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.     

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.     

Depositional processes of impactites from the YAX-1 drill core in the Chicxulub impact structure inferred from vertical profiles of PDF orientations and grain size distributions of shocked quartz

Core samples from the Chicxulub impact structure provide insights into the formation processes of a shallow-marine-target, complex crater. Although previous studies investigated the impactites (generally suevitic and polymict breccias) of the Yaxcopoil-1 (YAX-1) drill core in the Chicxulub impact structure, the interpretation of its deposition remains controversial. Here, we analyze planar deformation features (PDFs), grain size, and abundance of shocked quartz throughout the YAX-1 impactite sequence (794–895 m in depth). PDF orientations of most quartz grains in YAX-1 impactites show a distribution of both low angles ({104}, {103}, {102}) and high angles (orientations higher than 55° to c-axis), while the lower part of the impactite sequence contains quartz showing only PDF orientations of low angles. High-abundance, coarse-grained shocked quartz is found from the lower to middle parts of the impactites, whereas it abruptly changes to low-abundance, fine-grained shocked quartz within the upper part. In the uppermost part of the impactites, repeated oscillations in contents of these two components are observed. PDF orientation pattern suggests most of the shocked quartz grains experienced a range of shock pressure, except two samples in the lower part of impactites, which experienced only a high level of shock. We suggest that the base and lower part of the impactite sequence were formed by ejecta curtain and melt surge deposits, respectively. Our results are also consistent with the interpretation that the middle part of the impactite sequence is fallback ejecta from the impact plume. Additionally, we support the contention that massive seawater resurges into the crater occurred during the deposition of the upper and uppermost part of the impactites.     

Secondary alteration of the impactite and mineralization in the basal Tertiary sequence,Yaxcopoil‐1, Chicxulub impact crater,Mexico

Abstract The 65 Ma Chicxulub impact crater formed in the shallow coastal marine shelf of the Yucatán Platform in Mexico. Impacts into water‐rich environments provide heat and geological structures that generate and focus sub‐seafloor convective hydrothermal systems. Core from the Yaxcopoil‐1 (Yax‐1) hole, drilled by the Chicxulub Scientific Drilling Project (CSDP), allowed testing for the presence of an impact‐induced hydrothermal system by: a) characterizing the secondary alteration of the 100 m‐thick impactite sequence; and b) testing for a chemical input into the lower Tertiary sediments that would reflect aquagene hydrothermal plume deposition. Interaction of the Yax‐1 impactites with seawater is evident through redeposition of the suevites (unit 1), secondary alteration mineral assemblages, and the subaqueous depositional environment for the lower Tertiary carbonates immediately overlying the impactites. The least‐altered silicate melt composition intersected in Yax‐1 is that of a calc‐alkaline basaltic andesite with 53.4–56 wt% SiO_{2(volatile‐free)}. The primary mineralogy consists of fine microlites of diopside, plagioclase (mainly Ab 47), ternary feldspar (Ab 37 to 77), and trace apatite, titanite, and zircon. The overprinting alteration mineral assemblage is characterized by Mg‐saponite, K‐montmorillonite, celadonite, K‐feldspar, albite, Fe‐oxides, and late Ca and Mg carbonates. Mg and K metasomatism resulted from seawater interaction with the suevitic rocks producing smectite‐K‐feldspar assemblages in the absence of any mixed layer clay minerals, illite, or chlorite. Rare pyrite, sphalerite, galena, and chalcopyrite occur near the base of the impactites. These secondary alteration minerals formed by low temperature (0–150°C) oxidation and fixation of alkalis due to the interaction of glass‐rich suevite with down‐welling seawater in the outer annular trough intersected at Yax‐1. The alteration represents a cold, Mg‐K‐rich seawater recharge zone, possibly recharging higher temperature hydrothermal activity proposed in the central impact basin. Hydrothermal metal input into the Tertiary ocean is shown by elevated Ni, Ag, Au, Bi, and Te concentrations in marcasite and Cd and Ga in sphalerite in the basal 25 m of the Tertiary carbonates in Yax‐1. The lower Tertiary trace element signature reflects hydrothermal metal remobilization from a mafic source rock and is indicative of hydrothermal venting of evolved seawater into the Tertiary ocean from an impact‐generated hydrothermal convective system.     

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.     

The melt‐bearing impactites of the Ritland structure,Norway–Implications for melt formation in small impact craters

A melt‐bearing impactite unit is preserved in the 2.7 km diameter shallow marine Ritland impact structure. The main exposure of the melt‐bearing unit is in an approximately 100 m long cliff about 700 m southwest of the center of the structure. The melt and clast content vary through this maximum 2 m thick unit, so that lithology ranges from impact melt rock to suevite. Stratigraphic variations with respect to the melt content, texture, mineralogy, and geochemistry have been studied in the field, and by laboratory analysis, including thin section microscopy. The base of the melt‐bearing unit marks the transition from the underlying lithic basement breccia, and the unit may have been emplaced by an outward flow during the excavation stage. There is an upward development from a melt matrix‐dominated lower part, that commonly shows flow structures, to an upper part characterized by more particulate matrix with patchy melt matrix domains, commonly as deformed melt slivers intermingled with small lithic clasts. Melt and lithic fragments in the upper part display a variety of shapes and compositions, some of which possibly represent fallback material from the ejecta cloud. The upper boundary of the melt‐bearing impactite unit has been placed where the deposits are mainly clastic, probably representing slump and avalanche deposits from the modification stage. These deposits are therefore considered sedimentary and not impactites, despite the component of small melt fragments and shocked minerals within the lowermost part, which was probably incorporated as the debris moved down the steep crater walls.     

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.     

10Be content in clasts from fallout suevitic breccia in drill cores from the Bosumtwi impact crater,Ghana: Clues to preimpact target distribution

Rocks from drill cores LB‐07A (crater fill) and LB‐08A (central uplift) into the Bosumtwi impact crater, Ghana, were analyzed for the presence of the cosmogenic radionuclide ¹⁰Be. The aim of the study was to determine the extent to which target rocks of various depths were mixed during the formation of the crater‐filling breccia, and also to detect meteoric water infiltration within the impactite layer. ¹⁰Be abundances above background were found in two (out of 24) samples from the LB‐07A core, and in none of five samples from the LB‐08A core. After excluding other possible explanations for an elevated ¹⁰Be signal, we conclude that it is most probably due to a preimpact origin of those clasts from target rocks close to the surface. Our results suggest that in‐crater breccias were well mixed during the impact cratering process. In addition, the lack of a ¹⁰Be signal within the rocks located very close to the lake sediment–impactite boundary suggests that infiltration of meteoric water below the postimpact crater floor was limited. This may suggest that the infiltration of the meteoric water within the crater takes place not through the aerial pore‐space, but rather through a localized system of fractures.     

Geochemical identification of projectiles in impact rocks

Abstract— The three major geochemical methods for impactor identification are evaluated with respect to their potential and limitations with regards to the precise detection and identification of meteoritic material in impactites. The identification of a projectile component in impactites can be achieved by determining certain isotopic and elemental ratios in contaminated impactites. The isotopic methods are based on Os and Cr isotopic ratios. Osmium isotopes are highly sensitive for the detection of minute amounts of extraterrestrial components of even <<0.05 wt% in impactites. However, this only holds true for target lithologies with almost no chemical signature of mantle material or young mantle‐derived mafic rocks. Furthermore, this method is not currently suitable for the precise identification of the projectile type. The Cr‐isotopic method requires the relatively highest projectile contamination (several wt%) in order to detect an extraterrestrial component, but may allow the identification of three different groups of extraterrestrial materials, ordinary chondrites, an enstatite chondrites, and differentiated achondrites. A significant advantage of this method is its independence of the target lithology and post‐impact alteration. The use of elemental ratios, including platinum group elements (PGE: Os, Ir, Ru, Pt, Rh, Pd), in combination with Ni and Cr represents a very powerful method for the detection and identification of projectiles in terrestrial and lunar impactites. For most projectile types, this method is almost independent of the target composition, especially if PGE ratios are considered. This holds true even in cases of terrestrial target lithologies with a high component of upper mantle material. The identification of the projectile is achieved by comparison of the “projectile elemental ratio” derived from the slope of the mixing line (target‐projectile) with the elemental ratio in the different types of possible projectiles (e.g., chondrites). However, this requires a set of impactite samples of various degree of projectile contamination.     

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.     

Hydrothermal alteration in the core of the Yaxcopoil‐1 borehole,Chicxulub impact structure,Mexico

Abstract Petrographic, electron microprobe, and Raman spectrometric analyses of Yaxcopoil‐1 core samples from the Chicxulub crater indicate that the impact generated a hydrothermal system. Relative textural and vein crosscutting relations and systematic distribution of alteration products reveal a progression of the hydrothermal event in space and time and provide constraints on the nature of the fluids. The earliest calcite, halite, and gaylussite suggest that the impactite sequence was initially permeated by a low temperature saline brine. Subsequent development of a higher temperature hydrothermal regime is indicated by thermal metamorphic diopside‐hedenbergite (Aeg₃Fs_18‐33En_32‐11Wo_47‐53) after primary augite and widespread Na‐K for Ca metasomatic alkali exchange in plagioclase. Hydrothermal sphene, apatite, magnetite + (bornite), as well as early calcite (combined 3 to 8 vol%) were introduced with metasomatic feldspar. A lower temperature regime characterized by smectite after probable primary glass, secondary chlorite, and other pre‐existing mafic minerals, as well as very abundant calcite veins and open‐space fillings, extensively overprinted the early hydrothermal stage. The composition of early and late hydrothermal minerals show that the solution was chlorine‐rich (Cl/F >10) and that its Fe/Mg ratio and oxidation state increased substantially (4 to 5 logfO₂ units) as temperature decreased through time. The most altered zone in the impactite sequence occurs 30 m above the impact melt. The lack of mineralogical zoning about the impact melt and convective modeling constraints suggest that this unit was too thin at Yaxcopoil‐1 to provide the necessary heat to drive fluids and implies that the hydrothermal system resulted from the combined effects of a pre‐existing saline brine and heat that traveled to the Yaxcopoil‐1 site from adjacent areas where the melt sheet was thicker. Limonite after iron oxides is more common toward the top of the sequence and suggests that the impactite section was subjected to weathering before deposition of the Tertiary marine cover. In addition, scarce latest anatase stringers, chalcopyrite, and barite in vugs, francolite after apatite, and recrystallized halite are the likely products of limited post‐hydrothermal ambient‐temperature diagenesis, or ocean and/or meteoric water circulation.     

Impactites of the Yaxcopoil‐1 drilling site,Chicxulub impact structure: Petrography,geochemistry, and depositional environment

Abstract— The impact breccias encountered in drill hole Yaxcopoil‐1 (Yax‐1) in the Chicxulub impact structure have been subdivided into six units. The two uppermost units are redeposited suevite and suevite, and together are only 28 m thick. The two units below are interpreted as a ground surge deposit similar to a pyroclastic flow in a volcanic regime with a fine‐grained top (unit 3; 23 m thick; nuée ardente) and a coarse breccia (unit 4; ～15 m thick) below. As such, they consist of a mélange of clastic matrix breccia and melt breccia. The pyroclastic ground surge deposit and the two units 5 and 6 below are related to the ejecta curtain. Unit 5 (～24 m thick) is a silicate impact melt breccia, whereas unit 6 (10 m thick) is largely a carbonate melt breccia with some clastic‐matrix components. Unit 5 and 6 reflect an overturning of the target stratigraphy. The suevites of units 1 and 2 were deposited after emplacement of the ejecta curtain debris. Reaction of the super‐heated breccias with seawater led to explosive activity similar to phreomagmatic steam explosion in volcanic regimes. This activity caused further brecciation of melt and melt fragments. The fallback suevite deposit of units 1 and 2 is much thinner than suevite deposits at larger distances from the center of the impact structure than the 60 km of the Yax‐1 drill site. This is evidence that the fallback suevite deposit (units 1 and 2) originally was much thicker. Unit 1 exhibits sedimentological features suggestive of suevite redeposition. Erosion possibly has occurred right after the K/T impact due to seawater backsurge, but erosion processes spanning thousands of years may also have been active. Therefore, the top of the 100 m thick impactite sequence at Yaxcopoil, in our opinion, is not the K/T boundary.     

Lithium isotopes and light lithophile element abundances in shergottites: Evidence for both magmatic degassing and subsolidus diffusion

Degassed magmatic water was potentially the major source of surficial water on Mars. We measured Li, B, and Be abundances and Li isotope profiles in pyroxenes, olivines, and maskelynite from four compositionally different shergottites—Shergotty, QUE 94201, LAR 06319, and Tissint—using secondary ion mass spectrometry (SIMS). All three light lithophile elements (LLE) are incompatible: Li and B are soluble in H₂O‐rich fluids, whereas Be is insoluble. In the analyzed shergottites, Li concentration decreases and Be concentration increases from cores to rims in pyroxenes. However, B concentrations do not vary consistently with Li and Be abundances, except in QUE 94201 pyroxenes. Additionally, abundances of these three elements in olivines show a normal igneous‐fractionation trend consistent with the crystallization of olivine before magma ascent and degassing. We expect that kinetic effects would lead to fractionation of ⁶Li in the vapor phase compared to ⁷Li during degassing. The Li isotope profiles, with increasing δ⁷Li from cores to rims, as well as Li and B profiles indicate possible degassing of hydrous fluids only for the depleted shergottite QUE 94201, as also supported by degassing models. Conversely, Shergotty, LAR 06319, and Tissint appear to have been affected by postcrystallization diffusion, based on their LLE and Li isotope profiles, accompanied by diffusion models. This process may represent an overlay on a degassing pattern. The LLE profiles and isotope profiles in QUE 94201 support the hypothesis that degassing of some basaltic shergottite magmas provided water to the Martian surface, although evidence may be obscured by subsolidus diffusion processes.     

Evidence for ocean water invasion into the Chicxulub crater at the Cretaceous/Tertiary boundary

Abstract— The possibility of ocean water invasion into the Chicxulub crater following the impact at the Cretaceous/Tertiary boundary was investigated based on examination of an impactite between approximately 794.63 and 894.94 m in the Yaxcopoil‐1 (Yax‐1) core. The presence of cross lamination in the uppermost part of the impactite suggests the influence of an ocean current at least during the sedimentation of this interval. Abundant occurrence of nannofossils of late Campanian to early Maastrichtian age in the matrices of samples from the upper part of the impactite suggests that the carbonate sediments deposited on the inner rim margin and outside the crater were eroded and transported into the crater most likely by ocean water that invaded the crater after its formation. The maximum grain size of limestone lithics and vesicular melt fragments, and grain and bulk chemical compositions show a cyclic variation in the upper part of the impactite. The upward fining grain size and the absence of erosional contact at the base of each cycle suggest that the sediments were derived from resuspension of units elsewhere in the crater, most likely by high energy currents association with ocean water invasion.     

Evidence for ocean water invasion into the Chicxulub crater at the Cretaceous/Tertiary boundary

Abstract The possibility of ocean water invasion into the Chicxulub crater following the impact at the Cretaceous/Tertiary boundary was investigated based on examination of an impactite between approximately 794.63 and 894.94 m in the Yaxcopoil‐1 (Yax‐1) core. The presence of cross lamination in the uppermost part of the impactite suggests the influence of an ocean current at least during the sedimentation of this interval. Abundant occurrence of nannofossils of late Campanian to early Maastrichtian age in the matrices of samples from the upper part of the impactite suggests that the carbonate sediments deposited on the inner rim margin and outside the crater were eroded and transported into the crater most likely by ocean water that invaded the crater after its formation. The maximum grain size of limestone lithics and vesicular melt fragments, and grain and bulk chemical compositions show a cyclic variation in the upper part of the impactite. The upward fining grain size and the absence of erosional contact at the base of each cycle suggest that the sediments were derived from resuspension of units elsewhere in the crater, most likely by high energy currents association with ocean water invasion.     

Physical properties of the Yaxcopoil‐1 deep drill core,Chicxulub impact structure,Mexico

Abstract– The Chicxulub structure in Mexico, one of the largest impact structures on Earth, was formed 65 Ma by a hypervelocity impact that led to the large mass extinction at the K‐Pg boundary. The Chicxulub impact structure is well preserved, but is buried beneath a sequence of carbonate sediments and, thus, requires drilling to obtain subsurface information. The Chicxulub Scientific Drilling Program was carried out at Hacienda Yaxcopoil in the framework of the International Continental Scientific Drilling Program in 2001–2002. The structure was cored from 404 m down to 1511 m, through three intervals: 794 m of postimpact Tertiary sediments, a 100 m thick impactite sequence, and 616 m of preimpact Cretaceous rocks thought to represent a suite of megablocks. Physical property investigations show that the various lithologies, including the impactite units and the K‐Pg boundary layer, can be characterized by their physical properties, which depend on either changes in fabric or on mineralogical variations. The magnetic properties show mostly dia‐ or paramagnetic behavior, with the exception of the impactite units that indicate the presence of ferromagnetic, probably hydrothermally deposited magnetite and pyrrhotite. The magnetic fraction contributes mainly to enhanced magnetization in the impactite lithologies and, in this way, to the observed magnetic anomalies. The shape and orientation of the magnetic grains are varied and reflect inhomogeneous fabric development and the influence of impact‐related redeposition and hydrothermal activity. The Chicxulub impact occurred at the time of the reverse polarity geomagnetic chron 29R, and this finding is consistent with the age of the K‐Pg boundary.     

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.     

Shock‐metamorphosed zircon in terrestrial impact craters

Abstract— To ascertain the progressive stages of shock metamorphism of zircon, samples from three well‐studied impact craters were analyzed by optical microscopy, scanning electron microscopy (SEM), and Raman spectroscopy in thin section and grain separates. These samples are comprised of well‐preserved, rapidly quenched impactites from the Ries crater, Germany, strongly annealed impactites from the Popigai crater, Siberia, and altered, variably quenched impactites from the Chicxulub crater, Mexico. The natural samples were compared with samples of experimentally shock‐metamorphosed zircon. Below 20 GPa, zircon exhibits no distinct shock features. Above 20 GPa, optically resolvable planar microstructures occur together with the high‐pressure polymorph reidite, which was only retained in the Ries samples. Decomposition of zircon to ZrO₂ only occurs in shock stage IV melt fragments that were rapidly quenched. This is not only a result of post‐shock temperatures in excess of ?1700 °C but could also be shock pressure‐induced, which is indicated by possible relics of a high‐pressure polymorph of ZrO₂. However, ZrO₂ was found to revert to zircon with a granular texture during devitrification of impact melts. Other granular textures represent recrystallized amorphous ZrSiO₄ and reidite that reverted to zircon. This requires annealing temperatures >1100 °C. A systematic study of zircons from a continuous impactite sequence of the Chicxulub impact structure yields implications for the post‐shock temperature history of suevite‐like rocks until cooling below ?600 °C.