Diaplectic Quartz Research Articles

Revision and recalibration of existing shock classifications for quartzose rocks using low‐shock pressure (2.5–20 GPa) recovery experiments and mesoscale numerical modeling

AbstractA combination of shock recovery experiments and numerical modeling of shock deformation in the low‐shock pressure range from 2.5 to 20 GPa for two dry sandstone types of different porosity, a completely water‐saturated sandstone, and a well‐indurated quartzite provides new insights into strongly heterogeneous distribution of different shock features. (1) For nonporous quartzo‐feldspathic rocks, the traditional classification scheme (Stöffler ) is suitable with slight changes in pressure calibration. (2) For water‐saturated quartzose rocks, a cataclastic texture (microbreccia) seems to be typical for the shock pressure range up to 20 GPa. This microbreccia does not show formation of PDFs but diaplectic quartz glass/SiO2 melt is formed at 20 GPa (~1 vol%). (3) For porous quartzose rocks, the following sequence of shock features is observed with progressive increase in shock pressure (1) crushing of pores, (2) intense fracturing of quartz grains, and (3) increasing formation of diaplectic quartz glass/SiO2 melt replacing fracturing. The formation of diaplectic quartz glass/SiO2 melt, together with SiO2 high‐pressure phases, is a continuous process that strongly depends on porosity. This experimental observation is confirmed by our concomitant numerical modeling. Recalibration of the shock classification scheme results in a porosity versus shock pressure diagram illustrating distinct boundaries for the different shock stages.

Diaplectic quartz glass and SiO2 melt experimentally generated at only 5 GPa shock pressure in porous sandstone: Laboratory observations and meso-scale numerical modeling

A combination of shock recovery experiments and numerical modeling of shock deformation in the low pressure range from 2.5 to 17.5 GPa in dry, porous Seeberger sandstone provides new, significant insights with respect to the heterogeneous nature of shock distribution in such important, upper crustal material, for which to date no pressure-calibrated scheme for shock metamorphism exists. We found that pores are already completely closed at 2.5 GPa shock pressure. Whole quartz grains or parts of them are transformed to diaplectic quartz glass and/or SiO2 melt starting already at 5 GPa, whereas these effects are not observed below shock pressures of 30–35 and ∼45 GPa, respectively, in shock experiments with quartz single crystals. The appearance of diaplectic glass or melt is not restricted to the zone directly below the impacted surface but is related to the occurrence of pores in a much broader zone. The combined amount of these phases increases distinctly with increasing shock pressure from 0.03 vol.% at 5 GPa to ∼80 vol.% at 17.5 GPa. In accordance with a previous shock classification for silica phases in naturally shocked Coconino sandstone from Meteor Crater that was based on varied slopes of the Coconino sandstone Hugoniot curve, our observations allow us to construct a shock pressure classification for porous sandstone consistent with shock stages 1b–4 of the progressive shock metamorphism classification of Kieffer (1971).Numerical modeling at the meso-scale provides the explanation for the discrepancy of shock deformation in porous material and single-crystal quartz, in keeping with our experimental results. It confirms that pore space is completely collapsed at low nominal pressure and demonstrates that pore space collapse results in localized pressure amplification that can exceed 4 times the initial pressure. This provides an explanation for the formation of diaplectic quartz glass and lechatelierite as observed in the low-shock-pressure experiments. The numerical models predict an amount of SiO2 melt similar to that observed in the shock experiments. This also shows that numerical models are essential to provide information beyond experimental capabilities.

Chemical modification of projectile residues and target material in a MEMIN cratering experiment

Abstract– In the context of the MEMIN project, a hypervelocity cratering experiment has been performed using a sphere of the iron meteorite Campo del Cielo as projectile accelerated to 4.56 km s−1, and a block of Seeberger sandstone as target material. The ejecta, collected in a newly designed catcher, are represented by (1) weakly deformed, (2) highly deformed, and (3) highly shocked material. The latter shows shock‐metamorphic features such as planar deformation features (PDF) in quartz, formation of diaplectic quartz glass, partial melting of the sandstone, and partially molten projectile, mixed mechanically and chemically with target melt. During mixing of projectile and target melts, the Fe of the projectile is preferentially partitioned into target melt to a greater degree than Ni and Co yielding a Fe/Ni that is generally higher than Fe/Ni in the projectile. This fractionation results from the differing siderophile properties, specifically from differences in reactivity of Fe, Ni, and Co with oxygen during projectile‐target interaction. Projectile matter was also detected in shocked quartz grains. The average Fe/Ni of quartz with PDF (about 20) and of silica glasses (about 24) are in contrast to the average sandstone ratio (about 422), but resembles the Fe/Ni‐ratio of the projectile (about 14). We briefly discuss possible reasons of projectile melting and vaporization in the experiment, in which the calculated maximum shock pressure does not exceed 55 GPa.

The first MEMIN shock recovery experiments at low shock pressure (5–12.5 GPa) with dry, porous sandstone

Abstract– As part of the MEMIN research program this project is focused on shock deformation experimentally generated in dry, porous Seeberger sandstone in the low shock pressure range from 5 to 12.5 GPa. Special attention is paid to the influence of porosity on progressive shock metamorphism. Shock recovery experiments were carried out with a high‐explosive set‐up that generates a planar shock wave, and using the shock impedance method. Cylinders of sandstone of average grain size of 0.17 mm and porosity of about 19 vol%, and containing some 96 wt% SiO2, were shock deformed. Shock effects induced with increasing shock pressure include: (1) Already at 5 GPa the entire pore space is closed; quartz grains show undulatory extinction. On average, 134 fractures per mm are observed. Dark vesicular melt (glass) of the composition of the montmorillonitic phyllosilicate component of this sandstone occurs at an average amount of 1.6 vol%. (2) At 7.5 GPa, quartz grains show weak but prominent mosaicism and the number of fractures increases to 171 per millimeter. Two additional kinds of melt, both based on phyllosilicate precursor, could be observed: a light colored, vesicular melt and a melt containing large iron particles. The total amount of melt (all types) increased in this experiment to 2.4 vol%. Raman spectroscopy confirmed the presence of shock‐deformed quartz grains near the surface. (3) At 10 and 12.5 GPa, quartz grains also show weak but prominent mosaicism, the number of fractures per mm has reached a plateau value of approximately 200, and the total amount of the different melt types has increased to 4.8 vol%. Diaplectic quartz glass could be observed locally near the impacted surface. In addition, local shock effects, most likely caused by multiple shock wave reflections at sandstone‐container interfaces, occur throughout the sample cylinders and include locally enhanced formation of PDF, as well as shear zones associated with cataclastic microbreccia, diaplectic quartz glass, and SiO2 melt. Overall findings from these first experiments have demonstrated that characteristic shock effects diagnostic for the confirmation of impact structures and suitable for shock pressure calibration are rare. So far, they are restricted to the limited formation of PDF and diaplectic quartz glass at shock pressures of 10 GPa and above.

Fluid+melt intrusions in lechatelierite from the Popigai suevites: A product of dynamic interaction melts and fluids during the shock melting of the target gneiss

The paper presents data on lechatelierite form suevites of the Daldyn Formation in the Popigai astrobleme. Some of the lechatelierite samples show a complicated structure and contain block of diaplectic quartz glass and dynamic “intrusions” of glasses of types I, II, and III. The glasses of types I and II abound in fluid inclusions and display evidence of partial homogenization with lechatelierite. The glasses of type III are clearly separated from all other glasses but show evidence of dynamic interaction with them in the molten state. Fluid inclusions in the glasses of types I and II are syngenetic but have notably different densities from those of completely liquid or gaseous inclusions at 20°C. As is indicated by cryometric data, the liquid phase of the inclusions is aqueous solution of low salinity (5–8 wt % NaClequiv). The bulk petrochemistry of the glasses of type I characterizes them as highly silicic (96.04 wt % SiO2 on average), with elevated K and Na concentrations (Na2O + K2O = 0.72 wt % on average), with 0.73 wt % Al2O3 (on average) and analytical totals 1.97 wt % less than 100%. The glasses of type II are also rich in SiO2 (91.51 wt % SiO2 on average) but contain a broader spectrum of concentrations of major oxides (totaling 5.53 wt % on average) and deficient analytical totals (by 2.96 wt % on average). The glasses of type III are completely equal to impactites produced by melting gneisses of the Popigai astrobleme. The glasses of type I are interpreted to be the intrusion products of the “early” highly mobile and H2O-rich fluid+melt mixtures, whose protolithic material was K-Na feldspars of the target rocks. The derivation of these melts was associated with the capturing of much silica and water at a highly mobile behavior of K and Na and an inert behavior of Al. The glasses of type II were produced by the extensive mixing of silica and water at the limited involvement of apogneiss melts, and these glasses are sometimes deficient in Al. The glasses of type III are usual mixed apogneiss melts. Excess silica in the glasses of types I and II and their richness in water and deficiency in Al suggest impact anatexis and the selective separation of components during their derivation; the parental fluid-melt mixtures of these glasses were derived from such “hydrous” varieties of the target gneisses as diaphthorized and fractured rocks. The evolution and partial vitrification of lechatelierite and the glasses of types I and II proceeded under residual shock pressures, as follows from data on the dense (from ∼0.5 to 1 g/cm3) aqueous inclusions in these glasses, which suggest that the inclusions were captured in the glasses under pressures from ∼0.8 to 3.3 GPa. It follows that our lechatelierite samples have a complex multistage genesis, and their quenching facilitated the preservation of “intrusions” of various stages of shock melting, including the products of the “early” impact anatexis of the gneisses with the selective separation of components at the active participation of water.

Multiple fluvial reworking of impact ejecta—A case study from the Ries crater, southern Germany

Abstract— Impact ejecta eroded and transported by gravity flows, tsunamis, or glaciers have been reported from a number of impact structures on Earth. Impact ejecta reworked by fluvial processes, however, are sparsely mentioned in the literature. This suggests that shocked mineral grains and impact glasses are unstable when eroded and transported in a fluvial system. As a case study, we here present a report of impact ejecta affected by multiple fluvial reworking including rounded quartz grains with planar deformation features and diaplectic quartz and feldspar glass in pebbles of fluvial sandstones from the “Monheimer Höhensande” ?10 km east of the Ries crater in southern Germany.

Characterisation of ballen quartz and cristobalite in impact breccias: new observations and constraints on ballen formation

Ballen quartz and cristobalite in impactite samples from five impact structures (Bosumtwi, Chicxulub, Mien, Ries, and Rochechouart) were investigated by optical microscopy, scanning electron microscopy (SEM), cathodoluminescence (CL), transmission electron microscopy (TEM), and Raman spectroscopy to better understand ballen formation. The occurrence of so-called “ballen quartz” has been reported from about one in five of the known terrestrial impact structures, mostly from clasts in impact melt rock and, more rarely, in suevite. “Ballen silica”, with either α-quartz or α-cristobalite structure, occurs as independent clasts or within diaplectic quartz glass or lechatelierite inclusions. Ballen are more or less spheroidal, in some cases elongate (ovoid) bodies that range in size from 8 to 214 μm, and either intersect or penetrate each other or abut each other. Based mostly on optical microscopic observations and Raman spectroscopy, we distinguish five types of ballen silica: α-cristobalite ballen with homogeneous extinction (type I); ballen α-quartz with homogeneous extinction (type II), with heterogeneous extinction (type III), and with intraballen recrystallisation (type IV); chert-like recristallized ballen α-quartz (type V). For the first time, coesite has been identified within ballen silica – in the form of tiny inclusions and exclusively within ballen of type I. The formation of ballen involves an impact-triggered solid-solid transition from α-quartz to diaplectic quartz glass, followed by the formation at high temperature of ballen of β-cristobalite and/or β-quartz, and finally back-transformation to α-cristobalite and/or α-quartz; or a solid-liquid transition from quartz to lechatelierite followed by nucleation and crystal growth at high temperature. The different types of ballen silica are interpreted as the result of back-transformation of β-cristobalite and/or β-quartz to α-cristobalite and/or to α-quartz with time. In nature, ballen silica has not been found anywhere else but associated with impact structures and, thus, these features could be added to the list of impact-diagnostic criteria.

Impact melt rocks from the Paasselkä impact structure (SE Finland): Petrography and geochemistry

Abstract— Recently, samples of allochthonous melt rocks from the ˜10 km and ≤1.9 Ga Paasselkä impact structure, SE Finland, were obtained. In this study, we present a first detailed petrographic and geochemical description of clast‐rich Paasselkä impact melt rocks. Shock metamorphic features comprise shocked feldspar grains, intensely shocked and toasted quartz, marginally molten and recrystallized clasts thought to have been diaplectic quartz glass, largely fresh and recrystallized feldspar glasses, decomposed biotite flakes, recrystallized fluidal silica glass (originally probably lechatelierite) in partially molten sandstone clasts, all set into a glassy to cryptocrystalline melt matrix. The degree of shock metamorphism of clasts suggests initial whole‐rock melting at peak shock pressures of ≥35 GPa and post‐shock temperatures of up to ˜1500 °C. Glass components vary in geochemical composition corresponding to the mixed character of the crystalline target rock (i.e., representing different monomineralic and mixed‐mineral melts). Feldspar glasses and the fresh glassy to cryptocrystalline melt matrix indicate that the Paasselkä melt rocks are not intensely altered. The geochemical composition of the Paasselkä impact melt rocks is roughly consistent with the compositions of melt rocks from a number of impact structures located within the crystalline basement of the Baltic Shield.

Petrography, geochemistry, and alteration of country rocks from the Bosumtwi impact structure, Ghana

Abstract— Samples of the country rocks that likely constituted the target rocks at the 1.07 Myr old Bosumtwi impact structure in Ghana, West Africa, collected outside of the crater rim in the northern and southern parts of the structure, were studied for their petrographic characteristics and analyzed for their major‐ and trace‐element compositions. The country rocks, mainly meta‐graywacke, shale, and phyllite of the Early Proterozoic Birimian Supergroup and some granites of similar age, are characterized by two generations of alteration. A pre‐impact hydrothermal alteration, often along shear zones, is characterized by new growth of secondary minerals, such as chlorite, sericite, sulfides, and quartz, or replacement of some primary minerals, such as plagioclase and biotite, by secondary sericite and chlorite. A late, argillic alteration, mostly associated with the suevites, is characterized by alteration of the melt/glass clasts in the groundmass of suevites to phyllosilicates. Suevite, which occurs in restricted locations to the north and to the south‐southwest of the crater rim, contains melt fragments, diaplectic quartz glass, ballen quartz, and clasts derived from the full variety of target rocks. No planar deformation features (PDFs) in quartz were found in the country rock samples, and only a few quartz grains in the suevite samples show PDFs, and in rare cases two sets of PDFs. Based on a total alkali element‐silica (TAS) plot, the Bosumtwi granites have tonalitic to quartz‐dioritic compositions. The Nb versus Y and Ta versus Yb discrimination plots show that these granites are of volcanic‐arc tectonic provenance. Provenance studies of the metasedimentary rocks at the Bosumtwi crater have also indicated that the metasediments are volcanic‐arc related. Compared to the average siderophile element contents of the upper continental crust, both country rocks and impact breccias of the Bosumtwi structure show elevated siderophile element contents. This, however, does not indicate the presence of an extraterrestrial component in Bosumtwi suevite, because the Birimian country rocks also have elevated siderophile element contents, which is thought to result from regional hydrothermal alteration that is also related to widespread sulfide and gold mineralization.

Shock‐metamorphic petrography and microRaman spectroscopy of quartz in upper impactite interval, ICDP drill core LB‐07A, Bosumtwi impact crater, Ghana

Abstract— Standard and universal stage optical microscope and microRaman spectroscopic examination of quartz from the upper impactite interval of the International Continental Scientific Drilling Program (ICDP) Lake Bosumtwi crater drill core LB‐07A demonstrates widespread but heterogeneous evidence of shock metamorphism. In the upper impactite, which comprises interbedded polymict lithic breccia and suevite from a drilling depth of 333.4–415.7 m, quartz occurs as a major component within metasedimentary lithic clasts and as abundant, isolated, single‐crystal grains within matrix. The noted quartz shock‐metamorphic features include phenomena related to a) deformation, such as abundant planar microstructures, grain mosaicism, and reduced birefringence; b) phase transformations, such as rare diaplectic quartz glass and very rare coesite; c) melting, such as isolated, colorless to dark, glassy and devitrified vesicular melt grains; and d) secondary, post‐shock features such as abundant, variable decoration of planar microstructures and patchy grain toasting Common to abundant planar deformation features (PDFs) in quartz are dominated by ω{10ω13}‐equivalent crystallographic planes, although significant percentages of π{1012} and other higher index orientations also occur; notably, c(0001) planes are rare. Significantly, the quartz PDF orientations match most closely those reported elsewhere from strongly shocked, crystalline‐target impactites. Barometry estimates based on quartz alteration in the upper impactite indicate that shock pressures in excess of 20 GPa were widely reached; pressures exceeding 40–45 GPa were more rare. The relatively high abundances of decorated planar microstructures and grain toasting in shocked quartz, together with the nature and distribution of melt within suevite, suggest a water‐ or volatile‐rich target for the Bosumtwi impact event.

Lithostratigraphic and petrographic analysis of ICDP drill core LB‐07A, Bosumtwi impact structure, Ghana

Abstract— Lithostratigraphic and petrographic studies of drill core samples from the 545.08 m deep International Continental Scientific Drilling Program (ICDP) borehole LB‐07A in the Bosumtwi impact structure revealed two sequences of impactites below the post‐impact crater sediments and above coherent basement rock. The upper impactites (333.38–415.67 m depth) comprise an alternating sequence of suevite and lithic impact breccias. The lower impactite sequence (415.67–470.55 m depth) consists essentially of monomict impact breccia formed from meta‐graywacke with minor shale, as well as two narrow injections of suevite, which differ from the suevites of the upper impactites in color and intensity of shock metamorphism of the clasts. The basement rock (470.55–545.08 m depth) is composed of lower greenschist‐facies metapelites (shale, schist and minor phyllite), meta‐graywacke, and minor meta‐sandstone, as well as interlaminated quartzite and calcite layers. The basement also contains a number of suevite dikelets that are interpreted as injection veins, as well as a single occurrence of granophyric‐textured rock, tentatively interpreted as a hydrothermally altered granitic intrusion likely related to the regional pre‐impact granitoid complexes.Impact melt fragments are not as prevalent in LB‐07A suevite as in the fallout suevite facies around the northern crater rim; on average, 3.6 vol% of melt fragments is seen in the upper suevites and up to 18 vol% in the lower suevite occurrences. Shock deformation features observed in the suevites and polymict lithic breccias include planar deformation features in quartz (1 to 3 sets), rare diaplectic quartz glass, and very rare diaplectic feldspar glass. Notably, no ballen quartz, which is abundant in the fallout suevites, has been found in the within‐crater impact breccias. An overall slight increase in the degree of shock metamorphism occurs with depth in the impactites, but considerably lower shock degrees are seen in the suevites of the basement rocks, which show similar features to each other. The bulk of the suevite in LB‐07A appears to have been derived from the <35 GPa shock zone of the transient crater.

Drill core LB‐08A, Bosumtwi impact structure, Ghana: Petrographic and shock metamorphic studies of material from the central uplift

Abstract— During a recent drilling project sponsored by the International Continental Scientific Drilling Progam (ICDP), two boreholes (LB‐07A and LB‐08A) were drilled into the crater fill of the Bosumtwi impact structure and the underlying basement, into the deep crater moat and the outer flank of the central uplift, respectively. The Bosumtwi impact structure in Ghana (West Africa), which is 10.5 km in diameter and 1.07 Myr old, is largely filled by Lake Bosumtwi.Here we present the lithostratigraphy of drill core LB‐08A (recovered between 235.6 and 451.33 m depth below lake level) as well as the first mineralogical and petrographic observations of samples from this core. This drill core consists of approximately 25 m of polymict, clast‐supported lithic breccia intercalated with suevite, which overlies fractured/brecciated metasediment that displays a large variation in lithology and grain size. The lithologies present in the central uplift are metasediments composed dominantly of fine‐grained to gritty meta‐graywacke, phyllite, and slate, as well as suevite and polymict lithic impact breccia. The suevites, principally present between 235.6 and 240.5 m and between 257.6 and 262.2 m, display a fine‐grained fragmental matrix (about 39 to 45 vol%) and a variety of lithic and mineral clasts that include meta‐graywacke, phyllite, slate, quartzite, carbon‐rich organic shale, and calcite, as well as melt particles, fractured quartz, unshocked quartz, unshocked feldspar, quartz with planar deformation features (PDFs), diaplectic quartz glass, mica, epidote, sphene, and opaque minerals). The crater‐fill suevite contains calcite clasts but no granite clasts, in contrast to suevite from outside the northern crater rim.The presence of melt particles in suevite samples from the uppermost 25 meters of the core and in suevite dikelets in the basement is an indicator of shock pressures exceeding 45 GPa. Quartz grains present in suevite and polymict lithic impact breccia abundantly display 1 to (rarely) 4 sets of PDFs per grain. The shock pressures recorded by the PDFs in quartz grains in the polymict impact breccia range from 10 to ∼30 GPa. We also observed a decrease of the abundance of shocked quartz grains in the brecciated basement with increasing depth. Meta‐graywacke samples from the basement are heterogeneously shocked, with shock pressures locally ranging up to 25–30 GPa. Suevites from this borehole show a lower proportion of melt particles and diaplectic quartz glass than suevites from outside the northern crater rim (fallback impact breccia), as well as a lack of ballen quartz, which is present in the external breccias. Similar variations of melt‐particle abundance and shock‐metamorphic grade between impact‐breccia deposits within the crater and fallout impact breccia outside the crater have been observed at the Ries impact structure, Germany.

Petrographic studies of “fallout” suevite from outside the Bosumtwi impact structure, Ghana

Abstract— Field studies and a shallow drilling program carried out in 1999 provided information about the thickness and distribution of suevite to the north of the Bosumtwi crater rim. Suevite occurrence there is known from an ˜1.5 km2 area; its thickness is ≤15 m. The present suevite distribution is likely the result of differential erosion and does not reflect the initial areal extent of continuous Bosumtwi ejecta deposits.Here we discuss the petrographic characteristics of drill core samples of melt‐rich suevite. Macroscopic constituents of the suevites are melt bodies and crystalline and metasedimentary rock (granite, graywacke, phyllite, shale, schist, and possibly slate) clasts up to about 40 cm in size. Shock metamorphic effects in the clasts include multiple sets of planar deformation features (PDFs), diaplectic quartz and feldspar glasses, lechatelierite, and ballen quartz, besides biotite with kink bands. Basement rock clasts in the suevite represent all stages of shock metamorphism, ranging from samples without shock effects to completely shock‐melted material that is indicative of shock pressures up to ˜60 GPa.

The record of Miocene impacts in the Argentine Pampas

Argentine Pampean sediments represent a nearly continuous record of deposition since the late Miocene (~10 Ma). Previous studies described five localized concentrations of vesicular impact glasses from the Holocene to late Pliocene. Two more occurrences from the late Miocene are reported here: one near Chasico (CH) with an 40Ar/39Ar age of 9.24 ± 0.09 Ma, and the other near Baha Blanca (BB) with an age of 5.28 ± 0.04 Ma. In contrast with andesitic and dacitic impact glasses from other localities in the Pampas, the CH and BB glasses are more mafic. They also exhibit higher degrees of melting with relatively few xenoycrysts but extensive quench crystals. In addition to evidence for extreme heating (>1700 °C), shock features are observed (e.g., planar deformation features [PDFs] and diaplectic quartz and feldspar) in impact glasses from both deposits. Geochemical analyses reveal unusually high levels of Ba (~7700 ppm) in some samples, which is consistent with an interpretation that these impacts excavated marine sequences known to be at depth. These two new impact glass occurrences raise to seven the number of late Cenozoic impacts for which there is evidence preserved in the Pampean sediments. This seemingly high number of significant impacts over a 10^6 km^2 area in a time span of 10 Myr is consistent with the number of bolides larger than 100 m expected to enter the atmosphere but is contrary to calculated survival rates following atmospheric disruption. The Pampean record suggests, therefore, that either atmospheric entry models need to be reconsidered or that the Earth has received an enhanced flux of impactors during portions of the late Cenozoic. Evidence for the resulting collisions may be best preserved and revealed in rare dissected regions of continuous, low-energy deposition such as the Pampas. Additionally, the rare earth element (REE) concentrations of the target sediments and impact melts associated with the Chasic event resemble the HNa/K australites of similar age. This suggests the possibility that those enigmatic tektites could have originated as high-angle, distal ejecta from an impact in Argentina, thereby accounting for their rarity and notable chemical and physical differences from other Australasian impact glasses.

Shocked rocks and impact glasses from the El'gygytgyn impact structure, Russia

Abstract— The El'gygytgyn impact structure is about 18 km in diameter and is located in the central part of Chukotka, arctic Russia. The crater was formed in volcanic rock strata of Cretaceous age, which include lava and tuffs of rhyolites, dacites, and andesites. A mid‐Pliocene age of the crater was previously determined by fission track (3.45 ± 0.15 Ma) and 40Ar/39Ar dating (3.58 ± 0.04 Ma). The ejecta layer around the crater is completely eroded. Shock‐metamorphosed volcanic rocks, impact melt rocks, and bomb‐shaped impact glasses occur in lacustrine terraces but have been redeposited after the impact event.Clasts of volcanic rocks, which range in composition from rhyolite to dacite, represent all stages of shock metamorphism, including selective melting and formation of homogeneous impact melt. Four stages of shocked volcanic rocks were identified: stage I (≤35 GPa; lava and tuff contain weakly to strongly shocked quartz and feldspar clasts with abundant PFs and PDFs; coesite and stishovite occur as well), stage II (35–45 GPa; quartz and feldspar are converted to diaplectic glass; coesite but no stishovite), stage III (45–55 GPa; partly melted volcanic rocks; common diaplectic quartz glass; feldspar is melted), and stage IV (>55 GPa; melt rocks and glasses). Two main types of impact melt rocks occur in the crater: 1) impact melt rocks and impact melt breccias (containing abundant fragments of shocked volcanic rocks) that were probably derived from (now eroded) impact melt flows on the crater walls, and 2) aerodynamically shaped impact melt glass “bombs” composed of homogeneous glass. The composition of the glasses is almost identical to that of rhyolites from the uppermost part of the target. Cobalt, Ni, and Ir abundances in the impact glasses and melt rocks are not or only slightly enriched compared to the volcanic target rocks; only the Cr abundances show a distinct enrichment, which points toward an achondritic projectile. However, the present data do not allow one to unambiguously identify a meteoritic component in the El'gygytgyn impact melt rocks.

Kgagodi Basin: The first impact structure recognized in Botswana

Abstract— The 3.4 km wide, so‐called Kgagodi Basin structure, which is centered at longitude 27°34.4′ E and latitude 22°28.6′ S in eastern Botswana, has been confirmed as a meteorite impact structure. This crater structure was first recognized through geophysical analysis; now, we confirm its impact origin by the recognition of shock metamorphosed material in samples from a drill core obtained close to the crater rim. The structure formed in Archean granitoid basement overlain and intruded by Karoo dolerite. The crater yielded a gravity model consistent with a simple bowl‐shape crater form. The drill core extends to a depth of 274 m and comprises crater fill sediments to a depth of 158 m. Impact breccia was recovered only between 158 and 165 m depth, below which locally brecciated basement granitoids grade into fractured and eventually undeformed crystalline basement, from ∼250 m depth. Shock metamorphic effects were only found in granitoid clasts in the narrow breccia zone. This breccia is classified as suevitic impact breccia due to the presence of melt and glass fragments, at a very small abundance. The shocked grains are exclusively derived from granitoid target material. Shock effects include multiple sets of planar deformation features in quartz and feldspar; diaplectic quartz, and partially and completely isotropized felsic minerals, and rare melt fragments were encountered. Abundances of some siderophile elements and especially, Ir, in suevitic breccia samples are significantly elevated compared to the contents in the target rocks, which provides evidence for the presence of a small meteoritic component.Kgagodi is the first impact structure recognized in the region of the Kalahari Desert in southern Africa. Based on lithological and first palynological evidence, the age of the Kgagodi structure is tentatively assigned to the upper Cretaceous to early Tertiary interval. Thus, the crater fill has the potential to provide a long record of paleoclimatic conditions.

Shock metamorphism and shock barometry at a complex impact structure: Slate Islands, Canada

The Slate Islands archipelago is believed to represent the central uplifted portion of a complex impact structure. Planar microstructures in quartz and feldspars and shock vitrification of rocks are the most common shock metamorphic features encountered. No diaplectic quartz was identified in the exposed rocks, but minor maskelynite is present. Shatter cones occur on all islands of the archipelago suggesting minimum pressures of 4 ± 2 GPa. The relative frequency of low index planar microstructures of specific, optically determined crystallographic orientations in quartz are correlated with results from shock barometric experiments to estimate peak shock pressures experienced by the exposed target rocks. In general, there is a decrease in shock pressure recorded in the target rocks from about 20–25 GPa in east-central Patterson Island to about 5–10 GPa at the western shore of this island and on Mortimer Island. The shock attenuation gradient is ∼4.5 GPa/km across this section of the island group. However, the shock attenuation has a roughly concentric plan only over the western part of the archipelago. There is no distinct shock center and there are other deviations from circularity. This is probably the result of: (1) the shock wave not having expanded from a point or spherical source because of the ∼1. 0 to 1.5 km size of the impactor; (2) differential movement of large target rock blocks during the central uplift and crater modification phases of the impact process. The orientation of planar deformation features in quartz appears to be independent of the shock wave direction suggesting that crystal structure exerts the primary control on microstructure development. Based on the results of XRD analyses, residual, post-impact temperatures were high enough to cause annealing of submicroscopic damage in shocked quartz.

Shock experiments on pre-heated α- and β-quartz: I. Optical and density data

Discs of single crystal quartz, unheated, and pre-heated to 275°C and 540°C (i.e., α-quartz) and 630°C (i.e., β-quartz) were experimentally shocked to pressures ranging from 20 to 40 GPa, with the shock front propagating parallel to either (10 1 0) or (0001). Refractive indices, density and the orientation of planar deformation features (PDFs) were determined on the recovered quartz samples. Refractive indices of pre-heated quartz are unaffected up to 25 GPa but density starts to decrease slightly up to this pressure. Above 25 GPa, pre-heating causes drastic variations: Refractive indices and birefringence of quartz shocked at ambient temperature decrease continuously, until complete isotropization is reached at 35 GPa. In quartz shocked at 630°C, refractivity drops discontinuously in the interval from 25 to 26 GPa, and complete transformation to diaplectic glass is reached at 26 GPa. Density follows the trends demonstrated by the optical parameters, with higher pre-shock temperatures yielding lower density at a given shock pressure. These results indicate that the threshold pressure for the onset of transformation to diaplectic quartz glass is largely temperature-invariant, lying at 25 GPa, whereas the pressure limit for complete transformation decreases with increasing pre-shock temperature from ≈ 35 to ≈ 26 GPa. Quartz shocked parallel to (0001) always has a higher density and refractivity than that shocked parallel to (10 1 0), indicating a significant influence of the structural anisotropy. This is also evident from the distribution of PDF orientations. Pressures ⩾ 25 GPa cause, in quartz shocked parallel to (10 1 0), PDFs that are predominantly oriented parallel to {10 1 2}, while quartz shocked to the same pressures but parallel to (0001) contains almost exclusively PDFs parallel to {10 1 3}. PDF orientations in quartz shocked at ambient temperature parallel to (10 1 0) show the following characteristics: (i) The frequency of {10 1 3} is high at 20 GPa, declines with increasing pressure, and finally is totally absent at 32 GPa, (ii) {10 1 3} begins to occur at 25 GPa, increases, and finally is the only remaining orientation at 32 GPa, and (iii) {10 1 2} and {11 2 2} are only present at 20 and 25 GPa. In contrast, quartz shocked at 630°C parallel to (10 1 0) displays a broad PDF distribution with indistinct maxima, and {10 1 3} is totally absent. The small structural difference between α- and β-quartz, which is reflected in the PDF distribution, should allow, in principle, evaluation of the pre-shock temperature in naturally impacted metamorphic rocks. The results substantiate that formation of shock effects in quartz is dependent on (i) shock pressure, (ii) pre-shock temperature and (iii) shock wave direction. So far, the latter two parameters have not been considered in the application of experimental results to impact sites.

INFRARED SPECTROSCOPIC STUDIES OF EXPERIMENTALLY SHOCK‐LOADED QUARTZ

The infrared behavior of experimentally shock‐loaded quartz was studied in the wavenumber region 1400 to 100 cm−1. In agreement with results of X‐ray investigations reported in an earlier paper, the infrared studies indicate that solid‐state (diaplectic or thetomorphic) SiO2‐glass is formed upon release from shock pressures of about > 14.0 GPa; complete transformation occurs upon release from about 30.0 GPa.The structure of the solid‐state glass must be quite different from that of fused SiO2. While fused silica is supposed to consist of small “crystallites,” or of a network of SiO4‐tetrahedra groups of tridymite‐like short‐range order, the positions of the infrared absorption bands of the shock‐produced solid‐state quartz glass lie practically at the same wave numbers as crystalline quartz. We conclude that diaplectic quartz glass consists structurally of extremely small quartz‐like “crystallites.” These crystallites are mutually linked in a disordered but structurally more open manner as in α‐quartz.The formation of short‐range ordered quartz‐type solid‐state SiO2 glass is explained by the decomposition of a sixfold coordinated stishovite‐like high pressure phase upon pressure release at the relatively low shock temperatures (≤ 300°C at 30.0 GPa). The extremely short duration of the shock process may prevent the growth of the “quartz nuclei” to long‐range ordered crystallites.

Petrographie, Sto�wellenmetamorphose und Entstehung polymikter kristalliner Breccien im N�rdlinger Ries

Polymict cristalline breccias are typical impact products of the Ries crater. They occur within the Ries crater (Appetshofen, Lierheim, Leopold Meyers Keller), on its rim (Maihingen-Klostermuhle) and within the immediate vicinity of the crater (Itzing). Apart from very rare admixtures of sedimentary rock fragments the polymict cristalline breccias consist almost exclusively of fragments of various cristalline rocks, namely granites, gneisses and amphibolites. The petrographical and statistical investigations have shown that breccias from different localities have different composition. This reflects a possible difference in local compositions of the cristalline basement. The rocks in the breccias have been affected to various degress by shock metamorphism. The amphibolites could thus be shown to belong predominantly to stage I (diaplectic quartz and feldspar, ∼100–300 kb) and stage II (diaplectic quartz and feldspar glasses, ∼350–500 kb) whereas the granites and geisses can be attributed mostly to stage 0 (fractured quartz and feldspar, <100 kb) and stage I. This is in part the result of the bulk shock wave impedance of the rocks in question.