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1.
Abstract— Darwin glass formed about 800,000 years ago in western Tasmania, Australia. Target rocks at Darwin crater are quartzites and slates (Siluro‐Devonian, Eldon Group). Analyses show 2 groups of glass, Average group 1 is composed of: SiO2 (85%), Al2O3 (7.3%), TiO2 (0.05%), FeO (2.2%), MgO (0.9%), and K2O (1.8%). Group 2 has lower average SiO2 (81.1%) and higher average Al2O3 (8.2%). Group 2 is enriched in FeO (+1.5%), MgO (+1.3%) and Ni, Co, and Cr. Average Ni (416 ppm), Co (31 ppm), and Cr (162 ppm) in group 2 are beyond the range of sedimentary rocks. Glass and target rocks have concordant REE patterns (La/Lu = 5.9–10; Eu/Eu* = 0.55–0.65) and overlapping trace element abundances. 87Sr/86Sr ratios for the glasses (0.80778–0.81605) fall in the range (0.76481–1.1212) defined by the rock samples. ε‐Nd results range from –13.57 to –15.86. Nd model ages range from 1.2–1.9 Ga (CHUR) and the glasses (1.2–1.5 Ga) fall within the range defined by the target samples. The 87Sr/86Sr versus 87Rb/86Sr regression age (411 ± 42 Ma) and initial ratio (0.725 ± 0.016), and the initial 43Nd/144Nd ratio (0.51153 ± 000011) and regression age (451 ± 140 Ma) indicate that the glasses have an inherited isotopic signal from the target rocks at Darwin crater. Mixing models using target rock compositions successfully model the glass for all elementsexcept FeO, MgO, Ni, Co, and Cr in group 2. Mixing models using terrestrial ultramafic rocks fail to match the glass compositions and these enrichments may be related to the projectile.  相似文献   

2.
The Fe oxidation state and coordination number of 29 impact glass spherules recently recovered from the Transantarctic Mountains (Antarctica) have been determined by X‐ray absorption near edge structure (XANES) spectroscopy. Based on geochemical, isotopic, and fission track data, these spherules are considered as microtektites from the Australasian tektite/microtektite strewn field. Their find location is the farthest so far discovered from the possible source crater region, and their alkali content is the lowest compared with other published data on Australasian microtektite glasses. The Fe3+/(Fe2++Fe3+) ratio, determined from the analysis of the pre‐edge peak energy position and integrated intensity, is below 0.1 (±0.04) for all the samples, and is comparable to that of most tektites and microtektites from the Australasian strewn field. Also, the pre‐edge peak integrated intensity, which is sensitive to the average Fe coordination geometry, is comparable to that of other Australasian microtektites reported in the literature. The agreement of the Fe oxidation state and coordination number, between the Transantarctic Mountain microtektites (TAM) and the Australasian tektites and microtektites, further confirms the impact origin of these glass spherules and provides an independent suggestion that they represent a major extension southeastward of the Australasian strewn field. The fact that similar redox conditions are observed in tektites and microtektites within the Australasian strewn field regardless of the distance from the source crater area (up to approximately 11000 km) could be an important constraint for better understanding the different processes affecting microtektite formation and transport. The fact that the Fe oxidation state of microtektites does not increase with distance, as in the case of North American microtektites, means that thermal and redox histories of Australasian and TAM microtektites could differ significantly from those of North American microtektites.  相似文献   

3.
Coesite has been identified within ejected blocks of shocked basalt at Lonar crater, India. This is the first report of coesite from the Lonar crater. Coesite occurs within SiO2 glass as distinct ~30 μm spherical aggregates of “granular coesite” identifiable both with optical petrography and with micro‐Raman spectroscopy. The coesite+glass occurs only within former silica amygdules, which is also the first report of high‐pressure polymorphs forming from a shocked secondary mineral. Detailed petrography and NMR spectroscopy suggest that the coesite crystallized directly from a localized SiO2 melt, as the result of complex interactions between the shock wave and these vesicle fillings.  相似文献   

4.
Abstract— Three samples of Darwin Glass, an impact glass found in Tasmania, Australia at the edge of the Australasian tektite strewn field were dated using the 40Ar/39Ar single‐grain laser fusion technique, yielding isochron ages of 796–815 ka with an overall weighted mean of 816 ± 7 ka. These data are statistically indistinguishable from those recently reported for the Australasian tektites from Southeast Asia and Australia (761–816 ka; with a mean weighted age of 803 ± 3 ka). However, considering the compositional and textural differences and the disparity from the presumed impact crater area for Australasian tektites, Darwin Glass is more likely to have resulted from a distinct impact during the same period of time.  相似文献   

5.
Abstract— Five indochinites from Hainan Island and the Leizhou Peninsula, China were analyzed for noble gas abundances and isotopic ratios. These splash‐form tektites show vesiculation ranging from 0.4 vol% to 8 vol%, as determined by digital image analysis (software SXM®) on thin section photographs. To study the distribution of noble gases in vesicles and in glass, the gases were extracted by heating and by crushing, respectively, on 2 aliquots of the same sample. The results show that 5 to 53% of the total measured 20Ne resides in vesicles. The calculated concentration of neon dissolved in the glass is higher (0.7‐1 times 10?7 cm3STP/g) than that expected from solubility equilibrium (1 times 10?8 cm3STP/g), assuming solubility data from MORB glasses. The neon concentration of splash‐form tektites, those analyzed in this work and those from other strewn‐fields worldwide, is correlated with the SiO2 content of glass and with the nonbridging oxygen per tetrahedral cation in the melt (hereafter NBO/T ratio), the latter being an index of the free‐volume in the silica network where neon could be dissolved. These correlations suggest that the glass structure of tektite has a larger free‐volume available for dissolving noble gases than MORB glasses.  相似文献   

6.
Abstract— An examination of data collected over the last 30 years indicates that the percent of glass fragments vs. whole splash forms in the Cenozoic microtektite strewn fields increases towards the source crater (or source region). We propose that this is due to thermal stress produced when tektites and larger microtektites fall into water near the source crater while still relatively hot (>1150 °C). We also find evidence (low major oxide totals, frothing when melted) for hydration of most of the North American tektite fragments and microtektites found in marine sediments. High-temperature mass spectrometry indicates that these tektite fragments and microtektites contain up to 3.8 wt% H2O. The H2O-release behavior during the high-temperature mass-spectrometric analysis, plus high CI abundances (0.05 wt%), indicate that the North American tektite fragments and microtektites were hydrated in the marine environment (i.e., the H2O was not trapped solely on quenching from a melt). The younger Ivory Coast and Australasian microtektites do not exhibit much evidence of hydration (at least not in excess of 0.5 wt% H2O); this suggests that the degree of hydration increases with age. In addition, we find that some glass spherules (with <65 wt% SiO2) from the upper Eocene clinopyroxene-bearing spherule layer in the Indian Ocean have palagonitized rims. These spherules appear to have been altered in a similar fashion to the splash form K/T boundary spherules. Thus, our data indicate that tektites and microtektites that generally contain >65 wt% SiO2 can undergo simple hydration in the marine environment, while impact glasses (with <65 wt% SiO2) can also undergo palagonitization.  相似文献   

7.
Abstract— t‐Impact‐generated glasses from fallout suevite deposits at the Ries impact structure have been investigated using analytical scanning electron microscopy. Approximately 320 analyses of glass clasts were obtained. Four glass types are distinguished on the basis of composition and microtextures. Type 1 glasses correspond to the aerodynamically shaped glass bombs studied previously by many workers. Major oxide concentrations indicate the involvement of granitic rocks, amphibolites, and minor Al‐rich gneisses during melting. Type 2 glasses are chemically heterogeneous, even within individual clasts, with variations of several wt% in most of the major oxides (e.g., 57–70 wt% SiO2). This suggests incomplete mixing of: 1) mineral‐derived melts or 2) whole rock melts from a wide range of lithologies. Aluminium‐rich clinopyroxene and Fe‐Mg‐rich plagioclase quench crystals are present in type 1 and 2 glasses, respectively. Type 3 glasses contain substantial amounts of H2O (?12–17 wt%), low SiO2 (50–53 wt%), high Al2O3 (17–21 wt%), and high CaO (5–7 wt%) contents. This suggests an origin due to shock melting of part of the sedimentary cover. Type 4 glasses form a ubiquitous component of the suevites. Based on their high SiO2 content (?85–100 wt%), the only possible protolith are sandstones in the lowermost part of the sedimentary succession. Calcite forms globules within type 1 glasses, with which it develops microtextures indicative of liquid immiscibility. Unequivocal evidence also exists for liquid immiscibility between what are now montmorillonite globules and type 1, 2, and 4 glasses, indicating that montmorillonite was originally an impact melt glass. Clearly, the melt zone at the Ries must have incorporated a substantial fraction of the sedimentary cover, as well as the underlying crystalline basement rocks. Impact melts were derived from different target lithologies and these separate disaggregated melts did not substantially mix in most cases (type 2, 3, and 4 glasses and carbonate melts).  相似文献   

8.
Abstract— A layer of tektite glass and shock-metamorphosed grains found in an upper Eocene section of core 21 from DSDP Site 612 taken on the continental slope off New Jersey may belong to the North American tektite strewn field. However, the Site 612 glasses generally have higher K2O and lower Na2O contents for a given SiO2 content and different Sr and Nd isotopic compositions. In order to better define the layer, a series of samples was taken continuously through the layer at 1 cm intervals. Tektite fragments are in an 8 cm thick layer; microtektites are concentrated in the upper 4 cm, while spherules with “crystalline” textures (microkrystites) are concentrated in the lower half of the layer. Millimeter-size splash forms are mostly in the lower part of the tektite-bearing layer. Rock and mineral grains showing evidence of shock metamorphism are abundant in the upper half of the tektite-bearing layer. Coesite is abundant, and stishovite was found in one rock fragment. The size and abundance of the tektite glass and the abundance of shocked debris indicate that Site 612 is relatively close to the source crater, which may be to the north of Site 612 on the coastal plain or adjacent continental shelf.  相似文献   

9.
Abstract– Tektites, natural silica‐rich glasses produced during impact events, commonly contain bubbles. The paper reviews published data on pressure and composition of a gas phase contained in the tektite bubbles and data on other volatile compounds which can be released from tektites by either high‐temperature melting or by crushing or milling under vacuum. Gas extraction from tektites using high‐temperature melting generally produced higher gas yield and different gas composition than the low‐temperature extraction using crushing or milling under vacuum. The high‐temperature extraction obviously releases volatiles not only from the bubbles, but also volatile compounds contained directly in the glass. Moreover, the gas composition can be modified by reactions between the released gases and the glass melt. Published data indicate that besides CO2 and/or CO in the bubbles, another carbon reservoir is present directly in the tektite glass. To clarify the problem of carbon content and carbon isotopic composition of the tektite glass, three samples from the Central European tektite strewn field—moldavites—were analyzed. The samples contained only 35–41 ppm C with δ13C values in the range from ?28.5 to ?29.9‰ VPDB. This indicates that terrestrial organic matter was a dominant carbon source during moldavite formation.  相似文献   

10.
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   

11.
Abstract— We have analyzed fluorine and boron in nine tektites from all four strewn fields, and in a suite of impact glasses and target rocks from the Zhamanshin and Darwin impact craters, as well as Libyan Desert Glass and Aouelloul impact glass samples. Fluorine and boron are useful indicators for the volatilization and temperature history of tektites and impact glasses. Tektites from different strewn fields show a limited range of F and B contents and have F/B ratios near unity. Most splash-form tektites have lower average F and B contents than Muong Nong type tektites, which is similar to the relation between irghizites and zhamanshinites. The F and B contents in target rocks from the Zhamanshin and Darwin impact craters are similar to normal terrestrial sediments. Fluorine in impact glasses and tektites is more depleted compared to their (known or inferred) target rocks than is boron, which is caused by the higher volatility of F. The F/B ratios therefore decrease with increasing temperature of formation (suggesting that irghizites were formed at a higher temperature than zhamanshinites, and Muong Nong type tektites at a lower temperature than splash-form tektites). Mixing of local country rocks together with partial loss of the volatiles F and B can reproduce the F and B contents of impact glasses.  相似文献   

12.
Two irghizites, three zhamanshinites and one sample each of lechatelierite, vein-quartz, Palaeogene silty clay and Palaeogene quartzite were analyzed using neutron activation analysis. A silicate analysis of the Palaeogene silty clay has also been performed, as well as an incomplete analysis of the Palaeogene quartzite from the Zhamanshin impact crater. The REE abundances of irghizites resemble those of sedimentary rocks. On the Köhler and Raaz diagram all projection points of irghizites lie inside the field of tektites, and indicate that they were derived from terrestrial sedimentary rocks. The Zhamanshin impact glasses may be divided into three types: (a) silica-rich zhamanshinites (x?SiO2 = 73.89%), (b) zhamanshinites (x?SiO2 = 54.34%), and (c) silica-poor zhamanshinites (x?SiO2 = 39.64%). These are also characterized by varying proportions of alkalis and Al. Mn and Ca contents. Irghizites and silica-rich zhamanshinites display a depletion of Eu. Zhamanshinites do not show this Eu depletion. Partial melting is assumed to be an important process in the origin of zhamanshinites.  相似文献   

13.
Tasmanian Darwin glass has a fusion age sensibly identical with that of Australasian tektites and it is reasonable to assume all were produced in the same event. Recently a number of new Darwin glass localities and an associated crater have been discovered. The glass stewnfield covers at least 400 km2 and there is a strong positive correlation between glass fragment size and abundance and proximity to the crater. The glass was distributed from some point near the crater, with the smallest pieces traveling furthest. This structure is apparently an impact crater of rather unusual configuration and fortuitous location. Our gravity survey reveals a closed sedimentary basin about 1000 meters in diameter. A centrally located drill hole penetrated 60 meters of lacustrine clays and 40 meters of mixed clay, sand and rock fragments. The hole was terminated at 100 meters in loose sand containing sand-sized fragments of Darwin glass and lechetelierite. The 100 meters of cored sediments accounts for only about half of the observed 3.5 milligal negative anomaly and there must be a substantial additional thickness of low density material at depth. Further drilling is essential to confirm an impact origin and to delineate the subsurface crater configuration. This information would be of great calibration value for theoretical modeling studies of explosive cratering. The Darwin strewnfield characteristics support the theory that the distribution of Australasian tektites was aided by an impact-generated, atmospheric blast wave (or waves). The stratigraphic position of glass below 100 meters of lake sediments is strong evidence that the postulated stratigraphic age of the Australian land tektites is incorrect.  相似文献   

14.
Abstract— Impact‐metamorphosed CaCO3‐bearing sandstones at the Haughton structure have been divided into 6 classes, based to a large extent on a previous classification developed for sandstones at Meteor Crater. Class 1a sandstones (<3 GPa) display crude shatter cones, but no other petrographic indications of shock. At pressures of 3 to 5.5 GPa (class 1b), porosity is destroyed and well‐developed shatter cones occur. Class 2 rocks display planar deformation features (PDFs) and are characterized by a “jigsaw” texture produced by rotation and shear at quartz grain boundaries. Calcite shows an increase in the density of mechanical twins and undergoes micro‐brecciation in class 1 and 2 sandstones. Class 3 samples display multiple sets of PDFs and widespread development of diaplectic glass, toasted quartz, and symplectic intergrowths of quartz, diaplectic glass, and coesite. Textural evidence, such as the intermingling of silicate glasses and calcite and the presence of flow textures, indicates that calcite in class 3 sandstones has undergone melting. This constrains the onset of melting of calcite in the Haughton sandstones to > 10 < 20 GPa. At higher pressures, the original texture of the sandstone is lost, which is associated with major development of vesicular SiO2 glass or lechatelierite. Class 5 rocks (>30 GPa) consist almost entirely of lechatelierite. A new class of shocked sandstones (class 6) consists of SiO2‐rich melt that recrystallized to microcrystalline quartz. Calcite within class 4 to 6 sandstones also underwent melting and is preserved as globules and euhedral crystals within SiO2 phases, demonstrating the importance of impact melting, and not decomposition, in these CaCO3‐bearing sandstones.  相似文献   

15.
Abstract— A new 40Ar/39Ar data set is presented for tektites from the Central European strewn field (moldavites). This is the only strewn field that is entirely situated in a continental environment and still characterized by scattered ages (14–15.3 Myr). The main objectives of the study were to define more precisely the moldavite formation age and provide a good calibration for a glass standard proposed for fission‐track dating. The laser total fusion ages obtained on chips from 7 individual specimens from the Southern Bohemian and Moravian subfields are restricted to a narrow interval of time, with an average of 14.34 ± 0.08 Myr relative to the 27.95 ± 0.09 Myr of the Fish Canyon Tuff biotite. This result gives a more precise age not only for the tektite field but also for its producing impact. If the genetic link between the moldavites and the Nördlinger Ries impact crater is maintained, then this new age has to be considered a reliable estimate for the Ries crater also. This new value places the formation of Central European tektites within the Lower Serravallian period in the latest geologic timescales. Evidence of their impact products, such as glass spherules or shocked minerals, can, therefore, be sought in sedimentary marine formations in a more precisely defined age interval.  相似文献   

16.
The 455 Ma old Lockne crater in central Sweden is a well-preserved and accessible instance of marine impact crater. The process of formation of the over 7 km wide crater (referred to as inner crater) in crystalline Proterozoic basement is numerically modeled under the assumption of a 45° oblique impact of an asteroid-like impactor. The 3D version of the SOVA multi-material hydrocode is used to model the shock wave propagation through the target, transient crater growth, material ejection in water and basement target, and water and fragmented rock ejecta expansion. The model results in a crater formation with the greatest ejection and melting transferred in the downrange direction. The model reproduces the growth of the water crater accompanied by the growth of a “wall” of ejected water at its outer margin. The basement ejecta are mostly trapped in this transient “water wall”. Only the largest ejected rock fragments could break through this water wall and thus reach distances farther than about 6 km from the center of the target. The model predicts approximately of impact melt formation, less than 10% of which is ejected outside of the inner (basement) crater, whereas the rest is reckoned to have remained within the inner crater. We assume that most of the ejected melt occurs as sand-sized fragments in the resurge sediments that formed subsequent to the collapse of the water crater that resulted in the powerful backflow of water. The model results are in accordance with several important details of the known geology of the crater. The model also outlines the difference in the marine crater formation processes in contrast to a crater with similar size formed on land.  相似文献   

17.
Abstract— We report Sr-Nd isotope parameters, rare earth element (REE), and major element data for isolated findings of tektite-like objects from western Siberia (urengoites, South-Ural glass), as well as for two indochinites. The latter were recovered in Vietnam and their overall geochemical characteristics equal those of other tektites from the indochinite subgroup of the Australasian strewn field. The three urengoites (～24 Ma) are extremely silica-rich (89 to 96 wt% SiC2), and their REE abundances vary between 45 and 76 ppm. With LaN/YbN ranging from 7.6 to 10.4 and EuN/EU* between 0.69 and 0.75, their REE distribution patterns match that of average upper crust. The urengoites have present-day ?Sr of +155 to +174 and ?Nd ranging from ?18 to ?23. Their model ages in million years are: TSruR = 1200 up to 4060 and TNdcHUR = 1570 up to 2070. Data points for the urengoites plot colinearly in the Rb-Sr evolution diagram. The age corresponding to the slope is 183 ± 30 Ma (2s?), which is indistinguishable from the intercept age of 211 Ma in the TSrUR vs. l/fRb diagram. Rubidium-strontium and Sm-Nd systematics of the urengoites indicate a heterogeneous precursor material, derived from Paleoproterozoic continental crust, which underwent Rb/Sr fractionation and partial Sr isotope homogenization in Jurassic times. Any relation between the urengoites and the Haughton impact crater, having within 2s? errors an identical age, can be excluded on the basis of isotope relationships and geochemical data. The only known South-Ural glass (～6.2 Ma) is characterized by intermediate SiO2 (65 wt%), high Al2O3 (14 wt%) and CaO (12 wt%), and low FeOTOT (0.4 wt%) contents. This unique tektite-like object contains 110 ppm REE displaying a steeply negative C1 normalized distribution with LaN/YbN of 17, and EuN/Eu1 of 0.71. The Rb abundance (10 ppm) and Rb/Sr ratio are low, and combined with a “crustal” 87Sr/86Sr ratio of 0.722, yielding an unrealistic TSruR age of 2.5 Ga. The Rb-Sr systematics imply a rather recent parent/daughter element decoupling. The TNdCHUR age of the South-Ural glass is ～1690 Ma. Geochemical data suggest that urengoites and the South-Ural glass belong to two discrete groups of tektites, whose source craters remain to be discovered.  相似文献   

18.
The Morasko strewn field located near Poznań, Poland comprises seven impact craters with diameters ranging from 20 to 90 m, all of which were formed in glacial sediments around 5000 yr ago. Numerous iron meteorites have been recovered in the area and their distribution suggests a projectile with the trajectory from NE to SW. Similar impact events producing crater strewn fields on average happen every 500 yr and pose a serious risk for modern civilization, which is why it is of utmost importance to study terrestrial strewn fields in detail. In this work, we investigate the Morasko meteoroid passage through the atmosphere, the distribution of its fragments on the ground, and the process of forming individual craters by means of numerical modeling. By combining atmospheric entry modeling, Pi‐group scaling of transient crater size and hydrocode simulations of impact processes, we constructed a comprehensive model of the Morasko strewn field formation. We determined the preatmospheric parameters of the Morasko meteoroid. The entry mass is between 600 and 1100 tons, the velocity range is between 16 and 18 km s?1, and the trajectory angle is 30–40°. Such entry velocities and trajectory angles do not deviate from typical values for near‐Earth asteroids, although the initial mass we determined can be considered as small. Our studies on velocities and masses of crater‐forming fragments showed that the biggest Morasko crater was formed by a projectile about 1.5 m in diameter with the impact velocity ~10 km s?1. Environmental consequences of the Morasko impact event are very localized.  相似文献   

19.
Abstract– Despite its centennial exploration history, there are still unresolved questions about Meteor Crater, the first recognized impact crater on Earth. This theoretical study addresses some of these questions by comparing model results with field and laboratory studies of Meteor Crater. Our results indicate that Meteor Crater was formed by a high‐velocity impact of a fragmented projectile, ruling out a highly dispersed swarm as well as a very low impact velocity. Projectile fragmentation caused many fragments to fall separately from the main body of the impactor, making up the bulk of the Canyon Diablo meteorites; most of these fragments were engulfed in the expansion plume as they approached the surface without suffering high shock compression, and were redistributed randomly around the crater. Thus, the distribution of Canyon Diablo meteorites is not representative of projectile trajectory, as is usual for impactor fragments in smaller strewn fields. At least 50% of the main impactor was ejected from the crater during crater excavation and was dispersed mostly downrange of the crater as molten particles (spheroids) and highly shocked solid fragments (shrapnel). When compared with the known distribution, model results suggest an impactor from the SW. Overall, every model case produced much higher amounts of pure projectile material than observed. The projectile‐target mixing was not considered in the models; however, this process could be the main sink of projectile melt, as all analyzed melt particles have high concentrations of projectile material. The fate of the solid projectile fragments is still not completely resolved. Model results suggest that the depth of melting in the target can reach the Coconino sandstone formation. However, most of the ejected melt originates from 30–40 m depth and, thus, is limited to Moenkopi and upper Kaibab material. Some melt remains in the target; based on the estimated volume of the breccia lens at Meteor Crater, our models suggest at most a 2% content of melt in the breccia. Finally, a high water table at the time of impact could have aided strong dispersion of target and projectile melt.  相似文献   

20.
Shock-induced melting and vaporization of H2O ice during planetary impact events are widespread phenomena. Here, we investigate the mass of shock-produced liquid water remaining within impact craters for the wide range of impact conditions and target properties encountered in the Solar System. Using the CTH shock physics code and the new 5-phase model equation of state for H2O, we calculate the shock pressure field generated by an impact and fit scaling laws for melting and vaporization as a function of projectile mass, impact velocity, impact angle, initial temperature, and porosity. Melt production nearly scales with impact energy, and natural variations in impact parameters result in only a factor of two change in the predicted mass of melt. A fit to the π-scaling law for the transient cavity and transient-to-final crater diameter scaling are determined from recent simulations of the entire cratering process in ice. Combining melt production with π-scaling and the modified Maxwell Z-model for excavation, less than half of the melt is ejected during formation of the transient crater. For impact energies less than about 2 × 1020 J and impact velocities less than about 5 km s−1, the remaining melt lines the final crater floor. However, for larger impact energies and higher impact velocities, the phenomenon of discontinuous excavation in H2O ice concentrates the impact melt into a small plug in the center of the crater floor.  相似文献   

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