Geochemical and petrographic characteristics of impactites and Cretaceous target rocks from the Yaxcopoil‐1 borehole,Chicxulub impact structure,Mexico: Implications for target composition

Abstract— We present major and trace element data as well as petrographic observations for impactites (suevitic groundmass, bulk suevite, and melt rock particles) and target lithologies, including Cretaceous anhydrite, dolomite, argillaceous limestone, and oil shale, from the Yaxcopoil‐1 borehole, Chixculub impact structure. The suevitic groundmass and bulk suevite have similar compositions, largely representing mixtures of carbonate and silicate components. The latter are dominated by melt rock particles. Trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites; in contrast, major elements indicate a strong calcitic component in the impactites. The siliceous end‐member requires a mafic component in order to explain the low SiO₂ content. Multicomponent mixing of various target rocks, the high alteration state, and dilution by carbonate complicate the determination of primary melt particle compositions. However, two overlapping compositional groups can be discerned—a high‐Ba, low‐Ta group and a high‐Fe, high‐Zn, and high‐Hf group. Cretaceous dolomitic rocks, argillaceous limestone, and shale are typically enriched in U, As, Br, and Sb, whereas anhydrite contains high Sr contents. The oil shale samples have abundances that are similar to the North American Shale Composite (NASC), but with a comparatively high U content. Clastic sedimentary rocks are characterized by relatively high Th, Hf, Zr, As, and Sb abundances. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil‐1 drill core likely register a multistage deformation history that spans the period from pre‐ to post‐impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, we have found no evidence in our samples from a depth of 1347–1348 m for the presence of melt breccia. We favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event.     

Major and trace element compositions of melt particles and associated phases from the Yaxcopoil‐1 drill core,Chicxulub impact structure,Mexico

Abstract— Melt particles found at various depths in impactites from the Yaxcopoil‐1 borehole into the Chicxulub impact structure (Yucatán) have been analyzed for their major and trace element abundances. A total of 176 electron microprobe and 45 LA‐ICP‐MS analyses from eight different melt particles were investigated. The main purpose of this work was to constrain the compositions of precursor materials and secondary alteration characteristics of these melt particles. Individual melt particles are highly heterogeneous, which makes compositional categorization extremely difficult. Melt particles from the uppermost part of the impactite sequence are Ca‐ and Na‐depleted and show negative Ce anomalies, which is likely a result of seawater interaction. Various compositional groupings of melt particles are determined with ternary and binary element ratio plots involving major and trace elements. This helps distinguish the degree of alteration versus primary heterogeneity of melt phases. Comparison of the trace element ratios Sc/Zr, Y/Zr, Ba/Zr, Ba/Rb, and Sr/Rb with compositions of known target rocks provides some constraints on protolith compositions; however, the melt compositions analyzed exceed the known compositional diversity of possible target rocks. Normalized REE patterns are unique for each melt particle, likely reflecting precursor mineral or rock compositions. The various discrimination techniques indicate that the highly variable compositions are the products of melting of individual minerals or of mixtures of several minerals. Small, angular shards that are particularly abundant in units 2 and 3 represent rapidly quenched melts, whereas larger particles (>0.5 mm) that contain microlites and have fluidal, schlieric textures cooled over a protracted period. Angular, shard‐like particles with microlites in unit 5 likely crystallized below the glass transition temperature or underwent fragmentation during or after deposition.     

Impactites of the Haughton impact structure,Devon Island,Canadian High Arctic

Abstract— Contrary to the previous interpretation of a single allochthonous impactite lithology, combined field, optical, and analytical scanning electron microscopy (SEM) studies have revealed the presence of a series of impactites at the Haughton impact structure. In the crater interior, there is a consistent upward sequence from parautochthonous target rocks overlain by parautochthonous lithic (monomict) breccias, through allochthonous lithic (polymict) breccia, into pale grey allochthonous impact melt breccias. The groundmass of the pale grey impact melt breccias consists of microcrystalline calcite, silicate impact melt glass, and anhydrite. Analytical data and microtextures indicate that these phases represent a series of impact‐generated melts that were molten at the time of, and following, deposition. Impact melt glass clasts are present in approximately half of the samples studied. Consideration of the groundmass phases and impact glass clasts reveal that impactites of the crater interior contain shock‐melted sedimentary material from depths of >920 to <1880 m in the pre‐impact target sequence. Two principal impactites have been recognized in the near‐surface crater rim region of Haughton. Pale yellow‐brown allochthonous impact melt breccias and megablocks are overlain by pale grey allochthonous impact melt breccias. The former are derived from depths of >200 to <760 m and are interpreted as remnants of the continuous ejecta blanket. The pale grey impact melt breccias, although similar to the impact melt breccias of the crater interior, are more carbonate‐rich and do not appear to have incorporated clasts from the crystalline basement. Thus, the spatial distribution of the crater‐fill impactites at Haughton, the stratigraphic succession from target rocks to allochthonous impactites, the recognition of large volumes of impact melt breccias, and their probable original volume are all analogous to characteristics of coherent impact melt layers in comparatively sized structures formed in crystalline targets.     

Petrography and geochemistry of target rocks and impactites from the Ilyinets Crater,Ukraine

Abstract— The ～400 Ma old Ilyinets impact structure was formed in the Precambrian basement of the Ukrainian Shield and is now mostly covered by Quaternary sediments. Various impact breccias and melts are exposed in its southern section. The crater is a complex structure with a central uplift that is surrounded by an annular deposit of breccias and melt rocks. In the annulus, brecciated basement rocks are overlain by up to 80 m of glass-poor suevitic breccia, which is overlain (and partly intercalated) by glass-rich suevite with a thickness of up to 130 m. Impact-melt rocks occur within and on top of the suevites—in some cases in the form of devitrified bomb-shaped impact-glass fragments. We have studied the petrographic and geochemical characteristics of 31, mostly shocked, target rock samples (granites, gneisses, and one amphibolite) obtained from drill cores within the structure, and impact breccias and melt rock samples from drill cores and surface exposures. Multiple sets of planar deformation features (PDFs) are common in quartz, potassium feldspar, and plagioclase of the shocked target rocks. The breccias comprise more or less devitrified impact melt with shocked clasts. The impact-melt rocks (“bombs”) show abundant vesicles and, in some cases, glass is still present as brownish patches and schlieren. All impact breccias (including the melt rocks) are strongly altered and have significantly elevated K contents and lower Na contents than the target rocks. The alteration could have occurred in an impact-induced hydrothermal system. The bomb-shaped melt rocks have lower Mg and Ca contents than other rock types at the crater. Compared to target rocks, only minor enrichments of siderophile element contents (e.g., Ni, Co, Ir) in impact-melt rocks were found.     

The effect of target lithology on the products of impact melting

Abstract— Impact cratering is an important geological process on the terrestrial planets and rocky and icy moons of the outer solar system. Impact events generate pressures and temperatures that can melt a substantial volume of the target; however, there remains considerable discussion as to the effect of target lithology on the generation of impact melts. Early studies showed that for impacts into crystalline targets, coherent impact melt rocks or “sheets” are formed with these rocks often displaying classic igneous structures (e.g., columnar jointing) and textures. For impact structures containing some amount of sedimentary rocks in the target sequence, a wide range of impact‐generated lithologies have been described, although it has generally been suggested that impact melt is either lacking or is volumetrically minor. This is surprising given theoretical constraints, which show that as much melt should be produced during impacts into sedimentary targets. The question then arises: where has all the melt gone? The goal of this synthesis is to explore the effect of target lithology on the products of impact melting. A comparative study of the similarly sized Haughton, Mistastin, and Ries impact structures, suggests that the fundamental processes of impact melting are basically the same in sedimentary and crystalline targets, regardless of target properties. Furthermore, using advanced microbeam analytical techniques, it is apparent that, for the structures under consideration here, a large proportion of the melt is retained within the crater (as crater‐fill impactites) for impacts into sedimentary‐bearing target rocks. Thus, it is suggested that the basic products are genetically equivalent but they just appear different. That is, it is the textural, chemical and physical properties of the products that vary.     

Composition of impact melt particles and the effects of post‐impact alteration in suevitic rocks at the Yaxcopoil‐1 drill core,Chicxulub crater,Mexico

Abstract Petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m‐thick suevitic sequence at the Chicxulub Yax‐1 drill core was performed. The aim of this study is to determine the composition of the impact melt, the variation between different types of melt particles, and the effects of post‐impact hydrothermal alteration. We demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post‐impact hydrothermal alteration. Most melt particles are andesitic in composition. Clinopyroxene‐rich melt particles possess lower SiO₂ and higher CaO contents. These are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks. Multi‐stage post‐impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles. Following backwash of seawater into the crater, palagonitization of glassy melt particles likely caused depletion of SiO₂, Al₂O₃, CaO, Na₂O, and enrichment of K₂O and FeO_tot during an early alteration stage. Since glass is very susceptible to fluid‐rock interaction, the state of primary crystallization of the melt particles had a significant influence on the intensity of the post‐impact hydrothermal mass transfer and was more pronounced in glassy melt particles than in well‐crystallized particles. In contrast to other occurrences of Chicxulub impactites, the Yax‐1 suevitic rocks show strong potassium metasomatism with hydrothermal K‐feldspar formation and whole rock K₂0 enrichment, especially in the lower unit of the suevitic sequence. A late stage of hydrothermal alteration is characterized by precipitation of silica, analcime, and Na‐bearing Mg‐rich smectite, among other minerals. This indicates a general evolution from a silica‐undersaturated fluid at relatively high potassium activities at an early stage toward a silica‐oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction.     

Geological overview and cratering model for the Haughton impact structure,Devon Island,Canadian High Arctic

Abstract— The Haughton impact structure has been the focus of systematic, multi‐disciplinary field and laboratory research activities over the past several years. Regional geological mapping has refined the sedimentary target stratigraphy and constrained the thickness of the sedimentary sequence at the time of impact to ?1880 m. New ⁴⁰Ar–³⁹Ar dates place the impact event at ?39 Ma, in the late Eocene. Haughton has an apparent crater diameter of ?23 km, with an estimated rim (final crater) diameter of ?16 km. The structure lacks a central topographic peak or peak ring, which is unusual for craters of this size. Geological mapping and sampling reveals that a series of different impactites are present at Haughton. The volumetrically dominant crater‐fill impact melt breccias contain a calcite‐anhydrite‐silicate glass groundmass, all of which have been shown to represent impact‐generated melt phases. These impactites are, therefore, stratigraphically and genetically equivalent to coherent impact melt rocks present in craters developed in crystalline targets. The crater‐fill impactites provided a heat source that drove a post‐impact hydrothermal system. During this time, Haughton would have represented a transient, warm, wet microbial oasis. A subsequent episode of erosion, during which time substantial amounts of impactites were removed, was followed by the deposition of intra‐crater lacustrine sediments of the Haughton Formation during the Miocene. Present‐day intra‐crater lakes and ponds preserve a detailed paleoenvironmental record dating back to the last glaciation in the High Arctic. Modern modification of the landscape is dominated by seasonal regional glacial and niveal melting, and local periglacial processes. The impact processing of target materials improved the opportunities for colonization and has provided several present‐day habitats suitable for microbial life that otherwise do not exist in the surrounding terrain.     

Revisiting the Gow Lake impact structure,Saskatchewan, Canada

The ~5 km diameter Gow Lake impact structure formed in the Canadian Shield of northern Saskatchewan approximately 197 Myr ago. This structure has not been studied in detail since its discovery during a regional gravity survey in the early 1970s. We report here on field observations from a 2011 expedition that, when combined with subsequent laboratory studies, have revealed a wealth of new information about this poorly studied Canadian impact structure. Initially considered to be a prototypical central peak (i.e., a complex) impact structure, our observations demonstrate that Gow Lake is actually a transitional impact structure, making it one of only two identified on Earth. Despite its age, a well-preserved sequence of crater-fill impactites is preserved on Calder Island in the middle of Gow Lake. From the base upward, this stratigraphy is parautochthonous target rock, lithic impact breccia, clast-rich impact melt rock, red clast-poor impact melt rock, and green clast-poor impact melt rocks. Discontinuous lenses of impact melt-bearing breccia also occur near the top of the red impact melt rocks and in the uppermost green impact melt rocks. The vitric particles in these breccias display irregular and contorted outlines. This, together with their setting within crater-fill melt rocks, is indicative of an origin as flows within the transient cavity and not an airborne mode of origin. Following impact, a hydrothermal system was initiated, which resulted in alteration of the crater-fill impactites. Major alteration phases are nontronite clay, K-feldspar, and quartz.     

Geochemistry of drill core samples from Yaxcopoil‐1, Chicxulub impact crater,Mexico

Abstract— The chemical composition of suevites, displaced Cretaceous target rocks, and impact‐generated dikes within these rocks from the Yaxcopoil‐1 (Yax‐1) drill core, Chicxulub impact crater, Mexico, is reported and compared with the data from the Yucatán 6 (Y6) samples. Within the six suevite subunits of Yax‐1, four units with different chemical compositions can be distinguished: a) upper/lower sorted and upper suevite (depth of 795–846 m); b) middle suevite (depth of 846–861 m); c) brecciated impact melt rock (depth of 861–885 m); and d) lower suevite (depth of 885–895 m). The suevite sequence (a), (b), and (d) display an increase of the CaO content and a decrease of the silicate basement component from top to bottom. In contrast, the suevite of Y6 shows an inverse trend. The different distances of the Yax‐1 and Y6 drilling sites from the crater center (～60, and ～47 km, respectively) lead to different suevite sequences. Within the Cretaceous rocks of Yax‐1, a suevitic dike (depth of ～916 m) does not display chemical differences when compared with the suevite, while an impact melt rock dike (depth of ～1348 m) is significantly enriched in immobile elements. A clastic breccia dike (depth of ～1316 m) is dominated by material derived locally from the host rock, while the silicate‐rich component is similar to that found in the suevite. Significant enrichments of the K₂O content were observed in the Yax‐1 suevite and the impact‐generated dikes. All impactites of Yax‐1 and Y6 are mixtures of a crystalline basement and a carbonate component from the sedimentary cover. An anhydrite component in the impactites is missing (Yax‐1) or negligible (Y6).     

Target rocks,impact glasses,and melt rocks from the Lonar impact crater,India: Petrography and geochemistry

Abstract— The Lonar crater, India, is the only well‐preserved simple crater on Earth in continental flood basalts; it is excavated in the Deccan trap basalts of Cretaceous‐Tertiary age. A representative set of target basalts, including the basalt flows excavated by the crater, and a variety of impact breccias and impact glasses, were analyzed for their major and trace element compositions. Impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes. Comparable geochemical patterns of immobile elements (e.g., REEs) for glass, melt rock and basalt indicates minimal fractionation between the target rocks and the impactites. We found only little indication of post‐impact hydrothermal alteration in terms of volatile trace element changes. No clear indication of an extraterrestrial component was found in any of our breccias and impact glasses, indicating either a low level of contamination, or a non‐chondritic or otherwise iridium‐poor impactor.     

Petrographic studies of the impact melts from Meteor Crater,Arizona, USA

Abstract— We investigated the ballistically dispersed melts from Meteor Crater, Arizona, USA to determine the stratigraphic extent of its melt zone from the compositional relationship of melts and target rocks. Most melt particles are crystallized, hydrated, and oxidized; pristine glasses are rare. Hydration and oxidation occurred at ambient temperatures long after the impact. The preserved glasses are generally clear and texturally homogeneous, but unlike typical impact melts, they have unusually heterogeneous compositions, both within individual particles and from sample to sample. For example, the average SiO₂ for individual particles ranges from 43 to 65%. The projectile content is unusually high and it is distributed bimodally, with specific samples containing either 5–10% or 20–30% FeO. These compositional heterogeneities most likely reflect the high carbonate content of the target rocks and the release of copious CO₂ that dispersed the melts, thereby terminating melt flow and mixing. The high projectile content and the CO₂ depleted residue of purely sedimentary rocks produced mafic melts that crystallized fine‐grained olivine and pyroxene. The melts fall into three compositional groups reflecting variable proportions of the major target formations, Moenkopi, Kaibab, and Coconino. Least‐square mixing calculations revealed one group to contain 55% Moenkopi, 40% quartz‐rich, upper Kaibab, and 5% meteorite, suggesting a source depth of <30 m from the pre‐impact surface. The other two melt groups have higher contents of meteorite (15–20%) and Kaibab (50–70%) and contain more SiO₂ than average Kaibab. The additional quartz may have been derived from Coconino or the upper Kaibab, implying melt depths >90 m or <30 m, respectively. Additional studies, especially hydrocode calculations, are needed to better understand the source depth of these melts and their exceptionally high projectile content.     

Cathodoluminescence as a tool to discriminate impact melt,shocked and unshocked volcanics: A case study of samples from the El'gygytgyn impact structure

El'gygytgyn (Chukotka, Arctic Russia) is a well‐preserved impact structure, mostly excavated in siliceous volcanic rocks. For this reason, the El'gygytgyn structure has been investigated in recent years and drilled in 2009 in the framework of an ICDP (International Continental Scientific Drilling Program) project. The target rocks mostly consist of rhyodacitic ignimbrites and tuffs, which make it difficult to distinguish impact melt clasts from fragments of unshocked target rock within the impact breccia. Several chemical and petrologic attempts, other than dating individual clasts, have been considered to distinguish impact melt from unshocked volcanic rock of the targets, but none has proven reliable. Here, we propose to use cathodoluminescence (imaging and spectrometry), whose intensity is inversely correlated with the degree of shock metamorphism experienced by the investigated lithology, to aid in such a distinction. Specifically, impact melt rocks display low cathodoluminescence intensity, whereas unshocked volcanic rocks from the area typically show high luminescence. This high luminescence decreases with the degree of shock experienced by the individual clasts in the impact breccia, down to almost undetectable when the groundmass is completely molten. This might apply only to El'gygytgyn, because the luminescence in volcanic rocks might be due to devitrification and recrystallization processes of the relatively old (Cretaceous) target rock with respect to the young impactites (3.58 Ma). The alteration that affects most samples from the drill core does not have a significant effect on the cathodoluminescence response. In conclusion, cathodoluminescence imaging and spectra, supported by Raman spectroscopy, potentially provide a useful tool for in situ characterization of siliceous impactites formed in volcanic target.     

Geochemical studies of impact breccias and country rocks from the El'gygytgyn impact structure,Russia

The complex impact structure El'gygytgyn (age 3.6 Ma, diameter 18 km) in northeastern Russia was formed in ~88 Ma old volcanic target rocks of the Ochotsk‐Chukotsky Volcanic Belt (OCVB). In 2009, El'gygytgyn was the target of a drilling project of the International Continental Scientific Drilling Program (ICDP), and in summer 2011 it was investigated further by a Russian–German expedition. Drill core material and surface samples, including volcanic target rocks and impactites, have been investigated by various geochemical techniques in order to improve the record of trace element characteristics for these lithologies and to attempt to detect and constrain a possible meteoritic component. The bedrock units of the ICDP drill core reflect the felsic volcanics that are predominant in the crater vicinity. The overlying suevites comprise a mixture of all currently known target lithologies, dominated by felsic rocks but lacking a discernable meteoritic component based on platinum group element abundances. The reworked suevite, directly overlain by lake sediments, is not only comparatively enriched in shocked minerals and impact glass spherules, but also contains the highest concentrations of Os, Ir, Ru, and Rh compared to other El'gygytgyn impactites. This is—to a lesser extent—the result of admixture of a mafic component, but more likely the signature of a chondritic meteoritic component. However, the highly siderophile element contribution from target material akin to the mafic blocks of the ICDP drill core to the impactites remains poorly constrained.     

Target rocks,impact glasses,and melt rocks from the Lonar crater,India: Highly siderophile element systematics and Sr‐Nd‐Os isotopic signatures

The Lonar crater is a ~0.57‐Myr‐old impact structure located in the Deccan Traps of the Indian peninsula. It probably represents the best‐preserved impact structure hosted in continental flood basalts, providing unique opportunities to study processes of impact cratering in basaltic targets. Here we present highly siderophile element (HSE) abundances and Sr‐Nd and Os isotope data for target basalts and impactites (impact glasses and impact melt rocks) from the Lonar area. These tools may enable us to better constrain the interplay of a variety of impact‐related processes such as mixing, volatilization, and contamination. Strontium and Nd isotopic compositions of impactites confirm and extend earlier suggestions about the incorporation of ancient basement rocks in Lonar impactites. In the Re‐Os isochron plot, target basalts exhibit considerable scatter around a 65.6 Myr Re‐Os reference isochron, most likely reflecting weathering and/or magma replenishment processes. Most impactites plot at distinctly lower ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios compared to the target rocks and exhibit up to two orders of magnitude higher abundances of Ir, Os, and Ru. Moreover, the impactites show near‐chondritic interelement ratios of HSE. We interpret our results in terms of an addition of up to 0.03% of a chondritc component to most impact glasses and impact melt rocks. The magnitude of the admixture is significantly lower than the earlier reported 12–20 wt% of extraterrestrial component for Lonar impact spherules, reflecting the typical difference in the distribution of projectile component between impact glass spherules and bulk impactites.     

Melt‐bearing impactites (suevite and impact melt rock) within the Gardnos structure,Norway

Abstract– Melt‐bearing impactites dominated by suevite, and with a minor content of clast‐rich impact melt rock, are found within the central part of the Gardnos structure. They are preserved as the eroded remnants in the relatively small complex impact structure with a present diameter of 5 km. These rocks have been mapped in the field and in the Branden drill core, and described according to mineralogy/petrology, including matrix, litho clast, and melt content, as well as geochemistry. Based on our extensive field mapping, a simple 3‐D model of the original crater was constructed to estimate tentative volumes for the melt‐bearing impactites. The variations in lithic and melt fragment content and chemistry of suevite matrix can mostly be explained by incorporation of mafic rocks into a dominant mixture of granitic, gneissic, and quartzitic target rocks, reflecting mixing of material from different parts of the crater. Melt fragments within suevite occur with a variety of shapes and textures, probably related to different original target rock composition, to the various temperatures the individual fragments were subjected to during the impact event and deposition processes. This study discusses the impact‐related deposits based on a sedimentological approach. Their overall composition and structures indicate dominating gravity flow processes in the final transportation and deposition of the suevite.     

Incipient devitrification of impact melt particles at Bosumtwi crater,Ghana: Implications for suevite cooling history and melt dispersion

The petrographic, mineralogical, and geochemical compositions of the incipient devitrification products in impact melt fragments found in outer suevites at the Bosumtwi impact crater were studied to reconstruct the postimpact environmental constraints on the suevite formation and to refine its cooling history. Our study shows that devitrified melt/particles contain numerous microlitic crystals and crystal aggregates of different shapes derived from rapid cooling. The matrix of melt/particles in Bosumtwi suevites contains abundant Mg‐hercynite (pleonaste)‐type spinels with sizes rarely exceeding a few micrometers. High nucleation density of microlites suggests rapid crystallization under strong undercooling in the presence of abundant volatiles. Although the Bosumtwi impact event took place in a continental environment, the possible sources for elevated fluid/volatile content could have been the groundwater in the deeply weathered and fractured‐jointed Birimian basement, dewatering of abundant hydrous phases in weathered crust or hydrothermally altered basement, and the shale/phyllite–greywacke lithologies in the target rocks. Our results show that enough volatiles were present in the target rocks at the time of impact for the effective impact melt dispersion observed in Bosumtwi impactites.     

Geochemical evidence of an extraterrestrial component in impact melt breccia from the Paleoproterozoic Dhala impact structure,India

The Paleoproterozoic Dhala structure with an estimated diameter of ~11 km is a confirmed complex impact structure located in the central Indian state of Madhya Pradesh in predominantly granitic basement (2.65 Ga), in the northwestern part of the Archean Bundelkhand craton. The target lithology is granitic in composition but includes a variety of meta‐supracrustal rock types. The impactites and target rocks are overlain by ~1.7 Ga sediments of the Dhala Group and the Vindhyan Supergroup. The area was cored in more than 70 locations and the subsurface lithology shows pseudotachylitic breccia, impact melt breccia, suevite, lithic breccias, and postimpact sediments. Despite extensive erosion, the Dhala structure is well preserved and displays nearly all the diagnostic microscopic shock metamorphic features. This study is aimed at identifying the presence of an impactor component in impact melt rock by analyzing the siderophile element concentrations and rhenium‐osmium isotopic compositions of four samples of impactites (three melt breccias and one lithic breccia) and two samples of target rock (a biotite granite and a mafic intrusive rock). The impact melt breccias are of granitic composition. In some samples, the siderophile elements and HREE enrichment observed are comparable to the target rock abundances. The Cr versus Ir concentrations indicate the probable admixture of approximately 0.3 wt.% of an extraterrestrial component to the impact melt breccia. The Re and Os abundances and the ¹⁸⁷Os/¹⁸⁸Os ratio of 0.133 of one melt breccia specimen confirm the presence of an extraterrestrial component, although the impactor type characterization still remains inconclusive.     

Intra‐crater sedimentary deposits at the Haughton impact structure,Devon Island,Canadian High Arctic

Abstract— Detailed field mapping has revealed the presence of a series of intra‐crater sedimentary deposits within the interior of the Haughton impact structure, Devon Island, Canadian High Arctic. Coarse‐grained, well‐sorted, pale gray lithic sandstones (reworked impact melt breccias) unconformably overlie pristine impact melt breccias and attest to an episode of erosion, during which time significant quantities of impact melt breccias were removed. The reworked impact melt breccias are, in turn, unconformably overlain by paleolacustrine sediments of the Miocene Haughton Formation. Sediments of the Haughton Formation were clearly derived from pre‐impact lower Paleozoic target rocks of the Allen Bay Formation, which form the crater rim in the northern, western, and southern regions of the Haughton structure. Collectively, these field relationships indicate that the Haughton Formation was deposited up to several million years after the formation of the Haughton crater and that they do not, therefore, represent an immediate, post‐impact crater lake deposit. This is consistent with new isotopic dating of impactites from Haughton that indicate an Eocene age for the impact event (Sherlock et al. 2005). In addition, isolated deposits of post‐Miocene intra‐crater glacigenic and fluvioglacial sediments were found lying unconformably over remnants of the Haughton Formation, impact melt breccias, and other pre‐impact target rock formations. These deposits provide clear evidence for glaciation at the Haughton crater. The wealth and complexity of geological and climatological information preserved as intra‐crater deposits at Haughton suggests that craters on Mars with intra‐crater sedimentary records might present us with similar opportunities, but also possibly significant challenges.     

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.