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1.
This study demonstrates a relationship between changes of magnetic susceptibility and microstructure developing in minerals of a magnetite‐bearing ore, experimentally shocked to pressures of 5, 10, 20, and 30 GPa. Shock‐induced effects on magnetic properties were quantified by bulk magnetic susceptibility measurements while shock‐induced microstructures were studied by high‐resolution scanning electron microscopy. Microstructural changes were compared between magnetite, quartz, amphibole, and biotite grains. In the 5 GPa sample, a sharp drop of magnetic susceptibility correlates with distinct fragmentation as well as with formation of shear bands and twins in magnetite. At 10 GPa, shear bands and twins in magnetite are accompanied by droplet‐shaped nanograins. In this shock pressure regime, quartz and amphibole still show intensive grain fragmentation. Twins in quartz and foam‐shaped, highly porous amphibole are formed at 20 and 30 GPa. The formation of porous minerals suggests that shock heating of these mineral grains resulted in localized temperature spikes. The identified shock‐induced features in magnetite strongly advise that variations in the bulk magnetic susceptibility result from cooperative grain fragmentation, plastic deformation and/or localized amorphization, and probably postshock annealing. In particular, the increasing shock heating at high pressures is assumed to be responsible for a partial defect annealing which we suggest to be responsible for the almost constant values of magnetic susceptibility above 10 GPa.  相似文献   

2.
Abstract– We carried out shock experiments on macroscopic spherical samples of the L4 ordinary chondrite Saratov (natural shock stages S2–S3), using explosively generated spherical shock waves with maximum peak pressures of 400 GPa and shock‐induced temperatures >800 °C (up to several thousands °C). The evolution of shock metamorphism within a radius of the spherical samples was investigated using optical and scanning electron microscopy, microprobe and magnetic analyses as well as Mössbauer spectroscopy and X‐ray diffraction techniques. Petrographic analyses revealed a shock‐induced formation of three different concentric petrographic zones within the shocked samples: zone of total melting (I), zone of partial melting (II), and zone of solid‐state shock features (III). We found a progressive pressure‐induced oxidation of Fe‐Ni metal, whose degree increased with increasing shock peak pressure. The amount of FeO within zone I increased the factor of 1.4 with respect to its amount in the unshocked Saratov sample. This suggests that within zone I about 70 wt% of the initial metallic iron was oxidized, whereas magnetic analyses showed that about 10 wt% of it remained intact. This strongly supports the hypothesis that, in addition to oxidation, a migration of metallic iron from the central heavily shocked zone I toward less shocked peripheral zone took place as well (likely through shock veins where metallic droplets were observed). Magnetic analyses also showed a shock‐induced transformation of tetrataenite to taenite within all shocked subsamples, resulting in magnetic softening of these subsamples (decrease in remanent coercivity). These results have important implications for extraterrestrial paleomagnetism suggesting that due to natural impact processes, the buried crustal rocks of heavily cratered solid solar system bodies can have stronger remanent magnetism than the corresponding surface rocks.  相似文献   

3.
Abstract– Shock metamorphism can occur at transient pressures that reach tens of GPa and well over 1000 °C, altering the target material on both megascopic and microscopic scales. This study explores the effects of shock metamorphism on crystalline, quartzofeldspathic basement material from the Haughton impact structure on Devon Island, Arctic Canada. Shock levels were assigned to samples based on petrographic examination of main mineral phases. Conventional shock classification schemes proved to incompletely describe the Haughton samples so a modified shock classification system is presented. Fifty‐two crystalline bedrock samples from the clast‐rich impact melt rocks in the crater, and one reference site outside of the crater, were classified using this system. The shock levels range from 0 to 7 (according to the new shock stage classification proposed here, i.e., stages 0–IV after the Stöffler classification), indicating shock pressures ranging from 0 to approximately 80 GPa. The second aspect of this study involved measuring bulk physical characteristics of the shocked samples. The bulk density, grain density, and porosity were determined using a water displacement method, a bead displacement method, and a Hepycnometer. Results suggest a nonlinear, negative correlation between density and shock level such that densities of crystalline rocks with original densities of approximately 3 g cm?3 are reduced to <1.0 g cm?3 at high shock levels. The results also show a positive nonlinear correlation between porosity and shock level. These data illustrate the effect of shock on the bulk physical characteristics of crystalline rocks, and has implications for assessing the habitability of shocked rocks.  相似文献   

4.
Abstract— Demagnetized samples of cobalt precipitates in a copper matrix were shocked to 5, 10, and 20 GPa in a weak magnetic field of 7.7 μT to elucidate the origins of the natural remanent magnetization of meteorites and the magnetic anomalies of impact craters on the moon and Mars. The samples placed in the target acquired shock remanent magnetization (SRM) whose intensity increased up to 21.3 times compared with the demagnetized state, but SRM intensity and shock intensity were not correlated. The SRM direction was in most cases approximately perpendicular to the shock direction. The samples placed 4.8 mm from the impacted surface did not acquire significant magnetization, suggesting no plasma‐induced remanent magnetization (PIRM) up to 20 GPa. When the samples were divided into 8 sub‐samples, the SRM intensities of sub‐samples increased up to 40 times compared with bulk ones and their directions were scattered. Higher coercive force grains were magnetized perpendicular to the shock direction for shocks of 5 and 10 GPa, but at 20 GPa the directions were less systematically oriented. These results suggest that the proposed plasma‐induced magnetization of impactites should be reconsidered.  相似文献   

5.
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   

6.
A combination of shock recovery experiments and numerical modeling of shock deformation in the low‐shock pressure range from 2.5 to 20 GPa for two dry sandstone types of different porosity, a completely water‐saturated sandstone, and a well‐indurated quartzite provides new insights into strongly heterogeneous distribution of different shock features. (1) For nonporous quartzo‐feldspathic rocks, the traditional classification scheme (Stöffler 1984 ) is suitable with slight changes in pressure calibration. (2) For water‐saturated quartzose rocks, a cataclastic texture (microbreccia) seems to be typical for the shock pressure range up to 20 GPa. This microbreccia does not show formation of PDFs but diaplectic quartz glass/SiO2 melt is formed at 20 GPa (~1 vol%). (3) For porous quartzose rocks, the following sequence of shock features is observed with progressive increase in shock pressure (1) crushing of pores, (2) intense fracturing of quartz grains, and (3) increasing formation of diaplectic quartz glass/SiO2 melt replacing fracturing. The formation of diaplectic quartz glass/SiO2 melt, together with SiO2 high‐pressure phases, is a continuous process that strongly depends on porosity. This experimental observation is confirmed by our concomitant numerical modeling. Recalibration of the shock classification scheme results in a porosity versus shock pressure diagram illustrating distinct boundaries for the different shock stages.  相似文献   

7.
Abstract— This study examines the effects of shock metamorphism on fluid inclusions in crystalline basement target rocks from the Ries crater, Germany. The occurrence of two‐phase fluid inclusions decreases from shock stage 0 to shock stage 1, while single‐phase inclusions increase, likely as a result of re‐equilibration. In shock stages 2 and 3, both two‐phase and single‐phase inclusions decrease with increasing shock stage, indicating that fluid inclusion vesicles are destroyed due to plastic deformation and phase changes in the host minerals. However, quartz clasts entrained in shock stage 4 melts contain both single‐phase and two‐phase inclusions, demonstrating the rapid quenching of the melt and the heterogeneous nature of impact deformation. Inclusions in naturally shocked polycrystalline samples survive at higher shock pressures than those in single crystal shock experiments. However, fluid inclusions in both experimental and natural samples follow a similar trend in re‐equilibration at low to moderate shock pressures leading to destruction of inclusion vesicles in higher shock stages. This suggests that shock processing may lead to the destruction of fluid inclusions in many planetary materials and likely contributed to shock devolatilization of early planetesimals.  相似文献   

8.
MIL 11207 (R6) and LAP 04840 (R6) contain hornblende and phlogopite; MIL 07440 (R6) contains accessory titan‐phlogopite and no hornblende. All three meteorites have been shocked: MIL 11207 contains extensive sulfide veins, pyroxene that formed from dehydrated hornblende, and an extensive network of plagioclase glass; MIL 07440 contains chromite‐plagioclase assemblages, chromite veinlets and blebs, pincer‐shaped plagioclase patches, but no sulfide veins; LAP 04840 contains olivine grains with chromite‐bleb‐laden cores and opaque‐free rims, rare grains of pyroxene that formed from dehydrated hornblende, and no sulfide veins. These meteorites appear to have been heated to maximum temperatures of approximately 700–900 °C under conditions of moderately high PH2O (perhaps 250–500 bars). All three samples underwent postshock annealing. During this process, olivine crystal lattices healed (giving the rocks the appearance of shock‐stage S1), and diffusion of Fe and S from thin sulfide veins to coarse sulfide grains caused the veins to disappear in MIL 07440 and LAP 04840. This latter process apparently also occurred in most S1–S2 ordinary chondrites of high petrologic type. The pressure–temperature conditions responsible for forming the amphibole and mica in these rocks may have been present at depths of a few tens of kilometers (as suggested in the literature). A giant impact or a series of smaller impacts would then have been required to excavate the hornblende‐ and biotite‐bearing rocks and bring them closer to the surface. It was in that latter location where the samples were shocked, deposited in a hot ejecta blanket of low thermal diffusivity, and annealed.  相似文献   

9.
Abstract— Micrometeorites have been significantly altered or melted by heating, which has been mainly ascribed to aerodynamic drag during atmospheric entry. However, if a major fraction of micrometeorites are produced by impacts on porous asteroids, they may have experienced shock heating before contact with the Earth's atmosphere (Tomeoka et al. 2003). A transmission electron microscope (TEM) study of the matrix of Murchison CM chondrite experimentally shocked at pressures of 10–49 GPa shows that its mineralogy and texture change dramatically, mainly due to shock heating, with the progressive shock pressures. Tochilinite is completely decomposed to an amorphous material at 10 GPa. Fe‐Mg serpentine is partially decomposed and decreases in amount with increasing pressure from 10 to 30 GPa and is completely decomposed at 36 GPa. At 49 GPa, the matrix is extensively melted and consists mostly of aggregates of equigranular grains of Fe‐rich olivine and less abundant low‐Ca pyroxene embedded in Si‐rich glass. The mineralogy and texture of the shocked samples are similar to those of some types of micrometeorites. In particular, the samples shocked at 10 and 21 GPa are similar to the phyllosilicate (serpentine)‐rich micrometeorites, and the sample shocked at 49 GPa is similar to the olivine‐rich micrometeorites. The shock heating effects also resemble the effects of pulse‐heating experiments on the CI and CM chondrite matrices that were conducted to simulate atmospheric entry heating. We suggest that micrometeorites derived from porous asteroids are likely to go through both shock and atmospheric‐entry heating processes.  相似文献   

10.
The brecciation and shock classification of 2280 ordinary chondrites of the meteorite thin section collection at the Institut für Planetologie (Münster) has been determined. The shock degree of S3 is the most abundant shock stage for the H and LL chondrites (44% and 41%, respectively), while the L chondrites are on average more heavily shocked having more than 40% of rocks of shock stage S4. Among the H and LL chondrites, 40–50% are “unshocked” or “very weakly shocked.” Considering the petrologic types, in general, the shock degree is increasing with petrologic type. This is the case for all meteorite groups. The main criteria to define a rock as an S6 chondrite are the solid‐state recrystallization and staining of olivine and the melting of plagioclase often accompanied by the formation of high‐pressure phases like ringwoodite. These characteristics are typically restricted to local regions of a bulk chondrite in or near melt zones. In the past, the identification of high‐pressure minerals (e.g., ringwoodite) was often taken as an automatic and practical criterion for a S6 classification during chondrite bulk rock studies. The shock stage classification of many significantly shocked chondrites (>S3) revealed that most ringwoodite‐bearing rocks still contain more than 25% plagioclase (74%). Thus, these bulk chondrites do not even fulfill the S5 criterion (e.g., 75% of plagioclase has to be transformed into maskelynite) and have to be classified as S4. Studying chondrites on typically large thin sections (several cm2) and/or using samples from different areas of the meteorites, bulk chondrites of shock stage S6 should be extremely rare. In this respect, the paper will discuss the probability of the existence of bulk rocks of S6.  相似文献   

11.
Abstract— Previous X‐ray powder diffraction (XRD) studies revealed that shock deformed carbonates and quartz have broader XRD patterns than those of unshocked samples. Entire XRD patterns, single peak profiles and Rietveld refined parameters of carbonate samples from the Sierra Madera impact crater, west Texas, unshocked equivalent samples from 95 miles north of the crater and the Mission Canyon Formation of southwest Montana and western Wyoming were used to evaluate the use of X‐ray powder diffraction as a potential tool for distinguishing impact deformed rocks from unshocked and tectonically deformed rocks. At Sierra Madera dolostone and limestone samples were collected from the crater rim (lower shock intensity) and the central uplift (higher shock intensity). Unshocked equivalent dolostone samples were collected from well cores drilled outside of the impact crater. Carbonate rocks of the Mission Canyon Formation were sampled along a transect across the tectonic front of the Sevier and Laramide orogenic belts. Whereas calcite subjected to significant shock intensities at the Sierra Madera impact crater can be differentiated from tectonically deformed calcite from the Mission Canyon Formation using Rietveld refined peak profiles, weakly shocked calcite from the crater rim appears to be indistinguishable from the tectonically deformed calcite. In contrast, Rietveld analysis readily distinguishes shocked Sierra Madera dolomite from unshocked equivalent dolostone samples from outside the crater and tectonically deformed Mission Canyon Formation dolomite.  相似文献   

12.
Abstract— Asteroid and comet impacts on Earth are commonly viewed as agents of ecosystem destruction, be it on local or global scales. However, for some microbial communities, impacts may represent an opportunity for habitat formation as some substrates are rendered more suitable for colonization when processed by impacts. We describe how heavily shocked gneissic crystalline basement rocks exposed at the Haughton impact structure, Devon Island, Nunavut, Arctic Canada, are hosts to endolithic photosynthetic microorganisms in significantly greater abundance than lesser‐shocked or unshocked gneisses. Two factors contribute to this enhancement: (a) increased porosity due to impact fracturing and differential mineral vaporization, and (b) increased translucence due to the selective vaporization of opaque mineral phases. Using biological ultraviolet radiation dosimetry, and by measuring the concentrations of photoprotective compounds, we demonstrate that a covering of 0.8 mm of shocked gneiss can provide substantial protection from ultraviolet radiation, reducing the inactivation of Bacillus subtilis spores by 2 orders of magnitude. The colonisation of the shocked habitat represents a potential mechanism for pioneer microorganisms to invade an impact structure in the earliest stages of post‐impact primary succession. The communities are analogous to the endolithic communities associated with sedimentary rocks in Antarctica, but because they occur in shocked crystalline rocks, they illustrate a mechanism for the creation of microbial habitats on planetary surfaces that do not have exposed sedimentary units. This might have been the case on early Earth. The data have implications for the microhabitats in which biological signatures might be sought on Mars.  相似文献   

13.
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14.
Abstract— Sandstones are a common habitat for lithophytic microorganisms, including cryptoendoliths. We describe laboratory experiments on the colonization of impact metamorphosed sandstones from the Haughton impact structure, Canadian High Arctic. Colonization experiments with the coccoid cyanobacterium, Chroococcidiopsis sp. and the motile gram‐positive bacterium Bacillus subtilis, show that, in contrast to initially low porosity crystalline target rocks, which can become more porous as a result of impact bulking, by closing pore spaces the sedimentary cryptoendolithic habitat can be impoverished by impact. However, the heterogeneous distribution of collapsed pores, melt phases, and subsequent recrystallization, results in heterogeneous colonization patterns. Cavities and vesicles formed during melting can yield new habitats for both cryptoendoliths and chasmoendoliths, manifested in the natural cryptoendolithic colonization of shocked sandstones. By contrast, post‐impact thermal annealing and recrystallization of impact melt phases destroys the cavities and vesicles. In extreme cases, complete recrystallization of the rock fabric makes the material suitable only for epilithic, and potentially hypolithic, colonists. These experiments further our understanding of the influence of the target lithology on the effects of asteroid and comet impacts on habitats for lithophytic microorganisms.  相似文献   

15.
Abstract— The 4 km wide and 500 m deep circular Kärdla impact structure in Hiiumaa Island, Estonia, of middle Ordovician age (～455 Ma), is buried under Upper Ordovician and Quaternary sediments. To constrain the geophysical models of the structure, petrophysical properties such as magnetic susceptibility, natural remanent magnetization (NRM), density, electrical conductivity, porosity and P-wave velocity were measured on samples of crystalline and sedimentary rocks collected from drill cores in different parts of the structure and the surrounding area. The results were used to interpret the central gravity anomaly of ?3 mGal and the magnetic anomaly of ?100 nT and also the surrounding weak positive anomalies revealed by high precision survey data. The unshocked granitic rocks outside the structure have a mean density of ～2630 kgm?3. Their shocked counterparts have densities of ～2400 kgm?3 at a depth of ～500 m, increasing up to 2550 kgm?3 at a depth of 850 m. Porosity and electrical conductivity decrease, but P-wave velocity increases as density increases away from the impact point. Thus, the gradual changes in the physical properties of the rocks as a function of radial distance from the crater centre are consistent with an impact origin for Kärdla. As in many other impact structures, the magnetization of the shocked rocks are also clearly lower than those of unshocked target rocks. A new geophysical and geological model of the Kärdla structure is presented based on geophysical field measurements and data on gradual changes in petrophysical parameters of the shocked target and overlying rocks, together with structural data from numerous boreholes. An important feature of this model is the lack of an observable geophysical signature of the central uplift observed in drillcores.  相似文献   

16.
A study of pure, single crystal calcite shocked to pressures from 9.0 to 60.8 GPa was conducted to address contradictory data for carbonate shock behavior. The recovered materials were analyzed optically and by transmission electron microscopy (TEM), as well as by thermogravimetry (TGA), X‐ray diffraction (XRD), and Raman‐spectroscopy. In thin section, progressive comminution of calcite is observed although grains remain birefringent to at least 60.8 GPa. TGA analysis reveals a positive correlation between percent of mass loss due to shock and increasing shock pressure (R = 0.77) and suggests that shock loading leads to the modest removal of structural volatiles in this pressure range. XRD patterns of shocked Iceland spar samples produce peaks that are qualitatively and quantitatively less intense, more diffuse, and shift to lower o2θ. However, the regularity observed in these shocked powder patterns suggests that structures with very uniform unit cell separations persist to shock pressures as high as 60.8 GPa. Raman spectral analyses indicate no band asymmetry and no systematic peak shifting or broadening. TEM micrographs display progressively diminishing crystallite domain sizes. Selected area electron diffraction (SAED) patterns reveal no signatures of amorphous material. These data show that essentially intact calcite is recovered at shock pressures up to 60.8 GPa with only slight mass loss (~7%). This work suggests that the amount of CO2 gas derived from shock devolatilization of carbonate by large meteorite impacts into carbonate targets has been (substantially) overestimated.  相似文献   

17.
Abstract– The Lonar crater in Maharashtra state, India, has been completely excavated on the Deccan Traps basalt (approximately 65 Ma) at approximately 570 ± 47 ka by an oblique impact of a possible chondritic asteroid that struck the preimpact target from the east at an angle of approximately 30–45o to the horizon where the total duration of the shock event was approximately 1 s. It is shown by our early work that the distribution of ejecta and deformation of target rocks around the crater rim are symmetrical to the east–west plane of impact ( Misra et al. 2010 ). The present study shows that some of the rock magnetic properties of these shocked target basalts, e.g., low‐field anisotropy of magnetic susceptibility (AMS), natural remanent magnetization (NRM)/bulk susceptibility (χ), and high‐coercivity and high‐temperature (HC_HT) magnetization component, are also almost symmetrically oriented with reference to the plane of impact. Studies on the relative displacements of K3 (minimum) AMS axes of shocked basalts from around the crater rim and from the adjacent target rocks to the approximately 2–3 km west of the crater center suggest that the impact stress could have branched out into the major southwestward and northwestward components in the downrange direction immediately after the impact. The biaxial distribution of AMS axes in stereographic plots for the unshocked basalts transforms mostly into triaxial distribution for the shocked basalts, although transitional type distribution also exists. The degree of anisotropy (P′) of AMS ellipsoids of the shocked basalts decreases by approximately 2% when compared with those of the unshocked target (approximately 1.03). The NRM/χ (Am?1) values of the shocked basalts on the rim of the Lonar crater do not show much change in the uprange or downrange direction on and close to the east–west plane of impact, and the values are only approximately 1.5 times higher on average over the unshocked basalts around the crater. However, the values become approximately 1.4–16.4 times higher for the shocked basalts on the crater rim, which occur obliquely to the plane of impact. The target basalts at approximately 2–3 km west of the crater center in the downrange also show a significant increase (up to approximately 26 times higher) in NRM/χ. The majority of the shocked basalt samples (approximately 73%) from around the crater rim, in general, show a lowering of REM, except those from approximately 2–3 km west of the crater center in the downrange, where nearly half of the sample population shows a higher REM of approximately 3.63% in average. The shocked target basalts around the Lonar crater also acquired an HC_HT magnetization component due to impact. These HC_HT components are mostly oriented in the uprange direction and are symmetrically disposed about the east–west plane of impact, making an obtuse angle with the direction of impact. The low‐coercivity and low‐temperature (LC_LT) components of both the unshocked and shocked basalts are statistically identical to the present day field (PDF) direction. This could be chemical and/or viscous remanent magnetization acquired by the target basalts during the last 570 ± 47 ka, subsequent to the formation of the Lonar crater. The shocked Lonar target basalts appear to have remagnetized under high impact shock pressure and at low temperature of approximately 200–300 °C, where Ti‐rich titanomagnetite was the main magnetic remanence carrier.  相似文献   

18.
Baszkówka is an equilibrated, apparently low‐shock, unusually porous chondrite. Some earlier studies were undertaken to understand whether the porosity in Baszkówka, and similar porous chondrites, is a relic of a primordial feature or rather the effect of atypical reprocessing on the parent body. Neither of the studies reconstructed the accurate thermal and deformational evolution of chondrites, however, while it is known that shock‐induced compaction is the main means to affect chondritic porosity. Here we use a combination of 3‐D and 2‐D petrographic examination to understand how the evolution of pores correlates with thermal and shock history recorded in the Baszkówka chondrite. The grain framework silicates in Baszkówka contain healed shock fractures—a clear recorder of significant shock process and postshock annealing. Simultaneously, metal grains do not exhibit any preferred orientation or fabric, which would be expected to develop in response to the deformation as recorded by silicates. We interpret this as evidence for re‐agglomeration and annealing of shocked material. Pore spaces in Baszkówka are connected and decorated by fine‐grained plagioclase‐dominated mass and bulky euhedral olivine crystals, which exhibit growth steps on crystal surfaces. The euhedral olivine must have formed owing to the condensation of a vapor, while plagioclase most likely crystallized from melted chondritic matrix. During the shock event, fine‐grained matrix in Baszkówka was melted and vaporized. Vapor expansion added to ballistic velocity led to ejection and opening of the pore spaces. After re‐agglomeration in a hot ejecta blanket the rock was annealed, melted material circulated in created pore spaces and vapor condensed.  相似文献   

19.
Generation and propagation of shock waves by meteorite impact is significantly affected by material properties such as porosity, water content, and strength. The objective of this work was to quantify processes related to the shock‐induced compaction of pore space by numerical modeling, and compare the results with data obtained in the framework of the Multidisciplinary Experimental and Modeling Impact Research Network (MEMIN) impact experiments. We use mesoscale models resolving the collapse of individual pores to validate macroscopic (homogenized) approaches describing the bulk behavior of porous and water‐saturated materials in large‐scale models of crater formation, and to quantify localized shock amplification as a result of pore space crushing. We carried out a suite of numerical models of planar shock wave propagation through a well‐defined area (the “sample”) of porous and/or water‐saturated material. The porous sample is either represented by a homogeneous unit where porosity is treated as a state variable (macroscale model) and water content by an equation of state for mixed material (ANEOS) or by a defined number of individually resolved pores (mesoscale model). We varied porosity and water content and measured thermodynamic parameters such as shock wave velocity and particle velocity on meso‐ and macroscales in separate simulations. The mesoscale models provide additional data on the heterogeneous distribution of peak shock pressures as a consequence of the complex superposition of reflecting rarefaction waves and shock waves originating from the crushing of pores. We quantify the bulk effect of porosity, the reduction in shock pressure, in terms of Hugoniot data as a function of porosity, water content, and strength of a quartzite matrix. We find a good agreement between meso‐, macroscale models and Hugoniot data from shock experiments. We also propose a combination of a porosity compaction model (ε–α model) that was previously only used for porous materials and the ANEOS for water‐saturated quartzite (all pore space is filled with water) to describe the behavior of partially water‐saturated material during shock compression. Localized amplification of shock pressures results from pore collapse and can reach as much as four times the average shock pressure in the porous sample. This may explain the often observed localized high shock pressure phases next to more or less unshocked grains in impactites and meteorites.  相似文献   

20.
Abstract– Although the meteorite impact origin of the Keurusselkä impact structure (central Finland) has been established on the basis of the occurrence of shatter cones, no detailed microscopic examination of the impactites from this structure has so far been made. Previous microscope investigations of in situ rocks did not yield any firm evidence of shock features (Raiskila et al. 2008; Kinnunen and Hietala 2009). We have carried out microscopic observations on petrographic thin sections from seven in situ shatter cone samples and report here the discovery of planar fractures (PFs) and planar deformation features (PDFs) in quartz and feldspar grains. The detection and characterization of microscopic shock metamorphic features in the investigated samples substantiates a meteorite impact origin for the Keurusselkä structure. The crystallographic orientations of 372 PDF sets in 276 quartz grains were measured, using a universal stage (U‐stage) microscope, for five of the seven distinct shatter cone samples. Based on our U‐stage results, we estimate that investigated shatter cone samples from the Keurusselkä structure have experienced peak shock pressures from approximately 2 GPa to slightly less than 20 GPa for the more heavily shocked samples. The decoration of most of the PDFs with fluid inclusions also indicates that these originally amorphous shock features were altered by postimpact processes. Finally, our field observations indicate that the exposed surface corresponds to the crater floor; it is, however, difficult to estimate the exact diameter of the structure and the precise amount of material that has been eroded since its formation.  相似文献   

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