Revising the shock classification of meteorites

The current shock classification scheme of meteorites assigns shock levels of S1 (unshocked) to S6 (very strongly shocked) using shock effects in rock‐forming minerals such as olivine and plagioclase. The S6 stage (55–90 GPa; 850–1750 °C) relies solely on localized effects in or near melt zones, the recrystallization of olivine, or the presence of mafic high‐pressure phases such as ringwoodite. However, high whole rock temperatures and the presence of high‐pressure phases that are unstable at those temperatures and pressures of zero GPa (e.g., ringwoodite) are two criteria that exclude each other. Each type of high‐pressure phase provides a minimum shock pressure during elevated pressure conditions to allow the formation of this phase, and a maximum temperature of the whole rock after decompression to allow the preservation of this phase. Rocks classified as S6 are characterized not by the presence but by the absence of those thermally unstable high‐pressure phases. High‐pressure phases in or attached to shock melt zones form mainly during shock pressure decline. This is because shocked rocks (<60 GPa) experience a shock wave with a broad isobaric pressure plateau only during low velocity (<4.5 km s^?1) impacts, which rarely occur on small planetary bodies; e.g., the Moon and asteroids. The mineralogy of shock melt zones provides information on the shape and temporal duration of the shock wave but no information on the general maximum shock pressure in the whole rock.     

Two-Dimensional Simulation of the Dynamics of the Collapse of a Rotating Core with Formation of a Neutron Star on an Adaptive Triangular Grid in Lagrangian Coordinates

Results are presented from a two-dimensional numerical simulation of the collapse of a rotating core with formation of a neutron star that has strong differential rotation in its outer regions. A specially developed numerical method is used which is based on a fully conservative implicit operator difference scheme for gravitational gas dynamics problems in lagrangian coordinates on a variable-structure triangular grid. The recoil shock wave generated by the collapse causes ejection of a small amount of material. This cannot explain the explosion of type II supernovae. The strong differential rotation in the presence of even a weak initial magnetic field obtained in these calculations must lead to a rise in the magnetic pressure, formation of an MHD shock wave, and conversion of rotational energy into the energy of radial expansion (magnetorotational supernova explosion).     

Natural occurrence of reidite in the Xiuyan crater of China

The high‐pressure minerals of reidite and coesite have been identified in the moderately shock‐metamorphosed gneiss (shock stage II, 35–45 GPa) and the strongly shock‐metamorphosed gneiss (shock stage III, 45–55 GPa), respectively, from the polymict breccias of the Xiuyan crater, a simple impact structure 1.8 km in diameter in China. Reidite in the shock stage II gneiss displays lamellar textures developed in parental grains of zircon. The phase transformation of zircon to reidite likely corresponds to a martensitic mechanism. No coesite is found in the reidite‐bearing gneiss. The shock stage III gneiss contains abundant coesite, but no reidite is identified in the rock. Coesite occurs as acicular, dendritic, and spherulitic crystals characteristic of crystallization from shock‐produced silica melt. Zircon in the rock is mostly recrystallized. The postshock temperature in the shock stage III gneiss is too high for the preservation of reidite, whereas reidite survives in the shock stage II gneiss because of relatively low postshock temperature. Reidite does not occur together with coesite because of difference in shock‐induced temperature between the shock stage II gneiss and the shock stage III gneiss.     

A shock‐produced (Mg,Fe)SiO3 glass in the Suizhou meteorite

Abstract— Ovoid grains consisting of glass of stoichiometric (Mg, Fe)SiO₃ composition that is intimately associated with majorite were identified in the shock veins of the Suizhou meteorite. The glass is surrounded by a thick rim of polycrystalline majorite and is identical in composition to the parental low‐Ca pyroxene and majorite. These ovoid grains are surrounded by a fine‐grained matrix composed of majorite‐pyrope garnet, ringwoodite, magnesiowüstite, metal, and troilite. This study strongly suggests that some precursor pyroxene grains inside the shock veins were transformed to perovskite within the pyroxene due to a relatively low temperature, while at the rim region pyroxene grains transformed to majorite due to a higher temperature. After pressure release, perovskite vitrified at post‐shock temperature. The existence of vitrified perovskite indicates that the peak pressure in the shock veins exceeds 23 GPa. The post‐shock temperature in the meteorite could have been above 477 °C. This study indicates that the occurrence of high‐pressure minerals in the shock veins could not be used as a ubiquitous criterion for evaluating the shock stage of meteorites.     

Shock metamorphism of quartz in nature and experiment: II. Significance in geoscience*

Abstract— The occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies. Its utility is due to its almost ubiquitous occurrence in terrestrial rocks, its overall stability and the fact that a variety of shock metamorphic effects, occurring over a range of shock pressures, have been well documented. These shock effects have been generally duplicated in shock recovery experiments and, thus, serve as shock pressure barometers. After reviewing the general character of shock effects in quartz, the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored. The short pulse lengths in experiments may account for the difficulty in synthesizing the high-pressure polymorphs, coesite and stishovite, compared to natural occurrences. In addition, post-shock thermal effects are possible in natural events, which can affect shock altered physical properties, such as refractive index, and cause annealing of shock damage and recrystallization. The orientations of planar microstructures, however, are unaffected by post-impact thermal events, except if quartz is recrystallized, and provide the best natural shock barometer in terms of utility and occurrence. The nature of planar microstructures, particularly planar deformation features (PDFs), is discussed in some detail and a scheme of variations in orientations with shock pressure is provided. The effect of post-impact events on PDFs is generally limited to annealing of the original glass lamellae to produce decorated PDFs, resulting from the exsolution of dissolved water during recrystallization. Basal (0001) PDFs differ from other PDF orientations in that they are multiple, mechanical Brazil twins, which are difficult to detect if not partially annealed and decorated. The occurrence and significance of shock metamorphosed quartz and its other phases (namely, coesite, stishovite, diaplectic glass and lechatelierite) are discussed for terrestrial impact structures in both crystalline (non-porous) and sedimentary (porous) targets. The bulk of past studies have dealt with crystalline targets, where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures. In sedimentary targets, the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature, which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets. This is discussed, along with examples of variations in the relative abundances of planar microstructures and their orientations, which are attributed to textural variations in sedimentary target rocks. Examples of the shock metamorphism of quartz in distal ejecta, such as at the K/T boundary, and from nuclear explosions are illustrated and are equivalent to that of known impact structures, except with respect to characteristics that are due to long-term, post-shock thermal effects. Finally, the differences between the deformation and phase transformation of quartz by shock and by endogenic, tectonic and volcanic processes are discussed. We confirm previous conclusions that they are completely dissimilar in character, due to the vastly different physical conditions and time scales typical for shock events, compared to tectonic and volcanic events. Well-characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment.     

Effect of azimuthal magnetic field on the propagation of cylindrical shock waves in self-gravitating gas

A propagation of diverging cylindrical shock in a self-gravitating gas, having an initial density and azimuthal magnetic field distributions variable, has been studied for the two cases (i) when the shock is weak and (ii) when it is strong. Analytical relations for shock velocity and shock strength have been obtained. Lastly, the expressions for the pressure, the density and the particle velocity immediately behind the shock have been also obtained for both cases.     

Revision and recalibration of existing shock classifications for quartzose rocks using low‐shock pressure (2.5–20 GPa) recovery experiments and mesoscale numerical modeling

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.     

Shock jump relations for a dusty gas atmosphere

This paper presents simplified forms of jump relations for one dimensional shock waves propagating in a dusty gas. The dusty gas is assumed to be a mixture of a perfect gas and spherically small solid particles, in which solid particles are continuously distributed. The simplified jump relations for the pressure, the temperature, the density, the velocity of the mixture and the speed of sound have been derived in terms of the upstream Mach number. The expressions for the adiabatic compressibility of the mixture and the change-in-entropy across the shock front have also been derived in terms of the upstream Mach number. Further, the handy forms of shock jump relations have been obtained in terms of the initial volume fraction of small solid particles and the ratio of specific heats of the mixture, simultaneously for the two cases viz., (i) when the shock is weak and, (ii) when it is strong. The simplified shock jump relations reduce to the Rankine-Hugoniot conditions for shock waves in an ideal gas when the mass fraction (concentration) of solid particles in the mixture becomes zero. Finally, the effects due to the mass fraction of solid particles in the mixture, and the ratio of the density of solid particles to the initial density of the gas are studied on the pressure, the temperature, the density, the velocity of the mixture, the speed of sound, the adiabatic compressibility of the mixture and the change-in-entropy across the shock front. The results provided a clear picture of whether and how the presence of dust particles affects the flow field behind the shock front. The aim of this paper is to contribute to the understanding of how the shock waves behave in the gas-solid particle two-phase flows.     

Shock waves from point-sources in a heat-conducting gas

The shock wave produced by a point source has been studied in a heat-conducting gas medium. The shock is assumed to be strong enough to neglect the ambient gas pressure and the similarity method is used. The distribution of flow quantities behind the shock have been obtained by the numerical integration of a system of ordinary differential equations using the boundary conditions at the shock wave.     

Shock features in iron-nickel metal and troilite of L-group ordinary chondrites

Abstract— Shock metamorphic features in opaque minerals (FeNi metal and troilite) of 22 L chondrites have been studied petrographically and geochemically in an attempt to establish a connection between the present silicate-based shock classification scheme (Stöffler et al., 1991) and the peak-shock and postshock thermal history recorded in these minerals. Unshocked to weakly shocked (S1–S3) L chondrites contain FeNi metal and troilite that display textures related to normal, slow cooling. They may also contain rare disequilibrium shock features, which suggest localized departures from equilibrium shock conditions. Above shock stage S3, selected melting of FeNi metal and troilite produces melt droplets whose composition and abundance correspond to the maximum equilibrium shock state achieved by the sample. At these higher shock levels, the abundance of other shock-induced features, such as polycrystalline kamacite, sheared and fizzed troilite, coarse-grained pearlitic plessite, polycrystalline troilite, and polymineralic melt veins serve as textural criteria that can be used to establish peak-shock conditions. Minimum postshock temperatures obtained from analyses of plessite components show a systematic increase in temperature with an increase in shock stage, thereby providing additional information about the postshock thermal histories of L chondrites. At the highest shock levels recorded in L chondrites (S6 and above), melting and chemical homogenization of FeNi metal produces flattened Ni profiles that may partially to completely obscure any evidence for an earlier, slow-cooling history. All of these features serve as aids for shock classifying L chondrites as well as for quantifying minimum peak temperatures that resulted during shock metamorphism.     

Shock stage distribution of 2280 ordinary chondrites—Can bulk chondrites with a shock stage of S6 exist as individual rocks?

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.     

Shock metamorphism in plagioclase and selective amorphization

Plagioclase feldspar is one of the most common rock‐forming minerals on the surfaces of the Earth and other terrestrial planetary bodies, where it has been exposed to the ubiquitous process of hypervelocity impact. However, the response of plagioclase to shock metamorphism remains poorly understood. In particular, constraining the initiation and progression of shock‐induced amorphization in plagioclase (i.e., conversion to diaplectic glass) would improve our knowledge of how shock progressively deforms plagioclase. In turn, this information would enable plagioclase to be used to evaluate the shock stage of meteorites and terrestrial impactites, whenever they lack traditionally used shock indicator minerals, such as olivine and quartz. Here, we report on an electron backscatter diffraction (EBSD) study of shocked plagioclase grains in a metagranite shatter cone from the central uplift of the Manicouagan impact structure, Canada. Our study suggests that, in plagioclase, shock amorphization is initially localized either within pre‐existing twins or along lamellae, with similar characteristics to planar deformation features (PDFs) but that resemble twins in their periodicity. These lamellae likely represent specific crystallographic planes that undergo preferential structural failure under shock conditions. The orientation of preexisting twin sets that are preferentially amorphized and that of amorphous lamellae is likely favorable with respect to scattering of the local shock wave and corresponds to the “weakest” orientation for a specific shock pressure value. This observation supports a universal formation mechanism for PDFs in silicate minerals.     

Quantitative in situ XRD measurement of shock metamorphism in Martian meteorites using lattice strain and strain‐related mosaicity in olivine

All Martian meteorites have experienced shock metamorphism to some degree. We quantitatively determined shock‐related strain in olivine crystals to measure shock level and peak shock pressure experienced by five Martian meteorites. Two independent methods employing nondestructive in situ micro X‐ray diffraction (μXRD) are applied, i.e., (1) the lattice strain method, in which the lattice strain value (ε) for each olivine grain is derived from a Williamson–Hall plot using its diffraction pattern (peak width variation with diffraction angle) with reference to a best fit calibration curve of ε values obtained from experimentally shocked olivine grains; (2) the strain‐related mosaicity method, allowing shock stage to be estimated by measuring the streaking along the Debye rings of olivine grain diffraction spots to define their strain‐related mosaic spread, which can then be compared with olivine mosaicity in ordinary chondrites of known shock stage. In this study, both the calculated peak shock pressures and the estimated shock stages for Dar al Gani 476 (45.6 ± 0.6 GPa), Sayh al Uhaymir 005/8 (46.1 ± 2.2 GPa), and Nakhla (18.0 ± 0.6 GPa) compare well with literature values. Formal shock assessments for North West Africa 1068/1110 (53.9 ± 2.1 GPa) and North West Africa 6234 (44.6 ± 3.1 GPa) have not been reported within the literature; however, their calculated peak shock pressures fall within the range of peak shock pressures defining their estimated shock stages. The availability of nondestructive and quantitative μXRD methods to determine shock stage and peak shock pressure from olivine crystals provides a key tool for shock metamorphism analysis.     

Shock‐induced microtextures in lunar apatite and merrillite

Apatite and merrillite are the most common phosphate minerals in a wide range of planetary materials and are key accessory phases for in situ age dating, as well as for determination of the volatile abundances and their isotopic composition. Although most lunar and meteoritic samples show at least some evidence of impact metamorphism, relatively little is known about how these two phosphates respond to shock‐loading. In this work, we analyzed a set of well‐studied lunar highlands samples (Apollo 17 Mg‐suite rocks 76535, 76335, 72255, 78235, and 78236), in order of displaying increasing shock deformation stages from S1 to S6. We determined the stage of shock deformation of the rock based on existing plagioclase shock‐pressure barometry using optical microscopy, Raman spectroscopy, and SEM‐based panchromatic cathodoluminescence (CL) imaging of plagioclase. We then inspected the microtexture of apatite and merrillite through an integrated study of Raman spectroscopy, SEM‐CL imaging, and electron backscatter diffraction (EBSD). EBSD analyses revealed that microtextures in apatite and merrillite become progressively more complex and deformed with increasing levels of shock‐loading. An early shock‐stage fragmentation at S1 and S2 is followed by subgrain formation from S2 onward, showing consistent decrease in subgrain size with increasing level of deformation (up to S5) and finally granularization of grains caused by recrystallization (S6). Starting with 2°–3° of intragrain crystal‐plastic deformation in both phosphates at the lowest shock stage, apatite undergoes up to 25° and merrillite up to 30° of crystal‐plastic deformation at the highest stage of shock deformation (S5). Merrillite displays lower shock impedance than apatite; hence, it is more deformed at the same level of shock‐loading. We suggest that the microtexture of apatite and merrillite visualized by EBSD can be used to evaluate stages of shock deformation and should be taken into account when interpreting in situ geochemically relevant analyses of the phosphates, e.g., age or volatile content, as it has been shown in other accessory minerals that differently shocked domains can yield significantly different ages.     

The emergence of a spherical magnetogasdynamic shock wave at the surface of a star

The propagation of a magnetogasdynamic shock wave originating in a stellar interior, is ocnsidered when it approaches the surfaces of the star. The flow behind the shock wave is assumed isothermal rather than adiabatic to stimulate the conditions of large radiative transfer near the stellar surface. The product solution of McVittie has been used to obtain exact solution of the problem. It has been obtained that velocity, density, pressure and magnetic field increases as we move from shock surface towards the nucleus of the star.     

Shock experiments in range of 10–45 GPa with small multidomain magnetite in porous targets

Abstract– Physical properties of multidomain magnetite‐bearing porous pellets shocked up to 45 GPa were measured. The results show general magnetic softening as a result of shock. However, a relative magnetic hardening trend and slight magnetic susceptibility decrease is observed with increasing pressure among shocked samples. Initially, the shock also seems to cause a slight decrease in porosity, but at higher shock pressures macroscopic porosity increases progressively in our pellets. The microscopic porosity remains almost unchanged. Since our samples have distinctly higher initial porosity compared with samples used in previous studies, our results may be representative for impacts into highly porous magnetite‐bearing sedimentary or volcanic rocks and are relevant to impacts into such target rocks on Earth and Mars.     

Nonlinear Evolution of Thermal Structures. Numerical Approach

A numerical algorithm using a two stage, two level difference scheme has been developed to solve the heat transfer equation with nonlinear heat diffusion and bulk energy losses. The algorithm is an extension of the scheme developed by Meek and Norbury (1982). The first stage calculates an intermediate value which is used in a second stage to estimate a new value. The scheme is consistent, second-order convergent in space and almost second order in time. It has been applied to the nonlinear stability and time evolution of thermal structures constituted by optically thin plasmas with solar abundances. The configuration has been assumed to be heated at a rate T^m, cooled at a rate Tⁿ and a thermal conduction coefficient T^k. In particular, the second order analytical approximation considered in previous papers (Ibáñez and Rosenzweig, 1995; Steele and Ibáñez, 1997) has been worked out for arbitrary amplitude of the initial temperature disturbance. Particular cases of interest in Astrophysics are considered.     

Structural evidence from shock metamorphism in simple and complex impact craters: Linking observations to theory

Abstract— The structure of Canadian impact craters formed in crystalline rocks is analyzed using shock metamorphism and evidence for movement along shear zones. The analysis is based on an interpretation that, beyond the near field region, shock pressure attenuates down axis as P ? R^?2, in agreement with nuclear test and computed results, and as P ? R^?3 near the surface. In both simple and complex craters, the transient cavity is defined by the limit of fragmentation due to direct and reflected shock waves. The intersection of the transient cavity with hemispheric shock isobars indicates that the transient cavity has a parabolic form. Weakening by dilation during early uplift allows late stage slumping of the walls of simple craters. This is controlled by a spheroidal primary shear of radius r_s ～ 2d_t, where d_t is the depth of the transient crater due to excavation and initial compression. With increasing crater diameter, the size of the transient cavity decreases relative to the shock imprint, suggesting that fragmentation and excavation is limited by progressively earlier collapse of the margins under gravity. Central peak formation in complex craters may be initiated by relaxation of the shock‐compressed central parautochthone, so the primary shear, lubricated by friction melting, meets below the crater floor and drives the continuing upward motion.     

Estimating average shock pressures recorded by impactite samples based on universal stage investigations of planar deformation features in quartz—Sources of error and recommendations

Planar deformation features (PDFs) in quartz are the most widely used indicator of shock metamorphism in terrestrial rocks. They can also be used for estimating average shock pressures that quartz-bearing rocks have been subjected to. Here we report on a number of observations and problems that we have encountered when performing universal stage measurements and crystallographically indexing of PDF orientations in quartz. These include a comparison between manual and automated methods of indexing PDFs, an evaluation of the new stereographic projection template, and observations regarding the PDF statistics related to the c-axis position and rhombohedral plane symmetry. We further discuss the implications that our findings have for shock barometry studies. Our study shows that the currently used stereographic projection template for indexing PDFs in quartz might induce an overestimation of rhombohedral planes with low Miller–Bravais indices. We suggest, based on a comparison of different shock barometry methods, that a unified method of assigning shock pressures to samples based on PDFs in quartz is necessary to allow comparison of data sets. This method needs to take into account not only the average number of PDF sets/grain but also the number of high Miller–Bravais index planes, both of which are important factors according to our study. Finally, we present a suggestion for such a method (which is valid for nonporous quartz-bearing rock types), which consists of assigning quartz grains into types (A–E) based on the PDF orientation pattern, and then calculation of a mean shock pressure for each sample.     

The Evolution of Young Stars, Protostellar Jets & Bipolar Outflows - a Unification Scheme

Close links between jet evolution and protostellar evolution are beginning to be understood. Firstly, stellar jets are reviewed here, establishing the accretion-outflow connection. Then, outflows from young stars are reviewed, suggesting a synchronised development in the star and outflow. This yields a unification scheme in which rising molecular jets dominate the early protostellar epoch, followed by a jet-driven outflow stage and, finally, a bow-driven ballistic stage. This scheme is quantified, yielding the systematic changes in the bolometric, mechanical and shock luminosities and the cross-over phase from dense molecular jets to light atomic jets. This revised version was published online in July 2006 with corrections to the Cover Date.