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.     

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 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.     

Identification of a meteoritic component using chromium isotopic composition of impact rocks from the Lonar impact structure,India

The existence of mass‐independent chromium isotope variability of nucleosynthetic origin in meteorites and their components provides a means to investigate potential genetic relationship between meteorites and planetary bodies. Moreover, chromium abundances are depleted in most surficial terrestrial rocks relative to chondrites such that Cr isotopes are a powerful tool to detect the contribution of various types of extra‐terrestrial material in terrestrial impactites. This approach can thus be used to constrain the nature of the bolide resulting in breccia and melt rocks in terrestrial impact structures. Here, we report the Cr isotope composition of impact rocks from the ~0.57 Ma Lonar crater (India), which is the best‐preserved impact structure excavated in basaltic target rocks. Results confirm the presence of a chondritic component in several bulk rock samples of up to 3%. The impactor that created the Lonar crater had a composition that was most likely similar to that of carbonaceous chondrites, possibly a CM‐type chondrite.     

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.     

Major and trace element characteristics of impactites from the Yaxcopoil‐1 borehole,Chicxulub structure,Mexico

Abstract— Approximately 100 m of impactites were retrieved from the ICDP borehole Yaxcopoil‐1 (Yax‐1), located ～60 km south‐southwest from the center of the Chicxulub impact crater on the Yucatán Peninsula of Mexico. Here, we characterize and discuss this impact breccia interval according to its geochemical characteristics. Chemical analysis of samples from all five recognized breccia units reveals that the impactites are of heterogeneous composition with regard to both major and trace elements at the single sample (8–16 cm³) scale. This is primarily due to a strong mixing relationship between carbonate and silicate fractions. However, averaged compositions for suevitic units 1 to 3 are similar, and the silicate fraction (after removal of the carbonate component) indicates thorough mixing and homogenization. Analysis of the green melt breccia horizon, unit 4, indicates that it contains a distinct mafic component. Large brown melt particles (in units 2, 3, and 4) represent a mixture of feldspathic and mafic components, with high CaO abundances. Unit 5 shows the greatest compositional diversity, with highly variable abundances of SiO₂, CaO, and MgO. Inter‐sample heterogeneity is the result of small sample size combined with inherent heterogeneous lithological compositions, highly variable particle size of melt and lithic components, and post‐depositional alteration. In contrast to samples from the Y6 borehole from closer to the center of the structure, Yax‐1 impactites have a strong carbonate component. Elevated loss on ignition, Rb, and Cs contents in the upper two impactite units indicate strong interaction with seawater. The contents of the siderophile elements, including Ni, Co, Ir, and Cr, do not indicate the presence of a significant extraterrestrial component in the Yax‐1 impactites.     

The Obolon impact structure,Ukraine, and its ejecta deposits

Abstract— The Obolon impact structure, 18 km in diameter, is situated at the northeastern slope of the Ukrainian Shield near its margin with the Dnieper‐Donets Depression. The crater was formed in crystalline rocks of the Precambrian basement that are overlain by marine Carboniferous and continental Lower Triassic deposits. The post‐impact sediments comprise marine Middle Jurassic (Bajocian and Bathonian) and younger Mesozoic and Cenozoic deposits. Today the impact structure is buried beneath an about 300‐meter‐thick sedimentary rock sequence. Most information on the Obolon structure is derived from two boreholes in the western part of the crater. The lowest part of the section in the deepest borehole is composed by allogenic breccia of crystalline basement rocks overlain by clast‐rich impact melt rocks and suevites. Abundant shock metamorphic effects are planar deformation features (PDFs) in quartz and feldspars, kink bands in biotite, etc. Coesite and impact diamonds were found in clast‐rich impact melt rocks. Crater‐fill deposits are a series of sandstones and breccias with blocks of sedimentary rocks that are covered by a layer of crystalline rock breccia. Crystalline rock breccias, conglomeratic breccias, and sandstones with crystalline rock debris have been found in some boreholes around the Obolon impact structure to a distance of about 50 km from its center. Those deposits are always underlain by Lower Triassic continental red clay and overlain by Middle Jurassic marine clay. The K‐Ar age of impact melt glasses is 169 Ma, which corresponds to the Middle Jurassic (Bajocian) age. The composition of crater‐fill rocks within the crater and sediments outside the Obolon structure testify to its formation under submarine conditions.     

Through the impact glass: Insight into the evolution of melt at the Mistastin Lake impact structure

By analyzing impact glass, the evolution of the impact melt at the Mistastin Lake impact structure was investigated. Impact glass clasts are present in a range of impactites, including polymict breccias and clast‐rich impact melt rock, and from a variety of settings within the crater. From the glass clasts analyzed, three petrographic subtypes of impact glass were identified based on their clast content, prevalence of schlieren, color, texture, and habit. Several alteration phases were also observed replacing glass and infilling vesicles; however, textural observations and quantified compositional data allowed for the identification of pristine impact glass. Although the various types of glasses show significant overlap in their major oxide composition, several subtle variations in the major oxide chemistry of the glass were observed. To investigate this variation, a least‐squares mixing model was implemented utilizing the composition of the glass and the known target rock chemistry to model the initial melt composition. Additionally, image analysis of the glass clasts was used to investigate whether the compositional variations correlated to textural difference in the lithologies. We propose that the textural and compositional dichotomy observed is a product of the evolution, assimilation, and emplacement of the glass. The dichotomy is reflective of the melt either being ballistically emplaced (group 2 glasses: occurring in melt‐poor polymict breccias at lowermost stratigraphic position outside the transient crater) or the result of late‐stage melt flows (group 1 glasses, occurring in melt‐bearing polymict breccias and impact melt rocks at higher stratigraphic positions outside the transient crater).     

The Dhala structure,Bundelkhand craton,Central India—Eroded remnant of a large Paleoproterozoic impact structure

Abstract— The newly discovered Dhala structure, Madhya Pradesh State, India, is the eroded remnant of an impact structure with an estimated present‐day apparent diameter of about 11 km. It is located in the northwestern part of the Archean Bundelkhand craton. The pre‐impact country rocks are predominantly granitoids of ?2.5 Ga age, with minor 2.0–2.15 Ga mafic intrusive rocks, and they are overlain by post‐impact sediments of the presumably >1.7 Ga Vindhyan Supergroup. Thus, the age for this impact event is currently bracketed by these two sequences. The Dhala structure is asymmetrically disposed with respect to a central elevated area (CEA) of Vindhyan sediments. The CEA is surrounded by two prominent morphological rings comprising pre‐Vindhyan arenaceous‐argillaceous and partially rudaceous metasediments and monomict granitoid breccia, respectively. There are also scattered outcrops of impact melt breccia exposed towards the inner edge of the monomict breccia zone, occurring over a nearly 6 km long trend and with a maximum outcrop width of ?170 m. Many lithic and mineral clasts within the melt breccia exhibit diagnostic shock metamorphic features, including multiple sets of planar deformation features (PDFs) in quartz and feldspar, ballen‐textured quartz, occurrences of coesite, and feldspar with checkerboard texture. In addition, various thermal alteration textures have been found in clasts of initially superheated impact melt. The impact melt breccia also contains numerous fragments composed of partially devitrified impact melt that is mixed with unshocked as well as shock deformed quartz and feldspar clasts. The chemical compositions of the impact melt rock and the regionally occurring granitoids are similar. The Ir contents of various impact melt breccia samples are close to the detection limit (1–1.5 ppb) and do not provide evidence for the presence of a meteoritic component in the melt breccia. The presence of diagnostic shock features in mineral and lithic clasts in impact melt breccia confirm Dhala as an impact structure. At 11 km, Dhala is the largest impact structure currently known in the region between the Mediterranean and southeast Asia.     

First petrographic results on impactites from the Yaxcopoil‐1 borehole,Chicxulub structure,Mexico

Abstract— The ICDP Yaxcopoil‐1 (Yax‐1) borehole located 60 km south‐southwest of the center of the Chicxulub impact structure intercepted an interval of allogenic impactites (depth of 795–895 m). Petrographic analysis of these impactites allows them to be differentiated into five units based on their textural and modal variations. Unit 1 (795–922 m) comprises an apparently reworked, poorly sorted and graded, fine‐grained, clast‐supported, melt fragment‐bearing suevitic breccia. The interstitial material, similar to units 2 and 3, is permeated by numerous carbonate veinlets. Units 2 (823–846 m) and 3 (846–861 m) are groundmass‐supported breccias that comprise green to variegated angular and fluidal melt particles. The groundmass of units 2 and 3 comprises predominantly fine‐grained calcite, altered alkali element‐, Ca‐, and Si‐rich cement, as well as occasional lithic fragments. Unit 4 (861–885 m) represents a massive, variably devitrified, and brecciated impact melt rock. The lowermost unit, unit 5 (885–895 m), comprises highly variable proportions of melt rock particles (MRP) and lithic fragments in a fine‐grained, carbonate‐dominated groundmass. This groundmass could represent either a secondary hydrothermal phase or a carbonate melt phase, or both. Units 1 and 5 contain well‐preserved foraminifera fossils and a significantly higher proportion of carbonate clasts than the other units. All units show diagnostic shock deformation features in quartz and feldspar clasts. Our observations reveal that most felsic and all mafic MRP are altered. They register extensive K‐metasomatism. In terms of emplacement, we suggest that units 1 to 3 represent fallout suevite from a collapsing impact plume, whereby unit 1 was subsequently reworked by resurging water. Unit 4 represents a coherent impact melt body, the formation of which involved a significant proportion of crystalline basement. Unit 5 is believed to represent an initial ejecta/ground‐surge deposit.     

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.     

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.     

Hydrothermally enhanced magnetization at the center of the Haughton impact structure?

Haughton is a ~24 Myr old midsize (apparent diameter 23 km) complex impact structure located on Devon Island in Nunavut, Canada. The center of the structure shows a negative gravity anomaly of ?12 mGal coupled to a localized positive magnetic field anomaly of ~900 nT. A field expedition in 2013 led to the acquisition of new ground magnetic field mapping and electrical resistivity data sets, as well as the first subsurface drill cores down to 13 m depth at the top of the magnetic field anomaly. Petrography, rock magnetic, and petrophysical measurements were performed on the cores and revealed two different types of clast‐rich polymict impactites: (1) a white hydrothermally altered impact melt rock, not previously observed at Haughton, and (2) a gray impact melt rock with no macroscopic sign of alteration. In the altered core, gypsum is present in macroscopic veins and in the form of intergranular selenite associated with colored and zoned carbonate clasts. This altered core has a natural remanent magnetization (NRM) four to five times higher than materials from the other core but the same magnetic susceptibility. Their magnetization is still higher than the surrounding crater‐fill impact melt rocks. X‐ray fluorescence data indicate a similar proportion of iron‐rich phases in both cores and an enrichment in silicates within the altered core. In addition, alternating‐field demagnetization results show that one main process remagnetized the rocks. These results support the hypothesis that intense and possibly localized post‐impact hydrothermal alteration enhanced the magnetization of the clast‐rich impact melt rocks by crystallization of magnetite within the center of the Haughton impact structure. Subsequent erosion was followed by in situ concentration in the subsurface leading to large magnetic gradient on surface.     

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.     

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.     

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).     

Melt in the impact breccias from the Eyreville drill cores,Chesapeake Bay impact structure,USA

The center of the 35.3 Ma Chesapeake Bay impact structure (85 km diameter) was drilled during 2005/2006 in an ICDP–USGS drilling project. The Eyreville drill cores include polymict impact breccias and associated rocks (1397–1551 m depth). Tens of melt particles from these impactites were studied by optical and electron microscopy, electron microprobe, and microRaman spectroscopy, and classified into six groups: m1—clear or brownish melt, m2—brownish melt altered to phyllosilicates, m3—colorless silica melt, m4—melt with pyroxene and plagioclase crystallites, m5—dark brown melt, and m6—melt with globular texture. These melt types have partly overlapping major element abundances, and large compositional variations due to the presence of schlieren, poorly mixed melt phases, partly digested clasts, and variable crystallization and alteration. The different melt types also vary in their abundance with depth in the drill core. Based on the chemical data, mixing calculations were performed to determine possible precursors of these melt particles. The calculations suggest that most melt types formed mainly from the thick sedimentary section of the target sequence (mainly the Potomac Formation), but an additional crystalline basement (schist/gneiss) precursor is likely for the most abundant melt types m2 and m5. Sedimentary rocks with compositions similar to those of the melt particles are present among the Eyreville core samples. Therefore, sedimentary target rocks were the main precursor of the Eyreville melt particles. However, the composition of the melt particles is not only the result of the precursor composition but also the result of changes during melting and solidification, as well as postimpact alteration, which must also be considered. The variability of the melt particle compositions reflects the variety of target rocks and indicates that there was no uniform melt source. Original heterogeneities, resulting from melting of different target rocks, may be preserved in impactites of some large impact structures that formed in volatile‐rich targets, because no large melt body exists, in which homogenization would have taken place.     

Impact melt rocks from the Late Paleocene Hiawatha impact structure,northwest Greenland

Impact melt rocks formed during hypervelocity impact events are ideal for studying impact structures. Here, we describe impact melt rock samples collected proximal to the 31 km wide 58 Ma Hiawatha impact structure, northwest Greenland, which is completely covered by the Greenland Ice Sheet. The melt rocks contain diagnostic shock indicators (e.g., planar deformation features [PDF] in quartz and shocked zircon) and form three groups based on melt textures and chemistry: (i) hypocrystalline, (ii) glassy, and (iii) carbonate-based melt rocks. The exposed foreland directly in front of the structure consists of metasedimentary successions and igneous plutons; however, the carbonate-based impactites indicate a mixed target sequence with a significant carbonate-rich component. Well-preserved organic material in some melt rocks indicates that North Greenland at the time of impact was host to abundant organic material, likely a dense high-latitude temperate forest. Geochemical signatures of platinum-group elements in selected samples indicate an extraterrestrial component and support previous identification of a highly fractionated iron impactor in glaciofluvial sand. Our results illustrate the possibility to study impact structures hidden beneath a thick ice sheet based on transported samples and this opens a new avenue for identifying other potential impact craters in Greenland and Antarctica.