首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
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   

2.
The infrared behavior of experimentally shock-loaded quartz was studied in the wavenumber region 1400 to 100 cm?1. In agreement with results of X-ray investigations reported in an earlier paper, the infrared studies indicate that solid-state (diaplectic or thetomorphic) SiO2-glass is formed upon release from shock pressures of about > 14.0 GPa; complete transformation occurs upon release from about 30.0 GPa. The structure of the solid-state glass must be quite different from that of fused SiO2. While fused silica is supposed to consist of small “crystallites,” or of a network of SiO4-tetrahedra groups of tridymite-like short-range order, the positions of the infrared absorption bands of the shock-produced solid-state quartz glass lie practically at the same wave numbers as crystalline quartz. We conclude that diaplectic quartz glass consists structurally of extremely small quartz-like “crystallites.” These crystallites are mutually linked in a disordered but structurally more open manner as in α-quartz. The formation of short-range ordered quartz-type solid-state SiO2 glass is explained by the decomposition of a sixfold coordinated stishovite-like high pressure phase upon pressure release at the relatively low shock temperatures (≤ 300°C at 30.0 GPa). The extremely short duration of the shock process may prevent the growth of the “quartz nuclei” to long-range ordered crystallites.  相似文献   

3.
Abstract– As part of the MEMIN research program this project is focused on shock deformation experimentally generated in dry, porous Seeberger sandstone in the low shock pressure range from 5 to 12.5 GPa. Special attention is paid to the influence of porosity on progressive shock metamorphism. Shock recovery experiments were carried out with a high‐explosive set‐up that generates a planar shock wave, and using the shock impedance method. Cylinders of sandstone of average grain size of 0.17 mm and porosity of about 19 vol%, and containing some 96 wt% SiO2, were shock deformed. Shock effects induced with increasing shock pressure include: (1) Already at 5 GPa the entire pore space is closed; quartz grains show undulatory extinction. On average, 134 fractures per mm are observed. Dark vesicular melt (glass) of the composition of the montmorillonitic phyllosilicate component of this sandstone occurs at an average amount of 1.6 vol%. (2) At 7.5 GPa, quartz grains show weak but prominent mosaicism and the number of fractures increases to 171 per millimeter. Two additional kinds of melt, both based on phyllosilicate precursor, could be observed: a light colored, vesicular melt and a melt containing large iron particles. The total amount of melt (all types) increased in this experiment to 2.4 vol%. Raman spectroscopy confirmed the presence of shock‐deformed quartz grains near the surface. (3) At 10 and 12.5 GPa, quartz grains also show weak but prominent mosaicism, the number of fractures per mm has reached a plateau value of approximately 200, and the total amount of the different melt types has increased to 4.8 vol%. Diaplectic quartz glass could be observed locally near the impacted surface. In addition, local shock effects, most likely caused by multiple shock wave reflections at sandstone‐container interfaces, occur throughout the sample cylinders and include locally enhanced formation of PDF, as well as shear zones associated with cataclastic microbreccia, diaplectic quartz glass, and SiO2 melt. Overall findings from these first experiments have demonstrated that characteristic shock effects diagnostic for the confirmation of impact structures and suitable for shock pressure calibration are rare. So far, they are restricted to the limited formation of PDF and diaplectic quartz glass at shock pressures of 10 GPa and above.  相似文献   

4.
  相似文献   

5.
Abstract– The 3.8 km Steinheim Basin in SW Germany is a complex impact crater with central uplift hosted by a sequence of Triassic to Jurassic sedimentary rocks. It exhibits a well‐preserved crater morphology, intensely brecciated limestone blocks that form the crater rim, as well as distinct shatter cones in limestones. In addition, an impact breccia mainly composed of Middle to Upper Jurassic limestones, marls, mudstones, and sandstones is known from drilling into the impact crater. No impact melt lithologies, however, have so far been reported from the Steinheim Basin. In samples of the breccia that were taken from the B‐26 drill core, we discovered small particles (up to millimeters in size) that are rich in SiO2 (～50 wt%) and Al2O3 (～28 wt%), and contain particles of Fe‐Ni‐Co sulfides, as well as target rock clasts (shocked and unshocked quartz, feldspar, limestone) and droplet‐shaped particles of calcite. The particles exhibit distinct flow structures and relicts of schlieren and vesicles. From the geochemical composition and the textural properties, we interpret these particles as mixed silicate melt fragments widely recrystallized, altered, and/or transformed into hydrous phyllosilicates. Furthermore, we detected schlieren of lechatelierite and recrystallized carbonate melt. On the basis of impactite nomenclature, the melt‐bearing impact breccia in the Steinheim Basin can be denominated as Steinheim suevite. The geochemical character of the mixed melt particles points to Middle Jurassic sandstones (“Eisensandstein” Formation) that crop out at the center of the central uplift as the source for the melt fragments.  相似文献   

6.
Abstract— We have studied carbonate and associated oxides and glasses in a demountable section of Allan Hills 84001 (ALH 84001) using optical, scanning, and transmission electron microscopy (TEM) to elucidate their origins and the shock history of the rock. Massive, fracture‐zone, and fracture‐filling carbonates in typical locations were characterized by TEM, X‐ray microanalysis, and electron diffraction in a comprehensive study that preserved textural and spatial relationships. Orthopyroxene is highly deformed, fractured, partially comminuted, and essentially unrecovered. Lamellae of diaplectic glass and other features indicate shock pressures >30 GPa. Bridging acicular crystals and foamy glass at contacts of orthopyroxene fragments indicate localized melting and vaporization of orthopyroxene. Carbonate crystals are >5 mm in size, untwinned, and very largely exhibit the R3c calcite structure. Evidence of plastic deformation is generally found mildly only in fracture‐zone and fracture‐filling carbonates, even adjacent to highly deformed orthopyroxene, and appears to have been caused by low‐stress effects including differential shrinkage. High dislocation densities like those observed in moderately shocked calcite are absent. Carbonate contains impactderived glasses of plagioclase, silica, and orthopyroxene composition indicating brief localized impact heating. Stringers and lenses of orthopyroxene glass in fracture‐filling carbonate imply flow of carbonates and crystallization during an impact. Periclase (MgO) occurs in magnesite as 30–50 nm crystals adjacent to voids and negative crystals and as ?1 μm patches of 3 nm crystals showing weak preferred orientation consistent with (111)MgO//(0001)carb, as observed in the thermal decomposition of CaCO3 to CaO. Magnetite crystals that are epitaxially oriented at voids, negative crystals, and microfractures clearly formed in situ. Fully embedded, faceted magnetites are topotactically oriented, in general with (111)mag//(0001)carb, so that their oxygen layers are aligned. In optically opaque rims, magnetites are more irregularly shaped and, except for the smallest crystals, poorly aligned. All magnetite and periclase crystals probably formed by exsolution from slightly non‐stoichiometric, CO2‐poor carbonate following impact‐induced thermal decomposition. Any magnetites that existed in the rock before shock heating could not have preserved evidence for biogenic activity.  相似文献   

7.
Shatter cones are diagnostic for the recognition of meteorite impact craters. They are unambiguously identifiable in the field and the only macroscopic shock deformation feature. However, the physical boundary conditions and exact formation mechanism(s) are still a subject of debate. Melt films found on shatter cone surfaces may allow the constraint of pressure–temperature conditions during or immediately after their formation. Within the framework of the MEMIN research group, we recovered 24 shatter cone fragments from the ejecta of hypervelocity impact experiments. Here, we focus on silicate melt films (now quenched to glass) found on shatter cone surfaces formed in experiments with 20–80 cm sized sandstone targets, impacted by aluminum and iron meteorite projectiles of 5 and 12 mm diameter at velocities of 7.0 and 4.6 km s−1, respectively. The recovered shatter cone fragments vary in size from 1.2 to 9.3 mm. They show slightly curved, striated surfaces, and conical geometries with apical angles of 36°–52°. The fragments were recovered from experiments with peak pressures ranging from 46 to 86 GPa, and emanated from a zone within 0.38 crater radii. Based on iSale modeling and petrographic investigations, the shatter coned material experienced low bulk shock pressures of 0.5–5 GPa, whereas deformation shows a steep increase toward the shatter cone surface leading to localized melting of the rock, resulting in both vesicular as well as polished melt textures visible under the SEM. Subjacent to the melt films are zones of fragmentation and brittle shear, indicating movement away from the shatter cone apex of the rock that surrounds the cone. Smearing and extension of the melt film indicates subsequent movement in opposite direction to the comminuted and brecciated shear zone. We believe the documented shear textures and the adjacent smooth melt films can be related to frictional melting, whereas the overlying highly vesiculated melt layer could indicate rapid pressure release. From the observation of melting and mixing of quartz, phyllosilicates, and rutile in this overlying texture, we infer high, but very localized postshock temperatures exceeding 2000 °C. The melted upper part of the shatter cone surface cross-cuts the fragmented lower section, and is accompanied by PDFs developed in quartz parallel to the {112} plane. Based on the overprinting textures and documented shock effects, we hypothesize shatter cones start to form during shock loading and remain an active fracture surface until pressure release during unloading and infer that shatter cone surfaces are mixed mode I/II fracture surfaces.  相似文献   

8.
Kamil is a 45 m diameter impact crater identified in 2008 in southern Egypt. It was generated by the hypervelocity impact of the Gebel Kamil iron meteorite on a sedimentary target, namely layered sandstones with subhorizontal bedding. We have carried out a petrographic study of samples from the crater wall and ejecta deposits collected during our first geophysical campaign (February 2010) in order to investigate shock effects recorded in these rocks. Ejecta samples reveal a wide range of shock features common in quartz‐rich target rocks. They have been divided into two categories, as a function of their abundance at thin section scale: (1) pervasive shock features (the most abundant), including fracturing, planar deformation features, and impact melt lapilli and bombs, and (2) localized shock features (the least abundant) including high‐pressure phases and localized impact melting in the form of intergranular melt, melt veins, and melt films in shatter cones. In particular, Kamil crater is the smallest impact crater where shatter cones, coesite, stishovite, diamond, and melt veins have been reported. Based on experimental calibrations reported in the literature, pervasive shock features suggest that the maximum shock pressure was between 30 and 60 GPa. Using the planar impact approximation, we calculate a vertical component of the impact velocity of at least 3.5 km s?1. The wide range of shock features and their freshness make Kamil a natural laboratory for studying impact cratering and shock deformation processes in small impact structures.  相似文献   

9.
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   

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— The well‐preserved state and excellent exposure at the 39 Ma Haughton impact structure, 23 km in diameter, allows a clearer picture to be made of the nature and distribution of hydrothermal deposits within mid‐size complex impact craters. A moderate‐ to low‐temperature hydrothermal system was generated at Haughton by the interaction of groundwaters with the hot impact melt breccias that filled the interior of the crater. Four distinct settings and styles of hydrothermal mineralization are recognized at Haughton: a) vugs and veins within the impact melt breccias, with an increase in intensity of alteration towards the base; b) cementation of brecciated lithologies in the interior of the central uplift; c) intense veining around the heavily faulted and fractured outer margin of the central uplift; and d) hydrothermal pipe structures or gossans and mineralization along fault surfaces around the faulted crater rim. Each setting is associated with a different suite of hydrothermal minerals that were deposited at different stages in the development of the hydrothermal system. Minor, early quartz precipitation in the impact melt breccias was followed by the deposition of calcite and marcasite within cavities and fractures, plus minor celestite, barite, and fluorite. This occurred at temperatures of at least 200 °C and down to ?100–120 °C. Hydrothermal circulation through the faulted crater rim with the deposition of calcite, quartz, marcasite, and pyrite, occurred at similar temperatures. Quartz mineralization within breccias of the interior of the central uplift occurred in two distinct episodes (?250 down to ?90 °C, and <60 °C). With continued cooling (<90 °C), calcite and quartz were precipitated in vugs and veins within the impact melt breccias. Calcite veining around the outer margin of the central uplift occurred at temperatures of ?150 °C down to <60 °C. Mobilization of hydrocarbons from the country rocks occurred during formation of the higher temperature calcite veins (>80 °C). Appreciation of the structural features of impact craters has proven to be key to understanding the distribution of hydrothermal deposits at Haughton.  相似文献   

12.
The Terny impact structure, located in central Ukraine, displays a variety of diagnostic indicators of shock metamorphism, including shatter cones, planar deformation features in quartz, diaplectic glass, selective melting of minerals, and whole rock melting. The structure has been modified by erosion and subsequently buried by recent sediments. Although there are no natural outcrops of the deformed basement rocks within the area, mining exploration has provided surface and subsurface access to the structure, exposing impact melt rocks, shocked parautochthonous target rocks, and allochthonous impact breccias, including impact melt‐bearing breccias similar to suevites observed at the Ries structure. We have collected and studied samples from surface and subsurface exposures to a depth of approximately 750 m below the surface. This analysis indicates the Terny crater is centered on geographic coordinates 48.13° N, 33.52° E. The center location and the distribution of shock pressures constrain the transient crater diameter to be no less than approximately 8.4 km. Using widely accepted morphometric scaling relations, we estimate the pre‐erosional rim diameter of Terny crater to be approximately 16–19 km, making it close in original size to the well‐preserved El'gygytgyn crater in Siberia. Comparison with El'gygytgyn yields useful insights into the original morphology of the Terny crater and indicates that the amount of erosion Terny experienced prior to burial probably does not exceed 320 m.  相似文献   

13.
Abstract— Field studies and a shallow drilling program carried out in 1999 provided information about the thickness and distribution of suevite to the north of the Bosumtwi crater rim. Suevite occurrence there is known from an ?1.5 km2 area; its thickness is ≤15 m. The present suevite distribution is likely the result of differential erosion and does not reflect the initial areal extent of continuous Bosumtwi ejecta deposits. Here we discuss the petrographic characteristics of drill core samples of melt‐rich suevite. Macroscopic constituents of the suevites are melt bodies and crystalline and metasedimentary rock (granite, graywacke, phyllite, shale, schist, and possibly slate) clasts up to about 40 cm in size. Shock metamorphic effects in the clasts include multiple sets of planar deformation features (PDFs), diaplectic quartz and feldspar glasses, lechatelierite, and ballen quartz, besides biotite with kink bands. Basement rock clasts in the suevite represent all stages of shock metamorphism, ranging from samples without shock effects to completely shock‐melted material that is indicative of shock pressures up to ?60 GPa.  相似文献   

14.
Abstract— About 100 cobble-sized samples collected from the surface of the central polymict breccia formation of Haughton impact crater, Canada, have been studied microscopically and chemically. Breccia clasts derived from the 1700 m deep Precambian basement consist of 13 rock types which can be grouped into sillimanite- and garnet-bearing gneiss; alkali feldspar-rich aplitic or biotite-hornblende-bearing gneiss; biotite and hornblende gneiss; apatite-rich biotite and biotite-hornblende gneiss; calcitediopside gneiss; amphibolite; tonalitic orthogneiss; and basalts. The range of chemical compositions of these rocks is wide: e.g., SiO2 ranges from 40–85 wt.%; Al2O3 from 7–20 wt.%; CaO from 0.01–25 wt.%; or P2Os from <0.01–5 wt.%. Nearly all samples of crystalline rocks are shock metamorphosed up to about 60 GPa. Most conspicuous is the absence of whole-rock melts and the very rare occurrence of unshocked rocks. The 45 samples examined can be classified into the following shock stages: stage 0 (<5 GPa): 4.5%, stage Ia (10–20 GPa): 9.0%, stage Ib (20–35 GPa): 33%, stage II (35–45 GPa): 29%, stage III (45–55 GPa): 18%, stage III–IV (55–60 GPa): 6.5%. Among Paleozoic sedimentary rock clasts higher degrees of shock than within crystalline rocks were observed such as highly vesiculated, whole-rock melts of sandstones and shales. Within the northern and eastern sectors of the allochthonous breccia no distinct radial variation of the cobble-sized lithic clasts regarding abundance, rock type, and degree of shock was observed, with the exception that clasts of shock-melted sedimentary rocks and of highly shocked basement rocks (stage III–IV) are strongly concentrated near the center of the crater. Based on our field and laboratory investigations we conclude that vaporization and melting due to the Haughton impact affected the lower section of the sedimentary strata from about 900 to 1700 m depth (Eleanor River limestones and dolomites, Lower Ordovician and Cambrian limestones, dolomites, shales, and sandstones). The 60-GPa shock pressure isobar reached only the uppermost basement rocks so that whole rock melting of the crystalline rocks was not possible.  相似文献   

15.
Abstract— The central allochthonous polymict breccia of the Haughton impact structure is up to about 90 m thick and as much as 7.3 km in radial extent. It has been analyzed with respect to modal composition, grain-size characteristics, and degree of shock metamorphism for the grain-size ranges 10–～ 50, 1–10, 0.03–1, and <0.03 mm. The mineralogy of the breccia matrix is dominated by dolomite and calcite, with minor amounts of quartz, other silicate minerals, and rare melt particles. The following lithic clasts have been identified in the 1–10 mm size fraction (averages of vol.% given in parentheses): dolomitic rocks (51), limestones (29), crystalline rocks (10), sandstones and siltstones (3.7), chert (0.7), melt particles (1.9). The mineral clasts (1–0.03 mm) comprise (with decreasing frequency) dolomite, quartz, calcite, feldspar, biotite, amphibole, garnet, opaques, rounded quartz derived from sandstones and accessory minerals. Lithic and mineral clasts display various degrees of shock. Fragments of crystalline rocks are shocked in the 0–60 GPa range; whole rock melts from the crystalline basement are lacking and unshocked rocks are very rare. In contrast, shock-melted sandstones, shales, and chert were found in most samples. Large clasts of these melt rocks are highly concentrated near the center of the crater. Otherwise, no distinct change of the modal composition with radial range has been observed except that the frequency of limestone clasts increases slightly with radial range. The breccia near the center is more fine-grained than that beyond about 1 km radius and the sorting parameter increases somewhat with radial range. Except for the high concentration of shock-melted sedimentary rocks and highly shocked crystalline rocks near the center of the crater, the distribution of shock stages within the lithic clast population is quite uniform throughout the breccia formation. We conclude that the breccia constituents are derived from the lower part of the target stratigraphy (deeper than about 800 m) and that the total depth of excavation at Haughton is in the order of 2000 m. The mixing of sedimentary rocks of the Eleanor River Formation, Lower Ordovician, and Cambrian (～850 m thickness) with crystalline basement rocks is quite thorough and homogeneous throughout the breccia lens, at least for the analyzed part. This may require an air-borne mode of emplacement for the upper section of the breccia in analogy to the fall-back suevite in the Ries crater. A calculation of the excavation (Z-model) and of the shock pressure attenuation based on reasonable estimates of the energy and crater geometry of the Haughton impact confirms the observed maximum depth of excavation of about 2 km. Shock-melted crystalline basement rocks, if present at all, must be confined to the very center of the structure below the excavation cavity.  相似文献   

16.
Shock-induced recovery experiments were performed to investigate melt formation in porous sandstones in the low shock pressure regime between 2.5 and 17.5 GPa. The sandstone shocked at 2.5 and 5 GPa is characterized by pore closure, fracturing of quartz (Qtz), and compression and deformation of phyllosilicates; no melting was observed. At higher pressures, five different types of melts were generated around pores and alongside fractures in the sandstone. Melting of kaolinite (Kln), illite (Ill), and muscovite (Ms) starts at 7.5, 12, and 15 GPa, respectively. The larger the amount of water in these minerals (Kln ~14 wt%, Ill ~6–10 wt%, and Ms ~4 wt% H2O), the higher the shock compressibility and the lower the shock pressure required to induce melting. Vesicles in the almost dry silicate glasses attest to the loss of structural water during the short shock duration of the experiment. The compositions of the phyllosilicate-based glasses are identical to the composition of the parental minerals or their mixtures. Thus, this study has demonstrated that phyllosilicates in shocked sandstone undergo congruent melting during shock loading. In experiments at 10 GPa and higher, iron melt from the driver plate was injected into the phyllosilicate melts. During this process, Fe is partitioned from the metal droplets into the surrounding silicate melts, which induced unmixing of silicate melts with different chemical properties (liquid immiscibility). At pressures between 7.5 and 15 GPa, a pure SiO2 glass was formed, which is located as short and thin bands within Qtz grains. These bands were shown to contain tiny crystals of experimentally generated stishovite.  相似文献   

17.
Abstract– In the context of the MEMIN project, a hypervelocity cratering experiment has been performed using a sphere of the iron meteorite Campo del Cielo as projectile accelerated to 4.56 km s?1, and a block of Seeberger sandstone as target material. The ejecta, collected in a newly designed catcher, are represented by (1) weakly deformed, (2) highly deformed, and (3) highly shocked material. The latter shows shock‐metamorphic features such as planar deformation features (PDF) in quartz, formation of diaplectic quartz glass, partial melting of the sandstone, and partially molten projectile, mixed mechanically and chemically with target melt. During mixing of projectile and target melts, the Fe of the projectile is preferentially partitioned into target melt to a greater degree than Ni and Co yielding a Fe/Ni that is generally higher than Fe/Ni in the projectile. This fractionation results from the differing siderophile properties, specifically from differences in reactivity of Fe, Ni, and Co with oxygen during projectile‐target interaction. Projectile matter was also detected in shocked quartz grains. The average Fe/Ni of quartz with PDF (about 20) and of silica glasses (about 24) are in contrast to the average sandstone ratio (about 422), but resembles the Fe/Ni‐ratio of the projectile (about 14). We briefly discuss possible reasons of projectile melting and vaporization in the experiment, in which the calculated maximum shock pressure does not exceed 55 GPa.  相似文献   

18.
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   

19.
Libyan Desert Glass (LDG) is an enigmatic natural glass, about 28.5 million years old, which occurs on the floor of corridors between sand dunes of the southwestern corner of the Great Sand Sea in western Egypt, near the Libyan border. The glass occurs as centimeter‐ to decimeter‐sized, irregularly shaped, and strongly wind‐eroded pieces. The origin of the LDG has been the subject of much debate since its discovery, and a variety of exotic processes were suggested, including a hydrothermal sol‐gel process or a lunar volcanic source. However, evidence of an impact origin of these glasses included the presence of schlieren and partly or completely digested minerals, such as lechatelierite, baddeleyite (a high‐T breakdown product of zircon), and the presence of a meteoritic component in some of the glass samples. The source material of the glass remains an open question. Geochemical data indicate that neither the local sands nor sandstones from various sources in the region are good candidates to be the sole precursors of the LDG. No detailed studies of all local rocks exist, though. There are some chemical and isotopic similarity to rocks from the BP and Oasis impact structures in Libya, but no further evidence for a link between these structures and LDG was found so far. These complications and the lack of a crater structure in the area of the LDG strewn field have rendered an origin by airburst‐induced melting of surface rocks as a much‐discussed alternative. About 20 years ago, a few shocked quartz‐bearing breccias (float samples) were found in the LDG strewn field. To study this question further, several basement rock outcrops in the LDG area were sampled during three expeditions in the area. Here we report on the discovery of shock‐produced planar microdeformation features, namely planar fractures (PFs), planar deformation features (PDFs), and feather features (FFs), in quartz grains from bedrock samples. Our observations show that the investigated samples were shocked to moderate pressure, of at least 16 GPa. We interpret these observations to indicate that there was a physical impact event, not just an airburst, and that the crater has been almost completely eroded since its formation.  相似文献   

20.
The fundamental approach for the confirmation of any terrestrial meteorite impact structure is the identification of diagnostic shock metamorphic features, together with the physical and chemical characterization of impactites and target lithologies. However, for many of the approximately 200 confirmed impact structures known on Earth to date, multiple scale‐independent tell‐tale impact signatures have not been recorded. Especially some of the pre‐Paleozoic impact structures reported so far have yielded limited shock diagnostic evidence. The rocks of the Dhala structure in India, a deeply eroded Paleoproterozoic impact structure, exhibit a range of diagnostic shock features, and there is even evidence for traces of the impactor. This study provides a detailed look at shocked samples from the Dhala structure, and the shock metamorphic evidence recorded within them. It also includes a first report of shatter cones that form in the shock pressure range from ~2 to 30 GPa, data on feather features (FFs), crystallographic indexing of planar deformation features, first‐ever electron backscatter diffraction data for ballen quartz, and further analysis of shocked zircon. The discovery of FFs in quartz from a sample of the MCB‐10 drill core (497.50 m depth) provides a comparatively lower estimate of shock pressure (~7–10 GPa), whereas melting of a basement granitoid infers at least 50–60 GPa shock pressure. Thus, the Dhala impactites register a strongly heterogeneous shock pressure distribution between <2 and >60 GPa. The present comprehensive review of impact effects should lay to rest the nonimpact genesis of the Dhala structure proposed by some earlier workers from India.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号