Planar deformation features in quartz grains from the resurge deposit of the Lockne structure,Sweden

Abstract— Microscopic planar deformation features (PDFs) in quartz grains are diagnostic of shock meta-morphism during hypervelocity impact cratering. Measurements of the poles of sets of PDFs and the optic axis of 25 quartz grains were carried out for a sample of the Loftarsten deposit from the Lockne area, Sweden. The most abundant PDFs observed in the sample from the Lockne area correspond to those found at known impact craters (i.e., ω (1013} and π (1012). This study confirms the previous suggestion that the Lockne structure is an impact crater. The Loftarsten is, therefore, interpreted as the final stage of resurge deposition after a marine impact at Lockne in the Middle Ordovician.     

An experimental study of silver and palladium partitioning between solid and liquid metal,with applications to iron meteorites

Abstract— Solid metal/liquid metal partition coefficients for Ag and Pd were determined experimentally as a function of the S concentration of the metallic liquid. Silver is incompatible in solid metal and strongly sensitive to the S content of the metallic liquid; partition coefficients for Ag decrease more than an order of magnitude with increasing S content of the metallic liquid and can be expressed as: where k(Ag) is the molar solid metal/liquid metal partition coefficient and X_S is the molar S content of the metallic liquid. The partition coefficient of Pd is less variable but changes from modestly incompatible to modestly compatible in solid metal with increasing S content of the metallic liquid: With these new partition coefficients for Pd and a fractional crystallization model, Pd abundance trends recorded in iron meteorite groups are modeled successfully. Measured Ag distribution between troilite-rich nodules and adjacent metal in iron meteorites also agree well with experimental solid metal/liquid metal equilibrium values. However, observed Pd metal/nodule distributions do not agree with experimentally determined partition coefficients, which suggests a more complex history than simple solid metal/liquid metal equilibrium.     

Investigation of the Rhabdite/Kamacite Law of Intergrowth in iron meteorites with the aid of the Kossel technique

Abstract— X-ray microdiffraction measurements based on the Kossel effect have been used for orientation determinations of rhabdite (i.e., small prismatic schreibersite crystals) with respect to the kamacite matrix. For that purpose, polished specimens of the Toluca meteorite have been analyzed after surface etching. Kossel patterns of kamacite and rhabdite have been recorded and simulated. As the law of intergrowth for idiomorphic rhabdite crystals, we confirmed the relations: In comparison with typical line widths, the Kossel lines of kamacite are distinctly broadened. This is found for the meteorite Toluca and a for a second sample, the meteorite Morasko. This behaviour is probably connected with a high dislocation density, as shown by transmission electron microscope investigations.     

Shocked quartz grains in the polymict breccia of the Granby structure,Sweden–‐Verification of an impact

Abstract— The Middle Ordovician Granby structure in Sweden is generally considered the result of an asteroidal or cometary collision with Earth, although no hard evidence, i.e., shock metamorphic features or traces of the impactor, have been presented to date. In this study, drill core samples of a sedimentary breccia from the Granby structure have been investigated for microscopic shock metamorphic evidence in an attempt to verify the impact genesis of the structure. The finding of multiple sets of decorated planar deformation features (PDFs) in quartz grains in these samples provides unambiguous evidence that the structure is impact derived. Furthermore, the orientation of the PDFs, e.g., ω {101 }, π {101 } and r, z {101 }, is characteristic for impact deformation. The fact that a majority of the PDFs are decorated implies a water‐bearing target. The shocked quartz grains can be divided into two groups; rounded grains found in the breccia matrix likely originated from mature sandstone, and angular grains in fragments from crystalline target rocks. The absence of melt particles provides an estimated maximum shock pressure for the sedimentary derived quartz of 15–20 GPa and the frequency distribution of PDF orientations in the bedrock quartz implies pressures of the order of 10 GPa.     

Shock metamorphism of quartz at the submarine Mjølnir impact crater,Barents Sea

Abstract— Shock metamorphosed quartz grains have been discovered in a drill core from the central peak of the Late Jurassic, marine Mjølnir structure; this finding further corroborates the impact origin of Mjølnir. The intersected strata represent the Upper Jurassic Hekkingen Formation and underlying Jurassic and Upper Triassic formations. The appearance, orientation, and origin of shock features in quartz grains and their stratigraphic distribution within the core units have been studied by optical and transmission electron microscopy. The quartz grains contain planar fractures (PFs), planar deformation features (PDFs), and mechanical Brazil twins. The formation of PFs is the predominant shock effect and is attributed to the large impedance differences between the water‐rich pores and constituent minerals in target sediments. This situation may have strengthened tensional/extensional and shear movements during shock compression and decompression. The combination of various shock effects indicates possible shock pressures between 5 and at least 20 GPa for three core units with a total thickness of 86 m (from 74.00 m to 171.09 m core depth). Crater‐fill material from the lower part of the core typically shows the least pressures, whereas the uppermost part of the allochthonous crater deposits displays the highest pressures. The orientations of PFs in studied quartz grains seem to become more diverse as the pressure rises from predominantly (0001) PFs to a combination of (0001), , and orientations. However, the lack of experimental data on porous sedimentary rocks does not allow us to further constrain the shock conditions on the basis of PF orientations.     

Red Wing Creek structure,North Dakota: Petrographical and geochemical studies,and confirmation of impact origin

Abstract The 9 km diameter Red Wing Creek structure, North Dakota, is located within the oil-rich Williston Basin at 47°36′N and 103°33′W. Earlier geophysical studies indicated that this subsurface structure has a central uplift, surrounded by an annular crater moat, and a raised rim. Breccias were encountered during drilling between ～2000 and 2800 m depth in the central uplift area, and the presence of shatter cone fragments in drill core samples was suggested to indicate an impact origin of the Red Wing Creek structure. We studied the petrographic and geochemical characteristics of samples of well cuttings from two boreholes at the center of the structure: the True Oil 22–27 Burlington Northern and True Oil 11–27 Burlington Northern wells. We found planar deformation features (PDFs) in quartz with up to three sets of different crystallographic orientations in sandstone- and siltstone-dominated samples from the True Oil 11–27 borehole. U-stage measurements of the crystallographic orientations of the PDFs showed the occurrence of the shock-characteristic (0001), and orientations, with a dominance of (0001) and orientations. The relative frequencies of the orientations indicate a shock pressure of at least 12–20 GPa. These results provide unambiguous evidence for shock metamorphism at Red Wing Creek and confirm that the structure was formed by impact.     

A new titanium-bearing calcium aluminosilicate phase: II. Crystallography and crystal chemistry of grains formed in slowly cooled melts with bulk compositions of calcium- aluminum-rich inclusions

Abstract The crystallography and crystal chemistry of a new calcium-titanium-aluminosilicate mineral (UNK) observed in synthetic analogs to calcium-aluminum-rich inclusions (CAIs) from carbonaceous chondrites was studied by electron diffraction techniques. The unit cell is primitive hexagonal or trigonal, with a = 0.790 ± 0.002 nm and c = 0.492 ± 0.002 nm, similar to the lattice parameters of melilite and consistent with cell dimensions for crystals in a mixer furnace slag described by Barber and Agrell (1994). The phase frequently displays an epitactic relationship in which melilite acts as the host, with (0001)_UNK | (001)_mel and <10T0>_UNK | <100>_mel. If one of the two space groups determined by Barber and Agrell (1994) for their sample of UNK is applicable (P3ml or P31m), then the structure is probably characterized by puckered sheets of octahedra and tetrahedra perpendicular to the c-axis with successive sheets coordinated by planar arrays of Ca. In this likely structure, each unit cell contains three Ca sites located in mirror planes, one octahedrally coordinated cation located along a three-fold axis and five tetrahedrally coordinated cations, three in mirrors and two along triads. The octahedron contains Ti but, because there are 1.3–1.9 cations of Ti/formula unit, some of the Ti must also be in tetrahedral coordination, an unusual but not unprecedented situation for a silicate. Tetrahedral sites in mirror planes would contain mostly Si, with lesser amounts of Al while those along the triads correspondingly contain mostly Al with subordinate Ti. The structural formula, therefore, can be expressed as with Si + Ti = 4. Compositions of meteoritic and synthetic Ti-bearing samples of the phase can be described in terms of a binary solid solution between the end-members Ca₃TiAl₂Si₃O₁₄ and Ca₃Ti(AlTi)(AlSi₂)O₁₄. A Ti-free analog with a formula of Ca₃Al₂Si₄O₁₄ synthesized by Paque et al. (1994) is thought to be related structurally but with the octahedral site being occupied by Al, that is      

Experimental studies of magnetite formation in the solar nebula

Abstract— Oxidation of Fe metal and Gibeon meteorite metal to magnetite via the net reaction 3 Fe (metal) + 4 H₂O (gas) = Fe₃O₄ (magnetite) + 4 H₂ (gas) was experimentally studied at ambient atmospheric pressure at 91–442 °C in H₂ and H₂-He gas mixtures with H₂/H₂O molar ratios of ～4–41. The magnetite produced was identified by x-ray diffraction. Electron microprobe analyses showed 3.3 wt% NiO and 0.24 wt% CoO (presumably as NiFe₂O₄ and CoFe₂O₄) in magnetite formed from Gibeon metal. The NiO and CoO concentrations are higher than expected from equilibrium between metal and oxide under the experimental conditions. Elevated NiO contents in magnetite were also observed by metallurgists during initial stages of oxidation of Fe-Ni alloys. The rate constants for magnetite formation were calculated from the weight gain data using a constant surface area model and the Jander, Ginstling-Brounshtein, and Valensi-Carter models for powder reactions. Magnetite formation followed parabolic (i.e., diffusion-controlled) kinetics. The rate constants and apparent activation energies for Fe metal and Gibeon metal are: These rate constants are significantly smaller than the parabolic rate constants for FeS growth on Fe metal in H₂S-H₂ gas mixtures containing 1000 or 10 000 ppmv H₂S (Lauretta et al., 1996a). The experimental data for Fe and Gibeon metal are used to model the reaction time of Fe alloy grains in the solar nebula as a function of grain size and temperature. The reaction times for 0.1–1 μm radius metal grains are generally within estimated lifetimes of the solar nebula (0.1–10 Ma). However, the calculated reaction times are probably lower limits, and further study of magnetite formation at larger H₂/H₂O ratios, at lower temperatures and pressures, and as a function of metal alloy composition is needed for further modeling of nebular magnetite formation.     

Experimental study and TEM characterization of dusty olivines in chondrites: Evidence for formation by in situ reduction

Abstract— An analytical transmission electron microscopy (ATEM) study was undertaken in order to better understand the formation conditions of dusty olivines (i.e., olivines containing abundant tiny inclusions of Fe‐Ni metal) in primitive meteorites. Dusty olivines from type I chondrules in the Bishunpur chondrite (LL3.1) and from synthetic samples obtained by reduction of San Carlos olivines were examined. In both natural and experimental samples, micron size metal blebs observed in the dusty region often show preferential alignments along crystallographic directions of the olivine grains, have low Ni contents (typically <2 wt%), and are frequently surrounded by a silica‐rich glass layer. These features suggest that dusty olivines are formed by a sub‐solidus reduction of initially fayalitic olivines according to the following reaction: Some volatilization of SiO_gas may account for the apparent excess of metal relative to silica‐rich glass observed in both experimental and natural samples. Comparison with experimentally produced dusty olivines suggests that time scales of the order of minutes usually inferred for chondrule formation are also adequate for the formation of dusty olivines. These observations are in agreement with the hypothesis that at least part of the metal phase in chondrites originated from reduction during chondrule formation.     

Al‐Mg systematics of CAIs,POI, and ferromagnesian chondrules from Ningqiang

Abstract— We have made aluminum‐magnesium isotopic measurements on 4 melilite‐bearing calcium‐aluminum‐rich inclusions (CAIs), 1 plagioclase‐olivine inclusion (POI), and 2 ferromagnesian chondrules from the Ningqiang carbonaceous chondrite. All of the CAIs measured contain clear evidence for radiogenic ²⁶Mg* from the decay of ²⁶Al ( = 1.05 Ma). Although the low Al/Mg ratios of the melilites introduce large uncertainties, the inferred initial ²⁶Al/²⁷Al ratios for the CAIs are generally consistent with the value of 5 times 10^?5. There is clear evidence of ²⁶Al* in one POI and two chondrules, but with considerable uncertainties in the value of (²⁶Al/²⁷Al)₀. The (²⁶Al/²⁷Al)₀ ratios for the POI and the chondrules are 0.3–0.6 times 10^?5, roughly an order of magnitude lower than the canonical value. Ningqiang shows very little evidence of metamorphism as a bulk object and the (²⁶Al/²⁷Al)₀ ratios in its refractory inclusions and chondrules are consistent with those found in other unmetamorphosed chondrites of several different classes. Our observations and those of other workers support the view that ²⁶Al was widely and approximately homogeneously distributed throughout the condensed matter of the solar system. The difference in (²⁶Al/²⁷Al)₀ between CAIs and less refractory materials seems reasonably interpreted in terms of a ～2 million year delay between the formation of CAIs and the onset of formation of less refractory objects. The POI shows clear differences in ²⁵Mg/²⁴Mg between its constituent spinels and olivine, which confirms that they are partially reprocessed material from different sources that were rapidly quenched.     

Basaltic volcanism on the angrite parent body: Comparison with 4 Vesta

Carbon and nitrogen data from stepped combustion analysis of eight angrites, seven eucrites, and two diogenites, alongside literature data from a further nine eucrites and two diogenites, have been used to assess carbon and nitrogen incorporation and isotope fractionation processes on the angrite parent body (APB), for comparison with volatile behavior on the HED parent body (4 Vesta). A subset of the angrite data has been reported previously (Abernethy et al. 2013 ). Two separate families of volatile components were observed. They were (1) moderately volatile material (MVM), mostly combusting between ~500 and 750 °C and indistinguishable from terrestrial contamination and (2) refractory material (RM), mainly released above 750 °C and thought to be carbon (as ) and nitrogen (as N₂ or ) dissolved within the silicate lattice, fitting with the slightly oxidized (~IW to IW+2) angrite fO₂ conditions. Isotopic fractionation trends for carbon and nitrogen within the plutonic and basaltic (quenched) angrites suggest that the behavior of the two volatile elements is loosely coupled, but that the fractionation process differs between the two angrite subgroups. Comparison with results from eucrites and diogenites implies similarities between speciation of carbon and nitrogen on 4 Vesta and the APB, with the latter being more enriched in volatiles than the former.     

Vapor‐condensed phase processes in the early solar system

Abstract– Equilibrium thermodynamic calculations of the sequence of condensation of phases from a cooling gas of solar composition at total pressures thought to have prevailed in the inner part of the solar nebula successfully predict the primary mineral assemblages of refractory inclusions in CM2 and CV3 chondrites. Many refractory inclusions in CM2 chondrites contain a relatively SiO₂‐poor assemblage (spinel, hibonite, grossite, perovskite, corundum) that represents a high‐temperature stage of condensation, and some may be pristine condensates that escaped later melting. Compact Type A and Type B refractory inclusions, consisting of spinel, melilite, perovskite, Ca‐rich clinopyroxene ± anorthite, in CV3 chondrites are more SiO₂‐rich and equilibrated with the solar nebular gas at a slightly lower temperature. Textures of many of these objects indicate that they underwent melting after condensation, crystallizing into the same phase assemblage as their precursors. The Ti³⁺/Ti⁴⁺ ratio of their pyroxene indicates that this process occurred in a gas whose oxygen fugacity () was approximately 8.5 log units below that of the iron‐wüstite buffer, making them the only objects in chondrites known to have formed in a system whose composition was close to that of the sun. Relative to CI chondrites, these inclusions are uniformly enriched in a group of elements (e.g., Ca, REE, Zr, Ta, Ir) that are chemically diverse except for their high condensation temperatures in a system of solar composition. The enrichment factor, 17.5, can be interpreted to mean that these objects represent either the first 5.7 wt% of the condensable matter to condense during nebular cooling or the residue after vaporization of 94.3% of a CI chondrite precursor. The Mg and Si isotopic compositions of Types A and B inclusions are mass‐fractionated by up to 10 and 4 ‰/amu, respectively. When interpreted in terms of Rayleigh fractionation during evaporation of Mg and Si from the inclusions while they were molten, the isotopic compositions imply that up to 60% of the Mg and up to 25% of the Si were evaporated, and that approximately 80% of the enrichment in refractory (CaO+Al₂O₃) relative to more volatile (MgO+SiO₂) in the average inclusion is due to initial condensation and approximately 20% due to subsequent evaporation. The mineralogical composition, including the Ti³⁺/Ti⁴⁺ ratio of the pyroxene, in Inti, a particle sampled from Comet Wild 2 by the Stardust spacecraft, is nearly identical to that of a Type B inclusion, indicating that comets contain not only the lowest‐temperature condensates in the form of ices but the highest‐temperature condensates as well. The FeO/(FeO+MgO) ratios of olivine and pyroxene in the matrix and chondrules of carbonaceous and ordinary chondrites are too high to be made in a system of solar composition, requiring s only 1 or 2 log units below iron‐wüstite, more than 10⁵ times higher than that of a solar gas. Various ways have been devised to generate cosmic gases sufficiently oxidizing to stabilize significant FeO in olivine at temperatures above those where Fe‐Mg interdiffusion in olivine ceases. One is by vertical settling of dust toward the nebular midplane, enriching a region in dust relative to gas. Because dust is enriched in oxygen compared to carbon and hydrogen relative to solar composition, a higher results from total vaporization of the region, but the factor by which theoretical models have so far enriched the dust is 10 times too low. Another is by transporting icy bodies from the outer part of the nebula into the hot, inner part where vaporization of water ice occurs. Not only does this method fail to make the needed by a factor of 30–1000 but it also ignores simultaneous evaporation of carbon‐bearing ices that would make the even lower.     

Planar deformation features in quartz from impact‐produced polymict breccia of the Xiuyan crater,China

Abstract– The 1.8 km‐diameter Xiuyan crater is an impact structure in northeastern China, exposed in a Proterozoic metamorphic rock complex. The major rocks of the crater are composed of granulite, hornblendite, gneiss, tremolite marble, and marble. The bottom at the center of the crater covers about 100 m thick lacustrine sediments underlain by 188 m thick crater‐fill breccia. A layer of polymict breccia composed of clasts of granulite, gneiss, hornblendite, and fragments of glass as well as clastic matrix, occurs near the base, in the depth interval from 260 to 295 m. An investigation in quartz from the polymict breccia in the crater‐fill units reveals abundant planar deformation features (PDFs). Quartz with multiple sets of PDFs is found in clasts of granulite that consist of mainly quartz and feldspar, and in fine‐grained matrix of the impact‐produced polymict breccia. A universal stage was used to measure the orientation of PDFs in 70 grains of quartz from five thin sections made from the clasts of granulite of polymict breccia recovered at the depth of 290 m. Forty‐four percent of the quartz grains contain three sets of PDFs, and another 40% contain two sets of PDFs. The most abundant PDFs are rhombohedron forms of , , and with frequency of 33.5, 22.3, and 9.6%, respectively. A predominant PDF form of in quartz suggests a shock pressure >20 GPa. The occurrence of PDFs in quartz from the polymict breccia provides crucial evidence for shock metamorphism of target rocks and confirms the impact origin of this crater, which thus appears to be the first confirmed impact crater in China.     

Nonlinear simulations of magnetic instabilities in stellar radiation zones: The role of rotation and shear

Using the 3-dimensional ASH code, we have studied numerically the instabilities that occur in stellar radiation zones in presence of large-scale magnetic fields, rotation and large-scale shear. We confirm that some configurations are linearly unstable, as predicted by Tayler and collaborators, and we determine the saturation level of the instability. We find that rotation modifies the peak of the most unstable wave number of the poloidal instability but not its growth rate as much as in the case of the m = 1 toroidal instability for which it is changed to σ = /Ω. Further in the case with rotation and shear, we found no sign of the dynamo mechanism suggested recently by Spruit even though we possess the essential ingredients (Tayler's m = 1 instability and a large scale shear) supposedly at work. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the meaning and inapplicability of the Zeldovich relations of magnetohydrodynamics

Considering a plasma with an initially weak large scale field subject to nonhelical turbulent stirring, Zeldovich (1957), for two‐dimensions, followed by others for three dimensions, have presented formulae of the form 〈b²〉 = f(R_M) . Such “Zeldovich relations” have sometimes been interpreted to provide steady‐state relations between the energy associated with the fluctuating magnetic field and that associated with a large scale or mean field multiplied by a function f that depends on spatial dimension and a magnetic Reynolds number R_M. Here we dissect the origin of these relations and pinpoint pitfalls that show why they are inapplicable to realistic, dynamical MHD turbulence and that they disagree with many numerical simulations. For 2D, we show that when the total magnetic field is determined by a vector potential, the standard Zeldovich relation applies only transiently, characterizing a maximum possible value that the field energy can reach before necessarily decaying. In 3D, we show that the standard Zeldovich relations are derived by balancing subdominant terms. In contrast, balancing the dominant terms shows that the fluctuating field can grow to a value independent of R_M and the initially imposed , as seen in numerical simulations. We also emphasize that these Zeldovich relations of nonhelical turbulence imply nothing about the amount mean field growth in a helical dynamo. In short, by re‐analyzing the origin of the Zeldovich relations, we highlight that they are inapplicable to realistic steady‐states of large R_M MHD turbulence. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Zur kosmologischen Testung des Machschen Prinzips

The Machian models of isotropic expanding universes according to the “inertia-free” gravo-dynamics imply the equations between the instantan values H₀ and q₀ of the HUBBLE parameter H, the acceleration q, and the matter density o. Therefore, in Machian universes with linear expansion q₀ = 0 the energy integral E = -1/2ϵc² is zero and the matter density becomes (with H₀²R₀² = c²/3) (f₀ the Newtonian gravitational constant). This is the critical density in general relativistic cosmology.     

Depositional processes of impactites from the YAX-1 drill core in the Chicxulub impact structure inferred from vertical profiles of PDF orientations and grain size distributions of shocked quartz

Core samples from the Chicxulub impact structure provide insights into the formation processes of a shallow-marine-target, complex crater. Although previous studies investigated the impactites (generally suevitic and polymict breccias) of the Yaxcopoil-1 (YAX-1) drill core in the Chicxulub impact structure, the interpretation of its deposition remains controversial. Here, we analyze planar deformation features (PDFs), grain size, and abundance of shocked quartz throughout the YAX-1 impactite sequence (794–895 m in depth). PDF orientations of most quartz grains in YAX-1 impactites show a distribution of both low angles ({104}, {103}, {102}) and high angles (orientations higher than 55° to c-axis), while the lower part of the impactite sequence contains quartz showing only PDF orientations of low angles. High-abundance, coarse-grained shocked quartz is found from the lower to middle parts of the impactites, whereas it abruptly changes to low-abundance, fine-grained shocked quartz within the upper part. In the uppermost part of the impactites, repeated oscillations in contents of these two components are observed. PDF orientation pattern suggests most of the shocked quartz grains experienced a range of shock pressure, except two samples in the lower part of impactites, which experienced only a high level of shock. We suggest that the base and lower part of the impactite sequence were formed by ejecta curtain and melt surge deposits, respectively. Our results are also consistent with the interpretation that the middle part of the impactite sequence is fallback ejecta from the impact plume. Additionally, we support the contention that massive seawater resurges into the crater occurred during the deposition of the upper and uppermost part of the impactites.     

Dynamo coefficients from local simulations of the turbulent ISM

Observations in polarized emission reveal the existence of large‐scale coherent magnetic fields in a wide range of spiral galaxies. Radio‐polarization data show that these fields are strongly inclined towards the radial direction, with pitch angles up to 35° and thus cannot be explained by differential rotation alone. Global dynamo models describe the generation of the radial magnetic field from the underlying turbulence via the so called α ‐effect. However, these global models still rely on crude assumptions about the small‐scale turbulence. To overcome these restrictions we perform fully dynamical MHD simulations of interstellar turbulence driven by supernova explosions. From our simulations we extract profiles of the contributing diagonal elements of the dynamo α ‐tensor as functions of galactic height. We also measure the coefficients describing vertical pumping and find that the ratio between these two effects has been overestimated in earlier analytical work, where dynamo action seemed impossible. In contradiction to these models based on isolated remnants we always find the pumping to be directed inward. In addition we observe that depends on whether clustering in terms of superbubbles is taken into account. Finally, we apply a test field method to derive a quantitative measure of the turbulent magnetic diffusivity which we determine to be ∼2 kpckms^–1. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Shock metamorphism investigations of quartz grains in clasts from impact breccia of the Eyreville B drill core,Chesapeake Bay impact structure,USA

Abstract– The Chesapeake Bay impact structure, approximately 85 km in diameter, has been drilled in 2005–2006 at Eyreville (Virginia, USA), to a total depth of 1766 m. In the drill cores, the abundance of shock metamorphosed material is very variable with depth. Shocked mineral and lithic clasts, as well as melt particles, are most abundant in suevitic impact breccia section (1397–1451 m depth). Shocked quartz (i.e., quartz grains with planar fractures and/or planar deformation features) and melt particles, although rare, are also dispersed in the Exmore Formation unit (444–867 m depth). Other lithologies in the Eyreville drill cores show no clear evidence of shock metamorphism. Here, we report on the investigations of 40 samples from the impact breccia section. A total of more than 27,000 quartz grains were examined in about 200 clasts. The abundance of highly shocked clasts tends to decrease with increasing depth. Crystalline clasts derived from the crystalline basement are commonly only slightly shocked (contain generally <10 rel% of shocked quartz grains). The clasts of metamorphosed sediments show a low proportion of shocked quartz grains (mostly <10 rel%). Sedimentary clasts show a wide range of proportions of shocked quartz grains, with several of them being highly shocked clasts (most values between 0 and 40 rel%). Conglomerates show the highest proportion of shocked quartz grains of all types of clasts (up to 83 rel%). Polycrystalline quartz clasts are also commonly highly shocked (contain mostly between 10 and 40 rel% of shocked quartz grains). These hard nonporous clasts are possibly more liable to show evidence of shock. The investigations suggest that the intensity of shock metamorphism is the result of several parameters, such as original position in the target (both horizontal and vertical) and the properties of each lithology (e.g., grain size, porosity, and amount of matrix). According to the universal‐stage investigations, the dominant orientations of planar deformation features in quartz are , , and also .