to deform, whereas the impact melt breccia inclusions that probably had already cooled down behaved as rigid bodies in the flow.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Outline
  5. Samples and Analytical Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgments
  10. Editorial Handling
  11. References
  12. Appendix

The El'gygytgyn impact structure is located in the central part of the Chukotka Peninsula, Russia, and is centered at 67°30′ N and 172°34′ E (Fig. 1a). The impact origin of the El'gygytgyn structure was suggested to the international scientific community by Dietz and McHone (1976) and then confirmed by Gurov and co-authors after finding shock metamorphic effects in volcanic rocks from the crater (Gurov et al. 1978, 1979). The age of the El'gygytgyn impact structure was determined by 40Ar–39Ar plateau measurements on impact glasses, yielding a value of 3.58 ± 0.04 Ma (Layer 2000) that is in good agreement with earlier K-Ar and fission-track data (Gurov et al. 1979; Storzer and Wagner 1979). Dietz (1977) suggested that El'gygytgyn might be the source of the Australasian tektites, but as those tektites are much younger (0.8 Ma; e.g., Chalmers et al. 1976; Folco et al. 2011), this hypothesis was subsequently rejected.

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Figure 1. Geographic setting and geology of the El'gygytgyn impact structure. a) Geographic location of the El'gygytgyn structure with respect to the North Pole, on the Chukotka Peninsula, northeastern Siberia. b) Simplified geological map of the El'gygytgyn area (modified after Gurov and Koeberl 2004; Stone et al. 2009). Rhyol. ignimbrites = rhyolitic ignimbrites. The figure shows also the area where the glasses investigated here were collected, as well as the location of the ICDP scientific drilling.

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Recently, the El'gygytgyn structure became the subject of a detailed investigation within the framework of an International Continental Scientific Drilling Program (ICDP) project. The aims of the drilling project include paleoclimatic as well as impact-related objectives (Koeberl et al. 2013). A drill core of about/approximately 347 m length, with, on average, 63% recovery, was drilled through the postimpact lake sediments, the underlying impact-generated rocks, and fractured bedrock (Brigham-Grette et al. 2007; Melles et al. 2011). In this study, we focus on samples of impact glasses and melt breccias collected by one of us (C.K.) within the impact crater, during a field survey contemporary with the drilling operations in early 2009.

Impact melts with glassy appearance can form during different stages of crater formation. What is grouped as glasses includes small bodies, which may occur as inclusions in suevite (a polymict glass-bearing impact breccia; Stöffler and Grieve 2007), and impact spherules and tektites, which are mostly deposited as distal ejecta (e.g., French 1998). Such glass types show different microstructures and geochemical compositions that allow them to be classified as impact melts (see Dressler and Reimold 2001 for a review). The small glassy bodies exhibit a variety of compositions, reflecting the local target, and in general display typical shock features (e.g., ribbons of lechatelierite and inclusions of shocked minerals). Spherules and tektites form by melting of the near-surface target and are ejected over great distances. In addition, some tektites might retain geochemical signatures of the extraterrestrial impactor (e.g., Koeberl 1990; Koeberl and Shirey 1993).

Following on the work by Gurov and Koeberl (2004), in which glasses collected during past field surveys at the crater were briefly described and geochemically characterized, we present here the results of detailed petrographic investigations on 24 newly collected samples. This work is focused on the different types of impact glass and melt breccia that can be distinguished, the shock features locally present in clasts preserved from melting, local microstructures comparable with those in impact spherules, and the crystallization of microlites. Finally, our observations provide new constraints on the formation and on the rheological and thermal evolution of such melts during the impact process.

Geological Outline

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Outline
  5. Samples and Analytical Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgments
  10. Editorial Handling
  11. References
  12. Appendix

The El'gygytgyn structure consists of a circular depression, with a rim diameter of about 18 km (Fig. 1b). This depression is largely filled by a lake, which is 12 km in diameter and has a maximum depth of 170 m. The lake is surrounded by a complex system of lacustrine terraces (Gurov et al. 2005). The central uplift is not exposed, but has been revealed by geophysical investigations. It is centered on the crater rim rather than on the lake outline (Gebhardt et al. 2006). A system of radial and concentric faults surrounds the crater (Gurov et al. 2007).

The crater is excavated within the Late Cretaceous Okhotsk-Chukotsky Volcanic Belt, mainly involving the Pykarvaam Formation (88.5 ± 1.7 Ma; Stone et al. 2009). Laser 40Ar/39Ar dating of unshocked volcanic rocks in the crater yielded a range of ages from 89.3 to 83.2 Ma (Layer 2000). The volcanic sequence mainly includes lavas, tuffs, and ignimbrites of rhyolitic to dacitic composition. Locally, andesites and andesitic tuffs, as well as basalts, occur. The whole volcanic suite shows a general gentle dip of 6 to 10° to the east–southeast. The generalized preimpact stratigraphy, as it was observed at the crater (Gurov and Koeberl 2004), consists—from top to bottom—of: ignimbrites (250 m), rhyolitic tuffs and lava (200 m), andesitic tuffs and lava (70 m), ash tuffs and welded tuffs of rhyolitic and dacitic composition (≥100 m). Dacitic and andesitic lava and tuff predominate in the southeastern part of the crater (Fig. 1b). Finally, a basalt plateau (approximately 110 m thick) occurs above the rhyolites and ignimbrites in the northeastern part of the crater rim (Gurov and Koeberl 2004). This basalt probably belongs to the Koekvun Formation, which overlies the Pykarvaam Formation in the regional volcanic sequence (83.1 ± 0.4 Ma; Stone et al. 2009). As ignimbrites and tuffs of rhyolitic–dacitic composition are the most abundant rock types of the crater basement (approximately 89% by volume), the composition of rhyolitic ignimbrites is considered to be representative of the general target. The typical ignimbrite consists of mineral clasts (quartz, orthoclase (Or60–80), plagioclase (An20–30), biotite, and rarely amphibole) embedded in a fine-grained clastic matrix, which includes glass, quartz, and feldspar fragments. Textures range from fluidal-glassy to fine-grained granular and spherulitic (Gurov et al. 2005). The local andesites and andesite tuffs contain fragments and clasts of andesine (An45 to An40), clinopyroxene, and amphibole (Gurov and Koeberl 2004).

Impact melt rocks (mostly glasses) at El'gygytgyn have been found mainly in lacustrine terraces inside the crater. Such impact melt rocks probably were part of the fall-back ejecta that partly filled the crater and which now seem to be largely eroded. Aerodynamically shaped glass bombs occur together with shock metamorphosed rocks in terraces, both within the crater and in terraces along streams draining the lake. Although eroded, the impactites are generally fresh and most of them do not display significant postimpact hydrothermal alteration and weathering (Gurov et al. 2005). Some spherules have been reported by Smirnov et al. (2011) from alluvial sands of tributary streams around El'gygytgyn and in shallow excavations in blocks of impactites. These authors suggested that evidence of a meteoritic signature is present, although any direct connection between the spherules and the impact event could not be established. Also, such 0.5–2 mm sized spherules mainly consist either of titanomagnetite or of so-called black “magnetic glass,” both of which are slightly enriched in MgO compared to the felsic target rocks (Smirnov et al. 2011) and neither is similar to meteoritic material (cf. French and Koeberl 2010).

Samples and Analytical Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Outline
  5. Samples and Analytical Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgments
  10. Editorial Handling
  11. References
  12. Appendix

The samples of this study were collected in April of 2009 by one of us (C.K.) at five different localities within an area of approximately 3 km2 in the north-western part of the crater, on lacustrine terraces (Fig. 1b). As the exact locations of individual finds are not relevant for this work, and because the deposits were reworked, the 24 samples were numbered in no particular order. Optical and electron microscopy was carried out on 30 μm-thick polished thin sections. A description of samples at the meso- and microscale is given in the Appendix. We selected four representative polished thin sections for electron microscopy, using a FEI Inspect S50 scanning electron microscope (SEM) at the University of Vienna and a JEOL 6400 SEM at the Natural History Museum in Vienna. Both instruments are equipped for energy-dispersive spectrometry (EDS), which allowed semiquantitative analyses of the phases under investigation. Beam operating conditions were 10–15 kV at 10 mm working distance for the FEI Inspect and 15 kV at 37 mm working distance for the JEOL instrument, respectively. Quantification of some parameters, such as the relative porosity, was performed by image analysis with the software programs SXM-Image and ImageJ, applied to microphotographs.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Outline
  5. Samples and Analytical Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgments
  10. Editorial Handling
  11. References
  12. Appendix

The samples can be macroscopically divided into two main groups: type 1 that apparently consists only of homogeneous melt (Fig. 2a), and type 2 samples that contain unmelted portions of lithic breccia alternating with black-brownish homogeneous bands of glass (Fig. 2b).

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Figure 2. Representative samples of the two types of impactite found in the study area at El'gygytgyn. a) Samples of type 1 glass, consisting of homogeneous black melt. The sample on the left is highly vesiculated and resembles volcanic scoria, whereas the sample on the right is aerodynamically shaped and has a smooth surface. Samples 1-D and 1-F. Photograph of hand specimens. b) Type 2 sample, showing alternating layers of black glass and melt breccia. The black glass exhibits flow fabric and strong vesiculation. The impact melt breccia portions have lensoidal shapes and consist of lithic fragments in a reddish fine-grained matrix. Sample 2-D. Photograph of a fresh surface cut perpendicular to the layering.

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Type 1 Samples

Mesostructures

The samples of type 1 are more abundant in our collection, representing 16 of 24 samples, and possibly another three samples, but generally they are smaller in size (see Appendix for detailed descriptions). They include different subcategories, distinguished by color and porosity, which are (1) compact black melts, (2) porous black melts, (3) pumice-like whitish melts, and (4) mixed types. The compact black melts consist of homogeneous glass, with low porosity (about 2 vol%), closely resembling volcanic obsidian (Fig. 3a). They are generally aerodynamically shaped with smooth surfaces. In some cases the surface is decorated by millimeter-wide striations. Some larger samples are almost vesicle-free and exhibit conchoidal fracturing. The porous black melts are generally large (tens of centimeters) and are similar to lava scoria blocks. Porosity is high (>14 vol%), which results in rough surfaces. Whitish melts closely resemble volcanic pumice, with medium to high porosity (8–12 vol%) and rough surface. The internal structure reveals complex folding, due to flow and compaction when the melt had relatively low viscosity (Fig. 3b). Apparent layering is determined by different amounts of vesicles in the melt. Some samples have mixed character and contain portions characterized by different colors, where either the whitish or the blackish elements dominate (Figs. 3c and 3d). Locally, whitish melt fragments, characterized by sharp boundaries, are included in blackish melt. The blackish melt shows flow structures around the whitish melt inclusions. This is recognizable by the different orientations of elongated vesicles (Fig. 3c), which suggest different solidification times for the two glasses.

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Figure 3. Type 1 glass. a) Blackish melt with very low vesiculation and complex internal flow fabric. Schlieren of different color seem folded in the central part. The sample closely resembles volcanic obsidian. Sample 5-A. b) Complex flow texture in whitish, highly vesiculated glass, which resembles volcanic pumice. The layers marked by different color are determined by the amount of vesicles in the melt. Sample 4-A. c) Whitish melt fragment embedded in blackish melt. Vesiculation is high in both materials. Vesicles in the blackish melt are elongated subparallel to the margins of the whitish melt fragment, whereas vesicles in the whitish melt are subhorizontal with respect to the sample cut. Sample 4-B. d) Glass with alternating bands of whitish and blackish melts in random association. Sample 2-E. All pictures are scans of fresh cuts. Cutting was done normal to the anisotropy in the samples. The glassy appearance in (a) and (c) is caused by a film of water, which was used to enhance the internal characteristics.

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Microstructures

Under the optical microscope in transmitted light, the blackish homogeneous melts appear brownish to reddish in color. They locally show complex flow structures, which are enhanced by the occurrence of schlieren characterized by different color (Fig. 4). Such schlieren might reflect different contents of Fe and Si in the melt and, consequently, immiscibility. Locally, colored bands are the result of different crystallization stages or incomplete melting, as they include tiny crystals, as revealed by electron microscope observations. Angular fragments of minerals, mainly quartz, are rare. Silica clasts are generally isotropic, but preserve hypidiomorphic shape (Fig. 4d). Some grains display faint fluid inclusions trails, which resemble planar deformation features (PDFs), but the pervasive transformation of the crystals into diaplectic glass prevents any quantification of this feature. The microporosity is generally very low and is represented by rare vesicles. Vesicles are well rounded, with smooth surfaces, and are circular to slightly ellipsoidal in shape. The elongation of the vesicles is generally consistent with the apparent flow direction, which is also marked by the extension of compositional banding. Rounded vesicles and mineral fragments are generally aligned subparallel to the flow, with an angle depending on the aspect ratio of the object (Fig. 4d). The schlieren enclose such features and form pressure shadows at the termination, where locally devitrification effects and incipient spherulites formation occur.

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Figure 4. Type 1 glass: microstructures. a) Tight fold marked by thin schlieren. Vesicles are slightly elongated subparallel to the fold flanks. Sample 1-C. b) Flow fabric marked by alternating schlieren and partially unmelted material. Vesicles are rare and subcircular, apparently not affected by the flow. Sample 5-B. c) Complex flower-like flow structure. The center of the structure seems to be the generation point of the flow. Vesicles are rare and subcircular. Sample 4-F. d) Quartz clast, preserved by melting, enveloped in the flow. The grain is fractured and contains faint traces of fluid inclusions, apparently aligned resembling PFs. The glass exhibits spherulite formation and microlite crystallization in the pressure shadows. Sample 4-C. All pictures are plane-polarized light microphotographs.

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Darker portions of the glass, which are almost opaque under plane-polarized light, consist of fine-grained aggregates of microlites. They are variously arranged in spherulitic to bow-tie aggregates (Fig. 5a; for terminology of shapes for microlitic aggregates, see figs. 6.1 and 6.2 in Lin 2008). Locally, microlites grow radially from heterogeneities that act as crystallization nuclei (Fig. 5b). The heterogeneity is generally either a vesicle or tiny mineral fragment, but locally it can also consist of a crack generated by contraction during fast cooling. Locally, microlites show skeletal growth from a straight prismatic microlite of the same composition. Semiquantitative EDS analyses reveal that most microlites consist of pyroxene, but locally also plagioclase or biotite microlites occur. If the mafic microlites occur in dense aggregates, they appear almost opaque in the optical microscope and form a felt-like layer (Fig. 5b) that locally alternates with transparent lechatelierite (mostly silica, with minor amounts of alumina and alkali) melt bands. An uncommon feature is the occurrence of curved terminations in spider-like aggregates of pyroxene microlites (Fig. 5c). Pyroxenes with curved shapes were observed in volcanic obsidian and discussed by Ross (1962). He interpreted these spider-like shapes as the result of postemplacement crystallization, in contrast to straight prismatic microlites that form during magma ascent (decompression).

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Figure 5. Type 1 glass: microlites. a) Bow-tie aggregate of plagioclase microlites in a brownish melt. Sample 2-C (although this sample belongs to type 2, this particular type of microlite aggregate is here better developed in the quenched melt portions). Plane-polarized light microphotograph. b) Spherulitic aggregate of microlites, radially grown on a tiny clast. The brownish areas represent a felt-like aggregate of microlites, nucleated along microcracks in the glass, whereas the whitish area in the upper right corner might represent highly vesiculated silica melt (lechatelierite). Sample 2-C. Plane-polarized light microphotograph. c) Pyroxene microlites arranged in stars (spider-like) and locally showing curved termination (for comparison, see Ross 1962). Sample 1-D. Plane-polarized light microphotograph.

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Despite the ambiguous appearance in hand samples, which could indicate two or more layers of different compositions, the whitish pumice-like samples have homogeneous color and composition in the optical and electron microscope. They consist solely of solidified silica-rich melt. Although the composition should suggest a transparent appearance in plane-polarized light, such a glass is generally whitish. This is caused by microcrystallization, microvesiculation, and devitrification. In this type of glass, mineral clasts are never preserved. The porosity is high. Vesicles show a variety of sizes. The largest vesicles (up to millimeter-size) exhibit ameboidal shapes, with irregular embayments elongated in flow direction (e.g., Figs. 3b and 3c). The margins of the vesicles are rough because of microcrystallization, as well as incipient devitrification.

Type 2 Samples

Mesostructures

Type 2 samples cannot be simply classified as “glasses,” as they consist of alternating impact melt breccia and type 1-like glass. They are less abundant in our collection (for certain only 5 samples out of 24, with other samples not clearly classified), but are generally larger in size (up to 20 cm) than type 1 samples. The surface is corrugated and layering is recognizable in hand specimens. In some samples the internal structure is externally obliterated by dust coating. This might suggest alteration, but this was not confirmed after removal of the dust by washing and observing the samples on a fresh cut (e.g., Fig. 2a). Such a dusting is a characteristic feature of this sample type and probably results from the disaggregation of the fine-grained matrix of the breccia portions. The breccia portions are characterized by lenticular shapes (well recognizable in larger samples) and unsorted angular clasts, with random orientation, embedded in a brownish matrix (upper part of Fig. 6a). In contrast, the glassy portions exhibit strong flow fabric, which locally envelops isolated clasts or vesicles. The porosity has strong lateral variation, but it generally amounts to about 10–15 vol%.

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Figure 6. Type 2 samples: microstructures. a) Impact melt breccia, containing lithic and glass clasts (in the upper part of the picture) in contact with brownish glass (lower part of the picture). The brownish glass exhibits flow texture and internal layering, caused by different compositions as well as different microlite crystallization. The contact is irregular with fragments of the breccia included in the glass. Sample 2-C. Plane-polarized light microphotograph. b) Similar sample, with the impact melt breccia lens sandwiched between brownish glass layers on the right and left margins. Sample 2-D. Plane-polarized light microphotograph.

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Microstructures

Under the optical microscope, the glassy portions generally show similar features to those of type 1 glasses; thus, we describe here only the noticeable differences or the microstructures which are detectable by SEM. The glassy layers appear brownish in color, but vary from yellowish to dark brown (Fig. 6). As argued for type 1 glasses, the change in color might reflect minor compositional differences or a minor amount of crystallization in the melt. The glassy portions display strong flow fabric, which is marked by schlieren tens of micrometers in width. The brownish layers locally consist of closely spaced aggregates of microlites, which have mainly plagioclase composition (Fig. 7). Such a network is observable only with the SEM, as it appears almost opaque under plane-polarized light. Different amounts of microlites, despite having the same composition, control the apparent color under the optical microscope, rather than different melt compositions. Locally, tiny whitish (in SEM) melt droplets (likely consisting of Fe-oxides) occur (Fig. 7). They are generally concentrated along microlite margins or tiny cracks, suggesting immiscibility processes between melts of incompatible compositions (Magloughlin 2005). Because of their small size, such droplets are only observable with the SEM. They are a common feature in quenched melts (Magloughlin 2005). The porosity in the glassy portions is very low, similar to that observed in the dark type 1 glasses. The few vesicles are well-rounded, characterized by sharp and smooth margins, and have circular to slightly ellipsoidal shapes, which are locally elongated in flow direction. The glass appears quite fresh, and neither alteration nor devitrification are recognizable. Furthermore, the vesicles are not filled by secondary mineralization.

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Figure 7. Type 2 samples: crystallization texture of microlites in a glassy portion (px = pyroxene, plg = plagioclase). Whitish skeletal microlites consist of pyroxene, whereas light-gray microlites, in subparallel arrangement, consist of plagioclase. The Si-rich melt is represented by the dark-gray matrix, whereas the tiny rounded metallic droplets likely consist of Fe-Ti oxides or sulfides. Sample 1-E. Back-scattered electron (BSE) SEM image.

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The transition from the glassy portions to the melt breccia bands is locally marked by a layer (millimeter in thickness) of pervasively vesiculated, dark-brownish melt. Such a layer is generally characterized by extensive microlite crystallization. This reaction at the contact between the two lithologies resembles the chilled margin described from volcanic dykes (e.g., Huppert and Sparks 1989).

The melt breccia portions consist of lithic and glass fragments, as well as single mineral clasts, suspended in a glassy matrix (Fig. 8). The glassy matrix shows incipient devitrification and large lateral variation in composition. The average composition, qualitatively evaluated with EDS, is silica and alkali element rich. Locally, cataclastic domains, consisting mostly of micrometer-scale angular fragments of Si-rich glass, are preserved in the matrix (Fig. 8b), but glass shards (typically fragile and rarely preserved) are not observed. The lithic fragments cover a variety of volcanic lithologies, from rhyolitic ignimbrites to basalt, with andesitic particles common. All these lithologies are present in the target (Stone et al. 2009; Belyi 2010). In between lithic fragments, highly vesiculated melt clasts were observed (Fig. 6). Mineral fragments mainly consist of quartz and feldspar, which are generally refractory during melting, but locally biotite is preserved. Biotite fragments are difficult to recognize because of the occurrence of corrosion features, thermal breakdown, and formation of Ti-bearing oxides, especially along grain margins and cleavage planes. Most of the mineral clasts do not show any shock features (see also next section). The fragments are angular, with sharp margins. Except for biotite, no thermal reactions were observed along clast margins. Furthermore, spherulitic aggregates of microlites (typical of volcanic rocks) did not form at clast margins. Although the matrix locally shows flow fabric, the clasts have random orientations. This suggests that the impact melt breccia portions behaved like rigid lenses in the flowing glass. The porosity in the impact melt breccia is generally very high, also because the contribution of the pervasive microporosity in the clastic portions of the groundmass.

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Figure 8. Type 2 samples: matrix in the impact melt breccia portions. a) Detail of matrix in the impact melt breccia of sample 2-C. The matrix exclusively consists of Si-rich melt with high porosity. The K-feldspar clast exhibits sharp margins, without any reaction with the melt. BSE-SEM image. b) Local cataclastic portion of the matrix in the impact melt breccia. The clasts mainly consist of μm-sized glass fragments (gray in color), with minor amounts of mineral phases (similarly gray, almost undistinguishable in the image, and whitish grains, likely mica). Sample 1-E. BSE-SEM image.

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Shock Features

Preserved minerals in type 1 glasses and in the impact melt breccia portions of type 2 samples locally contain recognizable shock features. Shock features in quartz include one to multiple sets of planar deformation features (PDFs, e.g., in quartz, Fig. 9), mosaicism, isotropization (transformation into diaplectic glass), and melting (occurrence of lechatelierite layers, locally also as layers in individual quartz grains). Locally, Si-rich grains (likely quartz, approximately 300 μm in size) display faint traces of pervasively developed PDFs. In this case, the grains show incipient isotropization, which prevented either the clear classification of the phase, by the investigation techniques used here, and the PDF indexing. Unmelted K-feldspar grains exhibit intense fracturing, with some fractures similar to planar fractures (PFs). In contrast, we have not observed high pressure polymorphs of silica, which were already described from other samples of El'gygytgyn glasses (Gurov and Koeberl 2004).

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Figure 9. Shock feature in El'gygytgyn impact glass. Faint traces of one set of PDFs are visible in the right part of the quartz grain. Sample 4-C. Plane-polarized light microphotograph.

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In the glasses of both sample types, some spherical structures, most likely melt drops, are present. They are generally filled by prismatic microlites (Fig. 10a) of both pyroxene and plagioclase composition, crystallized from a Si-rich melt. The prismatic, and locally skeletal, pyroxene microlites are well developed in fine-grained and densely arranged plagioclase microlite aggregates. Pyroxene microlites seem to have a random distribution, whereas plagioclase microlites show radial growth. Such spherical structures closely resemble the so-called melt splash forms described in impact spherules by Sweeney and Simonson (2008). In homogeneous and microlite-free glass, aggregates of exsolved Fe-Ti-oxides occur locally (Fig. 10b). Similar exsolution textures of metallic oxides were described in spherules from the environs of the El'gygytgyn structure (Smirnov et al. 2011) (Fig. 7).

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Figure 10. a) Spherical structure, splash-form-like, preserved in the melt. The structure is almost completely crystallized, with randomly distributed pyroxene microlites (whitish phase) and radial plagioclase microlites (light gray). The dark gray groundmass consists of Si-rich glass. The apparent blackish phase surrounding the structure was caused by material removal during sample preparation. Sample 2-C. BSE-SEM image. b) Exsolution aggregate of Fe-Ti oxides in homogeneous silicate melt (dark gray). In the aggregate, the brighter phase contains Ti and Fe, whereas the light gray phase consists of TiO2 only. The melt is Si-rich, but contains also Al, Na, K, Ca, Ti, and Fe, in order of decreasing abundance. Sample 1-E. BSE-SEM image.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Outline
  5. Samples and Analytical Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgments
  10. Editorial Handling
  11. References
  12. Appendix

Type 1 Glasses

The impact glasses collected at the El'gygytgyn impact structure clearly belong to the mixed, clast-bearing impact melt derived through melting of various target rocks, mainly felsic volcanics. According to Gurov and Koeberl (2004), impact glasses at El'gygytgyn represent target rock shocked to stage IV, at pressures exceeding 55 GPa and temperatures probably higher than 1700 °C, allowing the melting of quartz at noneutectic conditions. The type 1 glasses described here seem to have experienced homogeneous shock pressure: apart from a few isolated crystals, melting was complete. Such surviving crystals are generally of silica composition, underwent a high degree of shock metamorphism (complete transformation to diaplectic glass), have reaction rims and locally rounded margins, and are aligned with the flow direction (with evidence of fast crystallization in pressure shadows). This suggests that minerals were more likely preserved during the melting rather than introduced from external unshocked material. How it is possible that some grains survived the shock melting can be explained by two processes: (i) heterogeneous shock pressure propagation in polymineralic rock, at the small scale, and (ii) refractory behavior of some minerals. These two processes may have both contributed to the preservation of selected mineral grains, even if they can possibly operate at different stages of impact cratering.

Shock propagation within a polymineralic rock is influenced by many factors (e.g., Langenhorst 2002; Baratoux and Melosh 2003). The occurrence of minerals with different crystallographic orientations and elastic properties, as well as preimpact porosity, may result in shock wave scattering, focusing, enhancement, or attenuation. As a consequence, the conditions required for complete shock melting of rocks (French 1998) might not be reached locally. This would apply during the early stages of the impact process and statistically minerals of every composition might survive. Furthermore, the occurrence of splash forms, Ti-oxide exsolution aggregates, and the quasi-absence of biotite microlites, which all are observed in local spherules (Smirnov et al. 2011), might suggest a coeval formation of these spherules and our type 1 glasses during the early stage of crater formation, although the quality of the data presented by Smirnov et al. (2011) makes such an assignment difficult.

In the case of different thermal responses of minerals, melting is considered from a thermodynamical point of view. The dominant lithology of the target at El'gygytgyn is a felsic tephra, which can be simplified as quartz and feldspar grains embedded in a glassy matrix. Although melting was almost instantaneous, the glassy matrix probably melted first, followed by refractory minerals, along a sequence from the mafic minerals to quartz. Melting large grains of quartz or feldspar requires that the amount of thermal energy stored in the rock after melting the fine-grained glassy matrix exceeds the latent heat of fusion of such phases. Locally, such a situation did not occur and silicate grains are preserved. This interpretation is also supported by the occurrence of a definite flow fabric, which envelops the surviving crystals, suggesting viscous deformation (flow) in a not-completely cooled (but already deposited) melt, and by the occurrence of pyroxene microlites with curved terminations (spider-like shapes), which have been interpreted as result of quenching after deposition (e.g., Ross 1962).

Information about the cooling process can be obtained from the evaluation of microlite crystallization in the melt, by comparison with volcanic glasses, where the occurrence of microlites is well documented. The formation of such microlites as a result of fast crystallization from a quenching melt is today well established (e.g., since Ross 1962). Microlite composition is mainly controlled by melt temperature, whereas the microlite shape is generally determined by the cooling rate, although other ambient parameters (such as local melt composition, volatile content, etc.) might influence shape and composition of microlites (see Ross 1962 and Sharp et al. 1996 for reviews). In our samples, applying a rough geothermometer based on the occurrence of pyroxene microlites in rhyolitic glasses, the crystallization temperature should have been higher than 900 °C (Sharp et al. 1996). Locally some microlites exhibit biotite-like composition. This might reflect local variation in water content in the glass. Although impact melts are usually characterized by extremely low water content (French 1998), the melt rocks at El'gygytgyn generally show an uncommonly high water content (Gurov and Koeberl 2004), which, on the other hand, might also result from postimpact alteration. Plagioclase microlites are common in the quenched melt (e.g., Figs. 7 and 10c), but their abundance is determined by the target composition and, consequently, is not particularly indicative for the cooling history of the melts.

In the magmatic process, microlite crystallization in felsic lavas has been related to decompression during the ascent of the magma from the magma chamber, rather than to cooling in the atmosphere (e.g., Szramek et al. 2010). In the impact process, the melt does not ascend along a conduit and, consequently, does not experience that type of “confined” decompression. The fast shock decompression is not comparable to the eruption-induced decompression of magmas, as the pressure drop and the velocity are several orders of magnitude higher in the impact process. It follows that the microlites we have observed probably crystallized during cooling of the superheated melts, partly in air and partly after deposition.

In conclusion, the exact process of formation of such glasses during the impact event cannot be unambiguously determined, but our observations support the idea that most of the melt was produced immediately after the contact stage and that further melting (almost under thermodynamic equilibrium conditions and thus allowing the preservation of the refractory crystals not melted at the first instance) and flow deformation might have occurred during cooling of the melt. The quenching of the melt did not affect the already formed high-temperature features but the rate of cooling controlled the shape and the composition of the high temperature microlite aggregates.

Type 2 Samples

Type 2 samples, as described above, contain lenses of impact melt breccia in apparently homogeneous glass. As the glass embedding the preserved lenses shows similar characteristics to those of the type 1 glasses, we discuss here only the process that leads to such a relationship between glass and impact melt breccia. Although the blackish glass experienced high shock (pressure stage IV according to Gurov and Koeberl 2004), some fragments of volcanic rocks and mineral clasts, which were only slightly affected by shock (shock stages I and II after Gurov and Koeberl 2004), are included in the impact melt breccia. In addition, the glassy portions show fluidal texture, whereas the impact melt breccia portions do not. This suggests that shocked and unshocked rocks were mixed in the air, after ejection and before deposition. After deposition, the Si-rich melt portions continued to flow until quenching, whereas the impact melt breccia lenses were probably already solidified. This is supported also by the local occurrence of chilled margins at the contact between impact glass and melt breccia portions (e.g., Fig. 6).

In contrast, the mineral and lithic clasts in the breccia have sharp boundaries and do not show any reaction rims with the glassy matrix, including any caused by thermal metamorphism. The matrix of the impact melt breccia is generally microlite-free and locally cataclastic domains are preserved. In addition, microlite-free, highly vesiculated glass clasts occur in the breccia, indicating rapid quenching. All these observations contribute to confirm a lower temperature of the breccia lenses, with respect to the enveloping fluidal hot melt. The features shown in the impact melt breccia lenses, such as the occurrence of highly porous glass fragments, the presence of shocked minerals and shocked volcanic rock clasts, etc., closely resemble those described from other impact melt samples at the El'gygytgyn structure (Gurov and Koeberl 2004; and Gurov et al. 2005), as well as in the suevitic breccia from the drill core (Pittarello et al. 2013). This supports a similar origin for the impact breccias and the inclusions in the type 2 samples, which can be considered fall-back breccia. The fact that the impact melt breccia had already cooled down when it was deposited was also postulated by Engelhardt et al. (1995) for the so-called fallout suevite at the Ries crater. They estimated an average temperature at deposition of 580–750 °C. The shock stage in the preserved portions (impact melt breccia lenses) can only be qualitatively evaluated, as quartz grains with measurable PDFs are rare.

In conclusion, we propose that impact melt breccia lenses were emplaced in the melt during ejection and then both phases were deposited already welded together. Consequently, the cooling histories after deposition were slightly different for the impact melt breccia (probably already solidified, at the temperature suggested by Engelhardt et al. 1995) and the melt. In the melt, which was still hot enough to be plastically deformed (flow structures) at the time of emplacement of the impact melt breccia lenses, the crystallization started at a temperature higher than 900 °C (as indicated by the occurrence of pyroxene microlites in rhyolitic melts, Sharp et al. 1996).

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Outline
  5. Samples and Analytical Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgments
  10. Editorial Handling
  11. References
  12. Appendix

Newly collected samples of impact glasses from the El'gygytgyn structure were investigated by optical and electron microscopy. We have observed two different types of samples: type 1 glasses, consisting of silica-rich quenched melt, and type 2 samples that consist of lenses of impact melt breccia floating in glass. Type 1 glasses locally contain preserved mineral fragments, most of which are moderately to highly shocked. The preservation of such mineral fragments implies variation in shock propagation and heterogeneous melting in polymineralic rock, an observation that is consistent with the formation of impact melts during the early stages of the impact process. Type 2 samples probably continued to undergo minor deformation after deposition, as portions of the glass were still hot enough, but this did not affect the general shape and extent of the breccia lenses, which behaved as rigid bodies. Other than rheology, the thermal history in such glasses was also different from that of type 1 glasses. As the completely molten portions cooled down from higher temperatures, microlites started to crystallize. In contrast, the impact melt breccia lenses were probably already solidified, as suggested by the features listed above and those observed in the impact melt breccia and suevite in the drill core (Pittarello et al. 2013).

Our observations contribute to a discussion of the formation of impact glasses during the cratering process. Type 1 glasses likely formed at the early stage of the impact event, and then were ejected. During ejection some of them might have incorporated impact melt breccia fragments, forming the type 2 material. Although microscopic analysis alone does not provide fully conclusive evidence for this interpretation and because we cannot constrain the formation time of both glass types, we try to establish a sequence of events. Type 2 material formed probably after type 1 glasses, as suggested by (1) type 1 glasses are aerodynamically shaped, indicating cooling during ejection and possible remelting upon re-entry into the atmosphere, (2) deformation of the melt in the type 2 samples, indicating relatively slow cooling after emplacement in the melt breccia lenses, (3) the fact that highly vesiculated glass fragments (likely of impact origin, as they neither were observed in the target rocks we analyzed nor would have survived the shock and mechanical fracturing by compaction after deposition) are embedded in the melt breccia within type 2 material, and (4) the limited observed variability in chemical composition of type 1 glasses and the melts from type 2 samples, with respect to the variety of target rock fragments included in the impact melt breccia lenses in type 2 samples.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Outline
  5. Samples and Analytical Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgments
  10. Editorial Handling
  11. References
  12. Appendix

F. Brandstätter and H. Rice are thanked for SEM assistance at the Natural History Museum Vienna and the University of Vienna, respectively. The present work was supported by the Austrian Science Foundation FWF, project P21821-N19. We are grateful to G. Pratesi, an anonymous reviewer, and associate editor W. U. Reimold for helpful reviews and comments.

References

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  2. Abstract
  3. Introduction
  4. Geological Outline
  5. Samples and Analytical Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgments
  10. Editorial Handling
  11. References
  12. Appendix
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Appendix

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Outline
  5. Samples and Analytical Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgments
  10. Editorial Handling
  11. References
  12. Appendix
Table 1. Detailed petrographic descriptions of the 24 samples (rock and thin section) investigated in this work. Samples are listed according to numbers given in the field.
Sample nameSample typeHand-sample descriptionPetrographic description
1A1–2Black glass (5 × 2 × 3 cm), apparently including a large lithic clast 1 cm in size. The contact is sharp and straight. The black glass has moderate porosity. The elongated vesicles have random orientation. The clast contains some whitish elongated domains defining a foliation.One half of the thin section is dominated by a whitish glass, the other by dark brown glass. The white part contains abundant rounded vesicles, whereas the interstitial glass shows flow structures. The dark part consists of brownish fresh glass injected into a dark gray, partially crystallized melt. The brownish glass contains schlieren with variations in color, rare but large rounded vesicles, dark brown microlites, probably of biotite or pyroxene, and rare clasts of fine-grained volcanic rock or impact melt.
1B1–2Glass block, several cm in size, with chaotic internal fabric, average porosity that locally defines a foliation, with rounded vesicles and some cm-sized lithic clasts. Rusty surface apparently due to alteration.The sample contains different domains (1) dark brown melt with schlieren, (2) homogeneous melt, and (3) with highly porous and microlite-bearing melt. Some unshocked mineral clasts a few mm in size occur, as well as fragments of volcanic/impact melts.
1C1Rounded glass sample (a few cm in size) with smooth surface and low porosity. The vesicles are mm-sized and elongated in flow directionTransparent, light brownish glass, with darker schlieren of different color intensity. The whole structure shows a low-angle fold and the vesicles are slightly elongated parallel to the flow direction. Darker spots in the glass might represent portions that were not completely melted.
1D1Small (3 cm in length), drop-like, black glass body, with smooth surface, very low porosity, and abundant small vesicles elongated in flow directionTransparent glass with thin blackish schlieren marking a dense foliation and abundant microlites. Prismatic microlites, locally with curved termination, are organized in spider-like aggregates. Locally, some partially unmelted portions are preserved.
1E2Fragment of black, highly porous, glass (6 × 5 × 4 cm in size), including cm sized lithic clasts (ignimbrite). The porosity is elongated defining a foliation.Breccia fragments, with lithic and mineral clasts, are included in a highly porous dark brown glass. The glass includes vesicle-free light brownish portions, characterized by strong flow fabric, and microlite-bearing highly porous portions. In lithic clasts, some preserved phenocrysts, mainly consisting of unshocked feldspar and quartz, occur. The breccia has high vesiculation and vesicles have irregular shapes and rough margins.
1F1Block of blackish, porous glass, 3 × 2 × 2 cm in size, with rounded shape and smooth surface. The porosity is intermediate, with perfectly circular vesicles.Light brownish glass with schlieren and well developed flow fabric. In darker domains, spherulitic aggregates of microlites occur. Vesicles are perfectly rounded and not affected by flow.
1G2Highly porous, dark-brown glass block (10 cm in length), with alternating layers that are extensively vesiculated. Some lithic clasts are recognizable.Dark brown glass with high porosity and containing melt breccia fragments. The melt breccia includes angular fragments of quartz and feldspar. The plagioclase is unshocked and exhibits dense albite-twinning. Some quartz grains have lowered birefringence. Some lithic clasts consist of strong vesiculated melt. The glass portions contain abundant vesicles of various shape and size. Locally, fine-grained aggregates of microlites occur.
2A1–2Black porous glass (4 cm in length) including some lithic clasts. The vesicles have random distribution, are small in size (<1 mm) and circular in shape.Strongly porous, dark-brownish glass, which contains abundant microlite aggregates and schlieren of brownish glass, aligned parallel to flow direction. Rare lithic clasts (such as basalt/andesite fragments) are characterized by a fine-grained groundmass and shocked grains (e.g., plagioclase is completely isotropized).
2B1Drop-shaped, black glass (a few cm in size) with few rounded vesicles, 1–2 mm in sizeTransparent glass with rare schlieren and rounded vesicles
2C2Blocky sample of several cm size, which consists of alternating layers of black glass with low porosity and lithic brecciaImpact melt breccia with lithic and mineral clasts, in contact with brownish glass. The glass consists of two melt types: (1) vesicle-free with schlieren and (2) highly porous with microlite aggregates. The melt breccia contains rock and mineral fragments, which are only weakly shocked. Locally, microlites have nucleated in the glassy groundmass.
2D2The sample (5 × 4 × 4 cm in size) consists of reddish melt breccia, which contains layers of black glass, and of a light gray breccia, which is characterized by abundant lithic clastsThe central part of the thin section is dominated by melt breccia, sandwiched by dark brownish glasses. The breccia has a glassy matrix and contains some lithic clasts. Most of the clasts are highly shocked, which prevents us from identifying the original lithology. Quartz grains are rare. The glass consists of alternating layers of strongly vesiculated and microlite-rich, blackish melt, and vesicle-free, brownish glass that exhibits schlieren and flow fabric.
2E1Pumice-like fragment (a few cm in size) that consists of alternating whitish and blackish, variously folded and deformed, glassPumice-like glass, which appears almost opaque in thin section because of extensive devitrification/microcrystallization. Vesicles are abundant and exhibit ameboidal shapes.
2F1Black glass (a few cm in size) with rare, small (<1 mm) vesicles that are strongly elongated. Locally mm-sized vesicles are filled by secondary minerals.Transparent glass, characterized by dense foliation and schlieren. The vesicles are strongly elongated and have whitish glass in pressure shadows. Rare mineral clasts are preserved and are tilted in the flow. They are strongly shocked, making mineral identification difficult.
3A1Block, 20 × 10 × 10 cm in size, of black glass, characterized by rough surfaces and high porosity, due to homogeneously distributed, subrounded vesicles. The sample closely resembles volcanic lava scoria.Strongly vesiculated blackish glass, which appears almost opaque under the optical microscope. Vesicles are abundant (>50 vol%) and have irregular shapes and various sizes. Some portions of vesicle-free, light brownish glass were also observed.
4A1Pumice-like sample (8 × 6 × 4 cm), which consists of alternating whitish and dark-gray layers, folded and kinked in a complex structureDark-gray glass characterized by extensive devitrification/microcrystallization. This makes the sample almost opaque in thin section. Vesicles with a variety of sizes are abundant and have ameboidal shapes.
4B1Black glass (a few cm in size) that includes a whitish, angular clast of glass. Porosity is high in both lithologies and vesicles are up to 1 mm in size.An angular fragment of light brownish glass is included in dark-gray glass, which contains abundant microlite aggregates. The dark-gray melt exhibits flow fabric around the glass fragment. Abundant brownish melt fragments are included in both lithologies. Vesicles are also abundant and have ameboidal shape and random orientation.
4C1Drop-shaped black glass (3 cm long), characterized by low porosity and narrow foliation. Vesicles are strongly elongated in flow direction.Brownish glass, characterized by narrow (tens of μm) schlieren in apparently linear flow. A single quartz clast is preserved. It exhibits rounded margins, reduced birefringence, and likely one to two sets of PDFs.
4D1Small (2 cm long), drop-shaped sample of weakly vesiculated, black glassTransparent glass, which consists of thin schlieren that are elongated in the flow or folded in complex structures. Within an embayment in the folded schlieren, a dark brown glass portion occurs.
4E2Black glass (few cm in size), with abundant mm-sized, rounded vesicles and some lithic, mm-sized clastsImpact melt breccia, which is characterized by layers of light brown glass, dark-brown glass with microlite aggregates, and abundant rounded lithic clasts. Lithic clasts consist of rock and mineral fragments. Rock fragments are strongly shocked. Locally, partially preserved andesite and rhyolite are recognizable. Mineral clasts, mainly quartz, are generally unshocked. The glass is characterized by schlieren elongated in a complex flow fabric and circular vesicles.
4F1Drop-shaped, black glass (2 cm in length), which contains very low porosity and no visible anisotropiesBrownish glass that contains schlieren of different color that are extensively folded in complex flower structures. Porosity is low. Few rounded vesicles are strongly elongated in the flow direction.
4G1Apparently homogeneous, black glass (a few cm in size) with few, rounded vesicles (mm in size)Transparent glass characterized by turbulent flow fabric. Vesicles are rare and locally surrounded by brownish schlieren.
4H1Blackish glass drop (3 cm long) characterized by abundant, rounded vesicles (1 mm in size) in random orientationLight brown glass that contains rare schlieren and small spherulitic microlite aggregates. Vesicles are abundant, with rounded margins and perfectly circular shape.
5A1Block of black glass (15 × 10 × 6 cm) that exhibits a very smooth surface. Vesiculation and any other anisotropy are apparently absent. Where broken, the sample shows conchoidal fracturing.Transparent glass, containing tiny, isolated, spider-like microlite aggregates. A dense flow fabric is marked by thin, blackish schlieren, which envelop elongated lenses of whitish glass.
5B1Small (2 cm long), drop-like sample of black glass, which has a smooth surface and low porosity. Vesicles are a few mm in size and perfectly rounded.Dark brown glass with thick schlieren, locally deformed in complex structures, such as flowers or folds. Vesicles are circular in shape and locally surrounded by brownish glass that is lighter in color than the matrix. Partially preserved, unmelted lithic fragments are rare.