核爆炸玻璃、撞击玻璃和玻璃陨石源岩

本文根据核爆炸岩石熔融玻璃的地球化学研究结果,与超速陨石撞击坑的熔岩进行比较,获得在远离热力学平衡条件下,各种玻璃和熔岩在化学成分上分布十分均匀的重要结论。岩石玻璃和熔岩是由基岩各组成岩石按一定比例混合熔融形成的。它们的主量元素和痕量元素丰度受基岩元素背景值制约。文中根据熔体和靶岩的化学成分,计算了熔岩各组成岩石的百分比。玻璃陨石是地壳岩石受撞击熔融形成的。同一撒布区的玻璃陨石化学成分相近,说明起源于同一源坑;而玻璃陨石化学成分的不同,则说明母岩组成分量的差异。因此,文中通过模拟计算,得出各玻璃陨石的组成源岩。澳大利亚撒布区的玻璃陨石,Al₂O₃,K₂O 和Na₂O 与 SiO₂及 K₂O/Na₂O 比值不完全相同,说明澳大利亚撒布区存在着几个不同的源岩和源坑,至少有印支、爪哇、菲律宾和澳大利亚四个相应撞击坑。     

Beryllium-10 in Australasian tektites: Constraints on the location of the source crater

By using accelerator mass spectrometry we have measured the ¹⁰Be concentrations of 86 Australasian tektites. Corrected to the time of tektite production ∼0.8 My ago, the ¹⁰Be concentrations (10⁶ atom/g) range from 59 for a layered tektite from Huai Sai, Thailand, to 280 for an australite from New South Wales, Australia. The average value is 143 ± 50. When tektites are sorted by country, their average measured ¹⁰Be concentrations increase slowly with increasing distance from Southeast Asia, the probable location of the tektite producing event, from 59 ± 9 for 6 layered tektites from Laos to 136 ± 20 for 20 splash-form tektites from Australia. The lowest ¹⁰Be concentrations for tektites fall on or within a contour centered off the shore of Vietnam, south of the Gulf of Tonkin (107°E; 17°N), but also encompassing two other locations in the area of northeastern Thailand previously proposed for the site of a single tektite-producing impact. The ¹⁰Be concentrations of layered tektites show only a weak anticorrelation (R ∼ −0.3) with the numbers of relict crystalline inclusions.Loosely consolidated, fine-grained terrestrial sediments or recently consolidated sedimentary rocks are the most likely precursor materials. Dilution of sediments with other kinds of rock raises problems in mixing and is not supported by petrographic data. Sedimentary columns that have the right range of ¹⁰Be concentrations occur off the coasts of places where sedimentation rates are high relative to those in the deep sea. A single impact into such a region, 15 to 300 m thick, could have propelled near-surface, high-¹⁰Be material farthest—to Australia—while keeping the deeper-lying, low-¹⁰Be layers closer to home. We do not rule out, however, other proposed mechanisms for tektite formation.     

Chemical analyses of Bosumtwi crater target rocks compared with the Ivory Coast tektites

The Bosumtwi crater, Ghana, was excavated in phyllites and greywackes with subordinate microgranite dykes and quartz veins of the 2000 Ma old Lower Birimian System with a granodiorite intrusion at Pepiakese on its northeastern side. New major and trace element analyses are presented for 7 phyllites, 5 greywackes, 2 microgranites, 3 Pepiakese intrusion rocks and 1 suevite using XRF and INNA.Means and standard deviations were calculated using all available modern analyses for each element in the Bosumtwi target rocks, Bosumtwi suevite glasses and Ivory Coast tektites. Good agreements between the means were found for the three groups with the suevite glasses and tektites having more limited compositional ranges than the target rocks. Least squares mixing between target rock types shows that the best fits to the tektite and suevite glass compositions require components of about a third or a quarter from the Pepiakese intrusion and some extra silica, derived from quartz veins, as well as the metasediments.The new data provide evidence for vapour phase fractionation of P₂O₅ and Na₂O in the tektites in addition to the previously reported Pb and Rb. Evidence for a meteoritic component in the tektites was found to be equivocal since the target rocks are probably a sufficient source of the meteorite indicator elements Ni and Ir.     

Tooting crater: Geology and geomorphology of the archetype large,fresh, impact crater on Mars

The 27.2 km diameter Tooting crater is the best preserved young impact crater of its size on Mars. It offers an unprecedented opportunity to study impact-related phenomena as well the geology of the crust in the Amazonis Planitia region of Mars. For example, the nearly pristine condition enables the partial reconstruction of the sequence of events for crater formation, as well as facilitates a comparison to deposits seen at the Ries crater in Germany. High-resolution images taken by the High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) on the Mars Reconnaissance Orbiter spacecraft have revealed a wealth of information on the distribution of features within the crater and beyond the rim: a large central peak, pitted material on the floor and terrace blocks, lobate flows interpreted to be sediment flows, impact melt sheets, four discrete layers of ejecta, and an asymmetric secondary crater field. Topographic data derived from the Mars Orbiter Laser Altimeter (MOLA) and stereo HiRISE and CTX images show that the central peak is ～1100 m high, the lowest point of the crater floor is 1274 m below the highest part of the rim, and the crater rim has ～600 m of variability around its perimeter. Layering within the cavity walls indicates ～260 m of structural uplift of the target material, which constitutes ～35% of the total relief of the rim. Abundant evidence is found for water flowing down the cavity walls, and on the surface of the ejecta layers, both of which took place sometime after the impact event. Thickness measurements of the ejecta layers reveal that the continuous blanket is remarkably thin (～3–5 m) in some places, and that the distal ramparts may be ～60 m high. Crater counts made on the ejecta layers indicate a model age of <3 Ma for the formation of Tooting crater, and that the target rocks have a model age of ～240–375 Ma. It is therefore possible that this may be the source of certain basaltic shergottite meteorites ejected at ～2.8 Ma that have crystallization ages which are comparable to those of the basaltic lava flows that formed the target materials for this impact event. The geology and geomorphology of Tooting crater may help in the interpretation of older large impact craters on Mars, as well as the potential role of target volatiles in the impact cratering process.     

Hypervelocity collisions into continental crust composed of sediments and an underlying crystalline basement: comparing the Ries (∼24 km) and Chicxulub (∼180 km) impact craters

The Chicxulub and Ries impact craters were excavated from layered continental terrains that were composed of carbonate-bearing sedimentary sequences and underlying crystalline silicate basement materials. The Chicxulub and Ries impact events were sufficiently large to produce complex peak-ring impact craters. The walls of transient craters and excavation cavities, with diameters of 12-16 km for the Ries and 90-100 km for Chicxulub, collapsed to form final crater diameters of ∼24 and ∼180 km, respectively. Debris from both the sedimentary and crystalline layers was ejected during crater formation, but the bulk of the melting occurred at depth, in the silicate basement. The volume of melt and proportion of melt among shock-metamorphosed debris was far larger at Chicxulub, producing a central melt sheet ∼3 km in depth. The central melt sheet was covered with melt-bearing polymict breccias and, at the Ries, similar breccias (crater suevites) filled the central cavity. Also at the Ries (and presumably at Chicxulub), large hill-size megablocks of crystalline basement material were deposited near the transient crater rim. Blocks and megablocks of sedimentary lithologies were ejected into the modification zone between the peak ring and final crater rim, while additional material was slumping inward during crater growth, and buried beneath a fallout deposit of melt-bearing polymict breccias. The melt and surviving clasts in the breccias are dominantly derived from the deeper, basement lithologies. At greater distances, however, the ejecta is dominated by near-surface sedimentary lithologies, large blocks of which landed with such high energy that they scoured and eroded the pre-existing surface. The excavation and ejecta pattern produced lithological and chemical variations with radial distance from the crater centers that evolve from basement components near the crater centers to sedimentary components far from the crater centers. In addition, carbonate (and anhydrite in the case of Chicxulub) was vaporized, producing environmentally active gases. The vaporized volume produced by the Ries impact event was too small to dramatically alter the evolution of life, but the vaporized volume produced by the Chicxulub impact event is probably a key factor in the Cretaceous-Tertiary boundary mass extinction event.     

Identification of the projectile at the Brent crater,and further considerations of projectile types at terrestrial craters

Impact melt samples from drill hole B1-59 at the 3.8 km diameter Brent crater (Ontario) have been analysed for siderophile trace elements indicative of meteoritic contamination. Samples from the basal melt zone at 823–857 m depth are enriched in Ir, Os, Pd, Ni, Co, Cr and Se over basement, with the abundance pattern suggesting a chondritic projectile for Brent. From a Ni-Cr correlation of 10 melt samples an L or LL chondrite is inferred. The contribution of an ultramafic country rock (alnoite) in the melt is too small to significantly influence its $\text{Ni}\text{Cr}$ ratio. Glass-rich breccias from the allochthonous breccias filling the crater also contain a meteoritic component. Interelement ratios (e.g. $\text{Ni}\text{Cr}$) are, however, fractionated relative to the melt zone samples. This, as well as the low Au content of all Brent samples, is probably a product of alteration.Additional data on impact melts from the 65 km diameter crater Manicouagan still did not reveal a meteoritic component, as also for the Mistastin crater (28 km diameter) where Cr analyses set an upper limit of 1% of an achondritic projectile component in the melt. Irghizites (tektite like glasses) from the Zhamanshin impact structure have been found to contain high Ni and Co concentrations, and our data show that Ir is also enriched. It is however not possible to define the projectile-type. Enrichment of an Ivory Coast tektite in Ir is confirmed. There are large differences in siderophile element concentrations among tektites, with otherwise similar chemical composition.There are now four known craters formed by chondrites (Clearwater East, Lapparjärvi, Wanapitei, and Brent), with Brent being the smallest of these. For smaller craters the projectiles appear to be limited to iron or stony-iron meteorites, because of atmospheric destruction of relatively small stony meteorites. It appears, however, that all major classes of meteorites are represented among the projectiles at terrestrial impact craters.     

Tektites: A post-Apollo view

Prior to the receipt of the lunar samples, it was the scientific consensus that tektites were melted and splashed material formed during large cometary or meteorite impact events. Whether the impact took place on the Earth or the Moon was the topic of a long-standing scientific debate, which raged with particular intensity during the decade previous to the lunar landings.Four definite and separate tektite-strewn fields are known: bediasites (North America, 34 m.y.); moldavites (Czechoslovakia, 14 m.y.); Ivory Coast (1.3 m.y.); and Southeast Asian and Australian fields (0.7 m.y.). A fifth possible occurrence, of high-Na australites, possibly 3–4 m.y. old, remains to be substantiated. The age of infall of the australites is not agreed upon. Radiometric and fission track dates agree with the magnetic stratigraphy for deep-sea core microtektite occurrences at about 0.7 m.y. Terrestrial stratigraphic evidence favours a recent (30,000 years) date.The chemistry of tektites appears to reflect that of the parent material, and losses during fusion appear to be restricted to elements and compounds more volatile than cesium. Terrestrial impact glasses provide small-scale analogues of tektite-forming events, and indicate that only the most volatile components are lost during fusion.The Apollo lunar missions provide critical evidence which refutes the hypothesis of lunar origin of tektites. Tektite chemistry is totally distinct from that observed in lunar maria basalts. These possess Cr contents which are two orders of magnitude higher than tektites, distinctive REE patterns with large Eu depletions, high Fe and low SiO₂ contents, low K/U ratios and many other diagnostic features, none of which are observed in the chemistry of tektites. The lunar uplands compositions, as shown by Apollo 14, 15 and 16 samples and the μ-ray and XRF orbiter data, are high-Al, low-SiO₂ compositions totally dissimilar to those of tektites. The composition of lunar rock 12013 shows typical lunar features and is distinct from that of tektites. The small amounts of lunar K-rich granitic material found in the soils have K/Mg and K/Na ratios 10–50 times those of tektites.The ages of the lunar maria (3.2–3.8 aeons) and uplands (> 4.0 aeons) are an order of magnitude older than the parent material of the Southeast Asian and Australian tektites, which yield Rb-Sr isochrons indicating ages of the order of 100–300 m.y. The lunar lead isotopic compositions are highly radiogenic whereas tektites have terrestrial Pb isotopic ratios. Lunar δ¹⁸ O values are low (< 7 per mil) compared with values of +9.6 to +11.5 per mil for tektites. In summary, a lunar impact origin for tektites is not compatible with the chemistry, age or isotopic composition of the lunar samples. A lunar volcanic origin, recently revived by O'Keefe (1970) encounters most of the same problems. Recent lunar volcanism (< 50 m.y.), if the source of tektites, should contribute tektite glass to the upper layers of the regolith. None has been found. The presence of meteoritic components in tektites, and the high pressure phase coesite, are more readily interpreted as evidence of impact.The element abundances and inter-element variations in tektites do not resemble those in terrestrial igneous rocks, but show a close similarity to terrestrial sandstones. The composition of the Southeast Asian tektites, australites and moldavites resembles that of micaceous sandstones or subgreywackes, the Ivory Coast tektite composition is similar to that of greywacke, and the bediasite chemistry is analogous to that of arkose.No suitable terrestrial impact site has been identified for the bediasites, Southeast Asian tektites and australites. It is suggested that a search for the source of these latter strewnfields be made using satellite photographs to look for wide shallow craters produced by super-Tunguska type events on areas of Mesozoic sandstones. The moldavites were possibly formed during the Ries Crater event but, if so, the precise source of the material remains to be identified. The Ivory Coast tektites are linked by chemistry, isotope and age evidence to the Bosumtwi Crater, Ghana. The overall evidence now supports the origin of tektites by cometary (or meteorite) impact on terrestrial sedimentary rocks.     

Meteoritic material at five large impact craters

Sixteen crater samples were analyzed by radiochemical neutron activation analysis for Ge, Ir, Ni, Os, Pd and Re. Two impact melt rock samples from Clearwater East (22 km) showed strong, uniform enrichments in all elements except Ge, corresponding to 7.4% C1 chondrite material. Interelement ratios suggest that the meteorite was a C1 (or C2) chondrite, not an iron, stony iron, or chondrite of another type. An Ivory Coast tektite (related to the 10 km Bosumtwi crater) was enriched in Ir + Os and Ni to about 0.04 and 1.6% of C1 chondrite levels, but in the absence of data on country rocks, the meteorite cannot yet be characterized.Impact melt rock samples from Clearwater West (32km), Manicouagan (70km), and Mistastin (28 km) showed no detectable meteoritic component. Upper limits, as Cl chondrite equivalent, were Os ≤ 2 × 10^?3% (~0.01 ppb), Ni ≤ 2 × 10^?1% (~20ppm). Possible causes are high impact velocity and/or a chemically inconspicuous meteorite (achondrite, Ir,Os-poor iron or stony iron). However, a more likely reason is that some fraction of the impact melt remains meteorite-free, especially at craters with central peaks.Clearwater East is the first terrestrial impact crater found to be associated with a stony meteorite. Apparently the consistent absence of stony projectiles at small craters (< 1 km diameter) reflects their destruction in the atmosphere, as proposed by Öpik.     

Multiring-forming large bolide impacts and evolution of planetary surfaces

In the past few years, meteoritic and cometary impacts have emerged as a major geological agent in the construction and evolution of planetary surfaces. Formation of complex central ring, peak ring and multiring craters involves excavation and melting of large volumes of crustal material. High-resolution geophysical mapping measuring gravity, magnetics, and topography of the Moon and Mars have recently provided information on the subsurface structure of large basins and aided in identifying buried giant craters. The terrestrial crater record has been significantly erased by tectonic, magmatic, and erosion processes and only a small proportion of impact structures remain. Record of multiring craters is limited to three examples: Vredefort, Sudbury and Chicxulub. Deep geophysical surveys and geochemical and isotopic studies of those craters provide means to evaluate the influence of large impacts on the lithospheric and crustal evolution by providing estimates of excavation depth and volume, amounts of material fragmented, ejected, vaporized and melted, and effects on the crustal stratigraphy and crustal thickness. Analyses on the melt from Vredefort, Sudbury, and Chicxulub indicate andesitic composition derived from lower-crustal material. The melt formed inside the lower transient cavity from lower crustal material that was then redistributed and emplaced in upper-crustal levels, resulting in crustal redistribution. Crystalline basement clasts fragmented and incorporated into the breccias show varying degrees of alteration but no significant thermal effects. Ejecta were deposited locally within the crater region and ballistic material and fine ejecta are globally distributed on the planetary surface. Impacts influence the crust–mantle boundary, with Moho uplift. Material from the mantle was not incorporated into the melt and impact breccias, indicating that the excavation cavities were confined to the lower crust. This is also apparently the case for the giant basins on the Moon, including the 2500 km diameter South Pole-Aitken Basin. Considering the numbers of large multiring basins, possible flux of large impacts, and effects on target surfaces, crustal scale redistribution of material during those large impacts has played a major role in the evolution of planetary surfaces.     

Iron local structure in tektites and impact glasses by extended X-ray absorption fine structure and high-resolution X-ray absorption near-edge structure spectroscopy

The local structure of iron in three tektites has been studied by means of Fe K-edge extended X-ray absorption fine structure (EXAFS) and high-resolution X-ray absorption near-edge structure (XANES) spectroscopy in order to provide quantitative data on <Fe-O> distance and Fe coordination number. The samples studied are a moldavite and two australasian tektites. Fe model compounds with known Fe oxidation state and coordination number were used as standards in order to extract structural information from the XANES pre-edge peak. EXAFS-derived grand mean <Fe-O> distances and Fe coordination numbers for the three tektite samples are constant within the estimated error (<Fe-O > =2.00 Å ± 0.02 Å, CN = 4.0 ± 0.4). In contrast to other data from the literature on Fe-bearing silicate glasses, the tektites spectra could not be fitted with a single Fe-O distance, but rather were fit with two independent distances (2 × 1.92 Å and 2 × 2.08 Å). High-resolution XANES spectra of the three tektites display a pre-edge peak whose intensity is intermediate between those of staurolite and grandidierite, thus suggesting a mean coordination number intermediate between 4 and 5. Combining the EXAFS and XANES data for Fe, we infer the mean coordination number to be close to 4.5.Comparison of the tektites XANES spectra with those of a suite of different impact glasses clearly shows that tektites display a relatively narrow range of Fe oxidation state and coordination numbers, whereas impact glasses data span a much wider range of Fe oxidation states (from divalent to trivalent) and coordination numbers (from tetra-coordinated to esa-coordinated). These data suggest that the tektite production process is very similar for all the known strewn fields, whereas impact glasses can experience a wide variety of different temperature-pressure-oxygen fugacity conditions, leading to different Fe local structure in the resulting glasses. These data could be of aid in discriminating between tektite-like impact glasses and impact glasses sensu strictu.     

泰国东北部猜也奔府Noen Sa-nga地区Yasothon土壤剖面中的澳大拉西亚玻陨石

A sedimentary profile exposed in soil quarries a few kilometers north of Noen Sa-nga district, Chaiyaphum province, NE Thailand, reveals a Quaternary geological history. The lower part of the soil profile is a gravel deposit characterized by sub-angular to well-rounded pebbles representing an abandoned river sedimentary deposit. The gravel bed in some places is coated by iron oxide as a thin layer of ferricrete at the uppermost part. The upper part of the soil profile is a bright reddish brown structureless sand deposit with fining upward sedimentary structure at the basal portion, the Yasothon soil series. A piece of tektite was discovered at the contact boundary between the two sedimentary units. It is characterized by an irregular shape with a smooth concaved surface regarding as an external mold of a piece of well-rounded pebble. This piece of tektite indicates that a solidified tektite had fallen from high sky then was remelted into a plastic form prior to reach and partly cover a piece of well-rounded pebble. These evidences suggest that there was a meteoritic or cometary impact on our earth surface somewhere in the region then catapulted numbers of melted silica ejecta with vast volume of dust into the sky. The melted silica ejecta were solidified into splash-form tektites with various shapes while they were in the high sky. After that the tektites had fallen down and remelted into a plastic form prior to reach the ground surface and then solidified as a tektite deposit followed by larger-sized sediments and angular quartz fragments forming a fining upward sedimentary structure. The finer sediments were gradually settled down forming a bright reddish brown structureless sand deposit, the Yasothon soil series. This meteoritic impact event occurred at about 0.77 Ma ago as the evidence of the previous tektite radiometric dating.     

Rare earth elements in tektites

Four moldavites and one sample each of an australite, billitonite, indochinite, philippinite, thailandite, Ivory Coast tektite, bediasite and a georgianite were analyzed using neutron activation analysis for La, Ce, Sm, Eu, Tb, Yb and Lu. The REE abundances resemble those of sedimentary rocks. Most of the tektites display a depletion of Eu, a characteristic feature of mature Phanerozoic continental sedimentary rocks. However, the Ivory Coast tektite and georgianite are relatively enriched in Eu, possibly due to the presence of plagioclase-rich source rocks.     

Impact cratering and distal ejecta: the Australian record

The Australian continent has one of the best-preserved impact-cratering records on Earth, closely rivalling that of North America and parts of northern Europe, and the rate of new discoveries remains high. In this review 26 impact sites are described, including five small meteorite craters or crater fields associated with actual meteorite fragments (Boxhole, Dalgaranga, Henbury, Veevers, Wolfe Creek) and 21 variably eroded or buried impact structures (Acraman, Amelia Creek, Connolly Basin, Foelsche, Glikson, Goat Paddock, Gosses Bluff, Goyder, Kelly West, Lawn Hill, Liverpool, Matt Wilson, Mt Toondina, Piccaninny, Shoemaker, Spider, Strangways, Tookoonooka, Woodleigh, Yallalie, Yarrabubba). In addition a number of possible impact structures have been proposed and a short list of 22 is detailed herein. The Australian cratering record is anomalously biased towards old structures, and includes the Earth's best record of Proterozoic impact sites. This is likely to be a direct result of aspects of the continent's unique geological evolution. The Australian impact record also includes distal ejecta in the form of two tektite strewn fields (Australasian strewn field, ‘high-soda’ tektites), a single report of 12.1?–?4.6 Ma microtektites, ejecta from the ca 580 Ma Acraman impact structure, and a number of Archaean to Early Palaeoproterozoic impact spherule layers. Possible impact related layers near the Eocene?–?Oligocene and the Permian?–?Triassic boundaries have been described in the literature, but remain unconfirmed. The global K?–?T boundary impact horizon has not been recognised onshore in Australia but is present in nearby deep-sea cores.     

The melt rocks of the Boltysh impact crater,Ukraine, USSR

The 100±12 m.y., 25 km diameter Boltysh impact crater was formed in Precambrian granites and granite gneisses of the Ukrainian Shield. The crater deposits have undergone minimal post-impact erosion and it is possible to study a complete vertical section of the underlying 200 m thick melt sheet. The melt rocks, as sampled in two drill holes, can be subdivided into two major textural classes: microcrystalline and glassy. The microcrystalline melt rocks form an uppermost and two lowermost units, with the glassy variety occupying the middle of the melt sheet. The microcrystalline units contain 25% zoned plagioclase phenocrysts set in a microcrystalline matrix of intergrown alkali feldspar and quartz. Pyroxene has been replaced by sheet-silicates. Mineral and lithic clasts make up 5–15% and show varying degrees of shock and resorption. The glassy melt rocks are characterized by 10–30% zoned plagioclase and 5–10% orthopyroxene set in a fresh to partially devitrified glassy matrix. Clast content is <5%. Chemically, the melt rocks are relatively homogeneous and correspond to a mixture of Kirovograd granites and gneisses in the ratio of 5 to 1, with Ni, Ir and Cr showing slight enrichments over the target rocks. There are minor differences in the Fe₂O₃/FeO ratio and the alkalis between the microcrystalline and glassy varieties. The increase in matrix crystallinity at the upper and lower contacts is contrary to observations at other impact melt sheets, where greater matrix crystallinity occurs in the interiors of the melt sheets. One possible explanation is that the melt matrix was originally glassy throughout, due to its high SiO₂ content, and the microcrystalline matrix is the result of extensive devitrification involving minor alkali exchange with circulating ground-waters.Contribution from the Geological Survey of Canada 40986     

Isotopic fractionation of Cu in tektites

Tektites are terrestrial natural glasses of up to a few centimeters in size that were produced during hypervelocity impacts on the Earth’s surface. It is well established that the chemical and isotopic composition of tektites is generally identical to that of the upper terrestrial continental crust. Tektites typically have very low water content, which has generally been explained by volatilization at high temperature; however, the exact mechanism is still debated. Because volatilization can fractionate isotopes, comparing the isotopic composition of volatile elements in tektites with those of their source rocks may help to understand the physical conditions during tektite formation.Interestingly, volatile chalcophile elements (e.g., Cd and Zn) seem to be the only elements for which isotopic fractionation is known so far in tektites. Here, we extend this study to Cu, another volatile chalcophile element. We have measured the Cu isotopic composition for 20 tektite samples from the four known different strewn fields. All of the tektites (except the Muong Nong-types) are enriched in the heavy isotopes of Cu (1.98 < δ⁶⁵Cu < 6.99) in comparison to the terrestrial crust (δ⁶⁵Cu ≈ 0) with no clear distinction between the different groups. The Muong Nong-type tektites and a Libyan Desert Glass sample are not fractionated (δ⁶⁵Cu ≈ 0) in comparison to the terrestrial crust. To refine the Cu isotopic composition of the terrestrial crust, we also present data for three geological reference materials (δ⁶⁵Cu ≈ 0).An increase of δ⁶⁵Cu with decreasing Cu abundance probably reflects that the isotopic fractionation occurred by evaporation during heating. A simple Rayleigh distillation cannot explain the Cu isotopic data and we suggest that the isotopic fractionation is governed by a diffusion-limited regime. Copper is isotopically more fractionated than the more volatile element Zn (δ^66/64Zn up to 2.49‰). This difference of behavior between Cu and Zn is predicted in a diffusion-limited regime, where the magnitude of the isotopic fractionation is regulated by the competition between the evaporative flux and the diffusive flux at the diffusion boundary layer. Due to the difference of ionic charge in silicates (Zn²⁺ vs. Cu⁺), Cu has a diffusion coefficient that is larger than that of Zn by at least two orders of magnitude. Therefore, the larger isotopic fractionation in Cu than in Zn in tektites is due to the significant difference in their respective chemical diffusivity.     

海南岛雷公墨(玻璃陨石)起源问题的初步探讨

玻璃陨石(tektite)或称熔融石是一种天然玻璃,它的成因是目前陨石学研究中激烈争论的问题。雷公墨是玻璃陨石的一种,发现于海南岛及雷州半岛等地。     

Homogeneous impact melts produced by a heterogeneous target?: Sr-Nd isotopic evidence from the Popigai crater, Russia

The 35.7 ± 0.2 Ma old Popigai crater, Siberia, with a diameter of about 100 km is one of the best preserved large terrestrial impact structures. The heterogeneous target at the impact site consists of Archean to Lower Proterozoic metamorphic rocks of the crystalline basement, Upper Proterozoic quartzites and other clastic deposits, as well as Cambrian to Cretaceous clastic sediments and sedimentary rocks, including carbonate rocks. Moreover, Proterozoic and Permo-Triassic dolerite dykes are found in the target area. We report major element, Sr and Nd isotope data for 13 of these target rocks and for various types of impactites. The 15 analysed impactite samples include tagamites (impact melt rocks), suevites and impact glass from small veins. Furthermore, two impact breccias and two impact glass-coated gneiss bombs were analysed. We discuss the relation of these impactites to the target lithologies, and evaluate on the basis of literature data the relation of microkrystites (and associated microtektites) in Upper Eocene sediments to the Popigai event.The impactites have SiO₂ abundances ranging from 59 to 66 wt.% and show significant variations in the content of Fe, Ca, and Ti. They have present day ⁸⁷Sr/⁸⁶Sr ratios between 0.7191 and 0.7369. Their Sr model ages T^Sr_UR range from 1.9 to 2.3 Ga. The ¹⁴³Nd/¹⁴⁴Nd ratios for the impactite samples cluster between 0.5113 and 0.5115. The Nd model ages T^Nd_CHUR range from 1.9 to 2.1 Ga.In an ε_CHUR(Nd)-ε_UR(Sr) diagram, the impactites and Upper Eocene microkrystites (and associated microtektites) plot in a field delimited by Popigai target lithologies. The impactites are restricted to the field of crystalline basement rocks and Upper Proterozoic quartzites, but they show different isotopic signatures in different crater sectors. Impactites and Upper Eocene microkrystites plot in different, only partly overlapping clusters. The leucocratic microkrystites and microtektites have a higher affinity to the post-Proterozoic rocks in the target area than the impactites. Seemingly, the melanocratic microkrystites originated mostly from crystalline basement. This data alignment supports the assumption that Popigai is the source crater for all three types of ejecta. For the first time, clear relations are established of the geochemically variable Upper Eocene microkrystites and associated microtektites to specific target lithologies at Popigai crater. Finally, the observed range in Sr and Nd isotope parameters determined for impact melt lithologies that originated during the Popigai event show a much higher variability than known from other craters. This result indicates that mixing of impact melt which later formed tagamite sheets and glass particles in different impact breccias, was incomplete at the time of ejecta dispersal.     

Lithium in tektites and impact glasses: Implications for sources, histories and large impacts

Lithium (Li) abundances and isotope compositions were determined in a representative suite of tektites (moldavites, Muong Nong-type tektites and an australite, Ivory Coast tektites and bediasites), impact-related glasses (Libyan Desert Glass, zhamanshinites and irghizites), a glass fragment embedded in the suevite from the Ries impact crater and sedimentary materials in order to test a possible susceptibility of Li to fractionation during hypervelocity impact events and to de-convolve links to their potential parental sources. The overall data show a large spread in Li abundance (4.7-58 ppm Li) and δ⁷Li values (−3.2‰ to 26.0‰) but individual groups of tektites and impact glasses have distinctive Li compositions.Most importantly, any significant high-temperature Li isotope fractionation can be excluded by comparing sedimentary lithologies from central Europe with moldavites. Instead, we suggest that Li isotope compositions in tektites and impact-related glasses are probably diagnostic of the precursor materials and their pre-impact geological histories. The Muong Nong-type tektites and australite specimen are identical in terms of Li concentrations and δ⁷Li and we tentatively endorse their common origin in a single impact event. Evidence for low-temperature Rayleigh fractionation, which must have operated prior to impact-induced melting and solidification, is provided for a subset of Muong Nong-type tektites. Although Li isotope variations in most tektites are broadly similar to those of the upper continental crust, Libyan Desert Glass carries high δ⁷Li ?24.7‰, which appears to mirror the previous fluvial history of parental material that was perhaps deposited in lacustrine environment or coastal seawater. Lithium isotopes in impact-related glasses from the Zhamanshin crater define a group distinct from all other samples and point to melting of chemically less evolved mafic lithologies, which is also consistent with their major and trace element patterns.Extreme shock pressures and the related extreme post-shock temperatures alone appear not to have any effect on the Li isotope systematics; therefore, useful information on parental lithologies and magmatic processes may be retrieved from analyses of Martian and lunar meteorites. Moreover, lack of significant Li depletion in tektites provides further constraints on the loss of moderately volatile elements during the Moon-forming impact.     

陨石撞击成因气凝球粒及撞击烟柱的化学特征

陨石撞击地球表面往往产生灾难性的影响,然而这种影响往往很快地被地表活跃的地质与生物营力所抹去。撞击事件所产生的烟柱,携带大量气体、液滴和尘埃抛溅到距离撞击坑很远的地方,这些烟柱冷凝后成为球形颗粒被保存到地层中,为还原地球早期撞击事件、评估其对环境的影响提供了重要证据。本研究聚焦于撞击烟柱中高温矿物气体冷凝而成的气凝球粒,仔细分析了其中原生尖晶石相的化学成分特征,并以此为窗口推测了烟柱中的气体成分、氧逸度等。我们的研究表明,由于气凝球粒中尖晶石Fe³⁺/Fe^TOT的比值往往很高,而单纯矿物气液平衡本身过于还原,因此气凝球粒的形成需要有撞击烟柱中混杂的空气参与作用。气凝球粒也因而在一定程度上提供了地球早期大气中氧逸度变化以及撞击烟柱不均一性的记录。这一推断与前人依据实验和地球化学分析得到的推测一致,为关于气凝球粒形成机制的数值模拟研究提供了重要启示。     

Resolving impact volatilization and condensation from target rock mixing and hydrothermal overprinting within the Chicxulub impact structure

This work presents isotopic data for the non-traditional isotope systems Fe, Cu, and Zn on a set of Chicxulub impactites and target lithologies with the aim of better documenting the dynamic processes taking place during hypervelocity impact events, as well as those affecting impact structures during the post-impact phase. The focus lies on material from the recent IODP-ICDP Expedition 364 Hole M0077A drill core obtained from the offshore Chicxulub peak ring. Two ejecta blanket samples from the UNAM 5 and 7 cores were used to compare the crater lithologies with those outside of the impact structure. The datasets of bulk Fe, Cu, and Zn isotope ratios are coupled with petrographic observations and bulk major and trace element compositions to disentangle equilibrium isotope fractionation effects from kinetic processes. The observed Fe and Cu isotopic signatures, with δ^56/54Fe ranging from ?0.95‰ to 0.58‰ and δ^65/63Cu from ?0.73‰ to 0.14‰, mostly reflect felsic, mafic, and carbonate target lithology mixing and secondary sulfide mineral formation, the latter associated to the extensive and long-lived (>10⁵ years) hydrothermal system within Chicxulub structure. On the other hand, the stable Zn isotope ratios provide evidence for volatility-governed isotopic fractionation. The heavier Zn isotopic compositions observed for the uppermost part of the impactite sequence and a metamorphic clast (δ^66/64Zn of up to 0.80‰ and 0.87‰, respectively) relative to most basement lithologies and impact melt rock units indicate partial vaporization of Zn, comparable to what has been observed for Cretaceous-Paleogene boundary layer sediments around the world, as well as for tektites from various strewn fields. In contrast to previous work, our data indicate that an isotopically light Zn reservoir (δ^66/64Zn down to ?0.49‰), of which the existence has previously been suggested based on mass balance considerations, may reside within the upper impact melt rock (UIM) unit. This observation is restricted to a few UIM samples only and cannot be extended to other target or impact melt rock units. Light isotopic signatures of moderately volatile elements in tektites and microtektites have previously been linked to (back-)condensation under distinct kinetic regimes. Although some of the signatures observed may have been partially overprinted during post-impact processes, our bulk data confirm impact volatilization and condensation of Zn, which may be even more pronounced at the microscale, with variable degrees of mixing between isotopically distinct reservoirs, not only at proximal to distal ejecta sites, but also within the lithologies associated with the Chicxulub impact crater.