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1.
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.  相似文献   

2.
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.  相似文献   

3.
Hyperspectral imaging can be used to rapidly identify and map the spatial distributions of many minerals. Here, hyperspectral mapping in three wavelength regions (visible and near‐infrared, shortwave infrared, and thermal infrared) was applied to drill cores (ST001, ST002, and ST003) penetrating a continuous sequence of crater‐fill breccias from the Steen River impact structure in Alberta, Canada. The combined data sets reveal distinct mineralogical layering, with breccias derived predominantly from sedimentary rocks overlying those derived from granitic basement. This stratigraphy demonstrates that the breccias were not appreciably disturbed following deposition, which is inconsistent with formation models of similar breccias (suevites) by explosive impact melt–fluid interaction. At Steen River, volatiles from sedimentary target rocks were an inherent part of forming these enigmatic breccias. Approximately three quarters of terrestrial impact structures contain sedimentary target rocks; therefore, the role of volatiles in producing so‐called suevitic breccias may be more widespread than previously realized. The hyperspectral maps, specifically within the SWIR wavelength region, also delineate minerals associated with postimpact hydrothermal activity, including ammoniated clay and feldspar minerals not detectable using traditional techniques. These nitrogen‐bearing minerals may have originated from microbial processes, associated with oil‐ and gas‐producing units in the crater vicinity. Such minerals may have important implications for the production of habitable environments by impact‐induced hydrothermal activity on Earth and Mars.  相似文献   

4.
Abstract— Clasts of deep-seated crystalline basement rocks in suevites of the Ries crater, Germany, were catalogued lithologically and classified with regard to their degree of shock metamorphism. The sample suite consisted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior. These clasts can be grouped into seven types of metamorphic and nine types of igneous rocks. One hundred forty-three clasts, representing these lithologies, were analyzed for major element bulk composition. The fallout suevite contains on average 4 vol% of crystalline basement clasts, 0.4 vol% of sedimentary rocks, 16 vol% of glass bodies (some of them aerodynamically shaped), and 79 vol% of groundmass. On average, 52% of all crystalline clasts are from metamorphic sources and 42% are of igneous origin. Using the shock classification of Stöffler (1974), 8% of all crystalline clasts appear unshocked (<10 Gpa), and 34, 30 and 27% of clasts are shocked to stages I (10–35 Gpa), II (35–45 GPa) and III (45–60 GPa), respectively. The bulk composition of suevite glasses is consistent with the modal proportions of crystalline rock types observed in the clast populations. This indicates that the glasses originate by shock-fusion of a similarly composed basement. The crater suevite contains the same crystalline rock types that occur in the fallout suevites. The bore hole “Nördlingen 1973” yields an average of 62 vol% metamorphic and 38 vol% igneous rocks. The crater suevite differs from fallout suevites by a higher clast/glass ratio, by preponderance (65–95%) of clasts shocked to stage I only, and by the absence of aerodynamically shaped glass bodies. The source of crystalline clasts and melt particles of suevites is a volume of rocks, located deep in the crystalline basement, to which the projectile transmittted most of its energy so that only rocks of the basement were shocked by pressures exceeding 10 GPa (deep-burst impact model). Fallout suevites were ejected, propelled by an expanding plume of vaporized rock, and withdrew preferentially from this volume melt and highly shocked clasts, leaving in the transient cavity the crater suevite with more clasts of modest shock levels and less melt.  相似文献   

5.
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.  相似文献   

6.
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.  相似文献   

7.
Abstract— The Kärdla crater is a 4 km‐wide impact structure of Late Ordovician age located on Hiiumaa Island, Estonia. The 455 Ma‐old buried crater was formed in shallow seawater in Precambrian crystalline target rocks that were covered with sedimentary rocks. Basement and breccia samples from 13 drill cores were studied mineralogically, petrographically, and geochemically. Geochemical analyses of major and trace elements were performed on 90 samples from allochthonous breccias, sub‐crater and surrounding basement rocks. The breccia units do not include any melt rocks or suevites. The remarkably poorly mixed sedimentary and crystalline rocks were deposited separately within the allochthonous breccia suites of the crater. The most intensely shockmetamorphosed allochthonous granitoid crystalline‐derived breccia layers contain planar deformation features (PDFs) in quartz, indicating shock pressures of 20–35 GPa. An apparent K‐enrichment and Ca‐Na‐depletion of feldspar‐ and hornblende‐bearing rocks in the allochthonous breccia units and sub‐crater basement is interpreted to be the result of early stage alteration in an impact‐induced hydrothermal system. The chemical composition of the breccias shows no definite sign of an extraterrestrial contamination. By modeling of the different breccia units with HMX‐mixing, the indigenous component was determined. From the abundances of the siderophile elements (Cr, Co, Ni, Ir, and Au) in the breccia samples, no unambiguous evidence for the incorporation of a meteoritic component above about 0.1 wt% chondrite‐equivalent was found.  相似文献   

8.
The Terny impact structure, located in central Ukraine, displays a variety of diagnostic indicators of shock metamorphism, including shatter cones, planar deformation features in quartz, diaplectic glass, selective melting of minerals, and whole rock melting. The structure has been modified by erosion and subsequently buried by recent sediments. Although there are no natural outcrops of the deformed basement rocks within the area, mining exploration has provided surface and subsurface access to the structure, exposing impact melt rocks, shocked parautochthonous target rocks, and allochthonous impact breccias, including impact melt‐bearing breccias similar to suevites observed at the Ries structure. We have collected and studied samples from surface and subsurface exposures to a depth of approximately 750 m below the surface. This analysis indicates the Terny crater is centered on geographic coordinates 48.13° N, 33.52° E. The center location and the distribution of shock pressures constrain the transient crater diameter to be no less than approximately 8.4 km. Using widely accepted morphometric scaling relations, we estimate the pre‐erosional rim diameter of Terny crater to be approximately 16–19 km, making it close in original size to the well‐preserved El'gygytgyn crater in Siberia. Comparison with El'gygytgyn yields useful insights into the original morphology of the Terny crater and indicates that the amount of erosion Terny experienced prior to burial probably does not exceed 320 m.  相似文献   

9.
Abstract— The 3.4 km wide, so‐called Kgagodi Basin structure, which is centered at longitude 27°34.4′ E and latitude 22°28.6′ S in eastern Botswana, has been confirmed as a meteorite impact structure. This crater structure was first recognized through geophysical analysis; now, we confirm its impact origin by the recognition of shock metamorphosed material in samples from a drill core obtained close to the crater rim. The structure formed in Archean granitoid basement overlain and intruded by Karoo dolerite. The crater yielded a gravity model consistent with a simple bowl‐shape crater form. The drill core extends to a depth of 274 m and comprises crater fill sediments to a depth of 158 m. Impact breccia was recovered only between 158 and 165 m depth, below which locally brecciated basement granitoids grade into fractured and eventually undeformed crystalline basement, from ～250 m depth. Shock metamorphic effects were only found in granitoid clasts in the narrow breccia zone. This breccia is classified as suevitic impact breccia due to the presence of melt and glass fragments, at a very small abundance. The shocked grains are exclusively derived from granitoid target material. Shock effects include multiple sets of planar deformation features in quartz and feldspar; diaplectic quartz, and partially and completely isotropized felsic minerals, and rare melt fragments were encountered. Abundances of some siderophile elements and especially, Ir, in suevitic breccia samples are significantly elevated compared to the contents in the target rocks, which provides evidence for the presence of a small meteoritic component. Kgagodi is the first impact structure recognized in the region of the Kalahari Desert in southern Africa. Based on lithological and first palynological evidence, the age of the Kgagodi structure is tentatively assigned to the upper Cretaceous to early Tertiary interval. Thus, the crater fill has the potential to provide a long record of paleoclimatic conditions.  相似文献   

10.
Abstract— Due to the effects of erosion, tectonism and burial, impact structures are often obscured or destroyed. Geophysical methods are increasingly being used in detecting the signatures of impact structures. While gravity lows associated with impact structures are well understood, associated magnetic anomaly lows are not. In this study, drill cores from three Canadian impact structures were analyzed for rock magnetic properties and mineralogy, in order to explain the magnetic anomaly lows associated with these structures. Samples from the drill cores were cut and measured for anisotropy of magnetic susceptibility (AMS) and natural remanent magnetization (NRM) parameters. Drill cores from the twin impact craters of the Clearwater structure exhibited different NRM characteristics, and samples from their respective drill cores were subject to demagnetization by alternating field and thermal techniques. The difference noted in their NRM characteristics was attributed to the acquisition of a viscous remanent magnetization (VRM) at depth in Clearwater East. At all three structures, both magnetic susceptibilities and remanent magnetizations are well below regional values in impact generated breccias, melt rocks, shocked crystalline rocks, and in postimpact sedimentary infill. The processes of brecciation, alteration, shock, and infill by nonmagnetic sediments contribute to the development of the magnetic lows. However, a significant contribution to the observed magnetic anomalies was found, by first-order forward modelling, to arise from basement rocks beneath the impact structures. This zone of reduced magnetization may be caused by the partial demagnetization of magnetite by the impact-induced transient stress wave traveling away from the point of impact.  相似文献   

11.
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12.
Abstract— The impact melt breccias from the Tenoumer crater (consisting of a fine‐grained intergrowth of plagioclase laths, pyroxene crystals, oxides, and glass) display a wide range of porosity and contain a large amount of target rock clasts. Analyses of major elements in impact melt rocks show lower contents of SiO2, Al2O3, and Na2O, and higher contents of MgO, Fe2O3, and CaO, than the felsic rocks (i.e., granites and gneisses) of the basement. In comparison with the bulk analyses of the impact melt, the glass is strongly enriched in Si‐Al, whereas it is depleted both in Mg and Fe; moreover, the impact melt rocks are variably enriched or depleted in some REE with respect to the felsic and mafic bedrock types. Gold is slightly enriched in the impact melt, and Co, Cr, and Ni abundances are possibly due to a contribution from mafic bedrock. Evidences of silicate‐carbonate liquid immiscibility, mainly as spherules and globules of calcite within the silicate glass, have been highlighted. HMX mixing calculation confirm that the impact melt rocks are derived from a mixing of at least six different target lithologies outcropping in the area of the crater. A large contribution is derived from granitoids (50%) and mica schist (17–19%), although amphibolites (?15%), cherty limestones (?10%), and ultrabasites (?6%) components are also present. The very low abundances of PGE in the melt rock seem to come mainly from some ultrabasic target rocks; therefore, the contamination from the meteoritic projectile appears to have been negligible.  相似文献   

13.
Abstract— The central uplift of the 40-km wide Araguainha impact structure, Brazil, consists of a ring, about 8 km in diameter, of up to 150-m high blocks of Devonian Furnas sandstone, which surround a central depression of elliptical shape (4.5 × 3.0 km). The depression is occupied by a pre-Devonian alkali-feldspar granite, shocked by pressures of 20–25 GPa and permeated by cataclastic shear zones and dikes of shocked granitic material. The granite is flanked and partly covered by several impact breccias: (1) Impact breccia with melt matrix overlies the granite in places and forms hills, bordering the granitic center in the S and SW. It is chemically identical with the granite and consists of thermally altered granitic clasts in a matrix of sanidine, quartz, biotite, muscovite, chlorite and riebeckite. (2) Polymict breccias form hills which border the central depression in the N and NW. Components are unshocked and shocked sediments, shock-melted sandstone, shocked granite and shock melt rocks in irregular masses and individual bodies, embedded in a fine-grained matrix. 40Ar/39Ar analyses show that the melt rocks solidified 246 Ma ago, indicating that the impact occurred at near the Permian-Triassic boundary, possibly when the area was covered by a shallow sea. The present chemistry and petrography of the melt rocks suggest that by reacting with seawater granitic impact melt was depleted of K and Rb and enriched in Na, and that later diagenetic processes produced replacement of feldspar by quartz and deposition of hematite. (3) Monomict breccias, consisting of unshocked, shocked and shock-fused quartz sandstones, form hills which surround the central depression in the SE and S. The Araguainha structure is an eroded complex crater, produced by an impact, 246 Ma ago. The depth of excavation was about 2.4 km, comprising Permian, Permo-Carboniferous and Devonian sediments and the granitic basement. The diameter of the transient crater was about 24 km. Erosion and weathering have removed most of the original crater fill and ejecta deposits, with the exception of remnants, preserved in the central uplift.  相似文献   

14.
Abstract— The suevite breccia of the Chicxulub impact crater, Yucatàn, Mexico, is more variable and complex in terms of composition and stratigraphy than suevites observed at other craters. Detailed studies (microscope, electron microprobe, SEM, XRF) have been carried out on a noncontinuous set of samples from the drill hole Yucatàn 6 (Y6) located 50 km SW from the center of the impact structure. Three subunits can be distinguished in the suevite: the upper unit is a fine‐grained carbonate‐rich suevite breccia with few shocked basement clasts, mostly altered melt fragments, and formerly melted carbonate material; the middle suevite is a coarse‐grained suevite with shocked basement clasts and altered silicate melt fragments; the lower suevite unit is composed of shocked basement and melt fragments and large evaporite clasts. The matrix of the suevite is not clastic but recrystallized and composed mainly of feldspar and pyroxene. The composition of the upper members of the suevite is dominated by the sedimentary cover of the Yucatàn target rock. With depth in well Y6, the amount of carbonate decreases and the proportion of evaporite and silicate basement rocks increases significantly. Even at the thin section scale, melt phases of different chemistry can be identified, showing that no widespread homogenization of the melt took place. The melt compositions also reflect the heterogeneity of the deep Yucatàn basement. Calcite with characteristic feathery texture indicates the existence of formerly pure carbonate melt. The proportion of carbonate to evaporite clasts is less than 5:1, except in the lower suevite where large evaporite clasts are present. This proportion constrains the amount of CO2 and SOX released by the impact event.  相似文献   

15.
Suevite and melt breccia compositions in the boreholes Enkingen and Polsingen are compared with compositions of suevites from other Ries boreholes and surface locations and discussed in terms of implications for impact breccia genesis. No significant differences in average chemical compositions for the various drill cores or surface samples are noted. Compositions of suevite and melt breccia from southern and northeastern sectors of the Ries crater do not significantly differ. This is in stark contrast to the published variations between within‐crater and out‐of‐crater suevites from northern and southern sectors of the Bosumtwi impact structure, Ghana. Locally occurring alteration overprint on drill cores—especially strong on the carbonate‐impregnated suevite specimens of the Enkingen borehole—does affect the average compositions. Overall, the composition of the analyzed impact breccias from Ries are characterized by very little macroscopically or microscopically recognized sediment‐clast component; the clast populations of suevite and impact melt breccia are dominated consistently by granitic and intermediate granitoid components. The Polsingen breccia is significantly enriched in a dioritic clast component. Overall, chemical compositions are of intermediate composition as well, with dioritic‐granodioritic silica contents, and relatively small contributions from mafic target components. Selected suevite samples from the Enkingen core have elevated Ni, Co, Cr, and Ir contents compared with previously analyzed suevites from the Ries crater, which suggest a small meteoritic component. Platinum‐group element (PGE) concentrations for some of the enriched samples indicate somewhat elevated concentrations and near‐chondritic ratios of the most immobile PGE, consistent with an extraterrestrial contribution of 0.1–0.2% chondrite‐equivalent.  相似文献   

16.
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.  相似文献   

17.
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.  相似文献   

18.
Abstract— Mafic, Th-rich impact-melt breccias, most of which are identified with the composition known as low-K Fra Mauro (LKFM), are the most common rock type in the nonmare regoliths of the Apollo lunar landing sites. The origin of mafic impact-melt breccias bears on many lunar problems: the nature of the late meteoroid bombardment (cataclysm); the spatial distribution of KREEP, both near the surface and at depth; the ages of the major basins; and the composition of the early crust of the nearside lunar highlands. Thus, it is crucial that the origin of mafic impact-melt breccias be accurately understood. Because of both intra- and intersite differences in compositions of mafic impact-melt breccia samples, apparent differences in crystallization age, and differences in siderophile-element ratios, previous studies have argued that either (1) most mafic impact-melt breccias are the products of several large craters local to the site at which they were found but that some are of basin origin or that (2) they are all from the Imbrium (Apollos 14 and 15), Nectaris (Apollo 16), and Serenitatis (Apollo 17) basins. Here, we reconsider the hypothesis that virtually all of the Th-rich, mafic impact-melt breccias from the Apollo missions are products of the Imbrium impact. Ejecta deposit modeling based on modern crater scaling indicates that the Imbrium event produced ejecta deposits that average hundreds of meters thick or more at all Apollo highland sites, which is thicker than some previous estimates. Substantial amounts of Imbrium ejecta should have been sampled at every Apollo highland site. We suggest that the mafic impact-melt breccias may be the principal form of those ejecta. The Imbrium projectile impacted into Th-rich material that we regard as part of a unique, mafic, lunar geochemical province we call the High-Th Oval Region. Based on the surface distribution of Th, only basins within the High-Th Oval Region excavated Th-rich material; the Th concentrations of the highlands as observed by the Apollo orbiting γ-ray experiments are consistent with the estimates from ejecta modeling. Of the younger basin-forming impacts, only Imbrium was large enough to produce the copious amount of melt required by the ubiquitous presence of mafic impact-melt breccias in the Apollo-sampled regolith. The High-Th Oval Region still may have been molten or hot at shallow depths ～4 Ga ago when the Imbrium projectile struck. We reason that compositional heterogeneity of ejected melt breccia is to be expected under these circumstances. We argue that siderophile-element “fingerprints” of mafic impact-melt breccias are not inconsistent with production of all common types by a single projectile. We suggest that the narrow range of ages of 3.7–4.0 Ga for all successfully dated mafic impact-melt breccias may reflect a single event whose age is difficult to measure precisely, rather than a number of discrete impact events closely spaced in time, such that reported age variations among mafic impact-melt breccias reflect the ability to measure 40Ar/39Ar ages with greater precision than the accuracy with which measured portions of mafic impact-melt breccias have recorded the time of their formation.  相似文献   

19.
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.  相似文献   

20.
The Målingen structure is an approximately 700 m wide, rimmed, sediment‐filled, circular depression in Precambrian crystalline basement approximately 16.2 km from the concentric, marine‐target Lockne crater (inner, basement crater diameter approximately 7.5 km, total diameter in sedimentary strata approximately 13.5 km). We present here results from geologic mapping, a 148.8 m deep core drilling from the center of the structure, detailed biostratigraphic dating of the structure's formation and its age correlation with Lockne, chemostratigraphy of the sedimentary infill, and indication for shock metamorphism in quartz from breccias below the crater infill. The drill core reveals, from bottom to the top, approximately 33 m of basement rocks with increased fracturing upward, approximately 10 m of polymict crystalline breccia with shock features, approximately 97 m of slumped Cambrian mudstone, approximately 4.7 m of a normally graded, polymict sedimentary breccia that in its uppermost part grades into sandstone and siltstone (cf. resurge deposits), and approximately 1.6 m of secular sediments. The combined data set shows that the Målingen structure formed in conjunction with the Lockne crater in the same marine setting. The shape and depth of the basement crater and the cored sequence of crystalline breccias with shocked quartz, slumped sediments, and resurge deposits support an impact origin. The stratigraphic and geographic relationship with Lockne suggests the Lockne and Målingen craters to be the first described doublet impact structure by a binary asteroid into a marine‐target setting.  相似文献   

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