Post-shock Temperature Research Articles

Shock metamorphism in sillimanite from the Haughton impact structure, Devon Island, Canada

Clasts of shocked garnet-sillimanite gneisses comprise a minor fraction of the allochthonous breccia at the Haughton impact structure. Refractive indices of the diaplectic and fused components of the gneisses, and reduced specific gravity indicate shock pressures from 35 to 55±5 GPa and effective post-shock temperatures from 500° to 1,000° C in a suite of selected samples.

Rotational distribution of para-H2 in He shock wave

The rotational distribution and collision number Zr of para-H2 in the He shock wave are studied using the master equation and the quantum mechanical rate constant obtained by Rabitz and Zarur. For high Mach number, at the initial stage, the rotational distribution indicates a merging pattern of two Boltzmann distributions at the lower and higher rotational levels corresponding to the pre-shock temperature T1 and the reduced temperature T2Tc/(T2+Tc), where T2 and Tc are the post-shock and rate-constant temperatures, respectively. The rotational distribution at the final stage indicates a non-Boltzmann distribution with underpopulation at the higher levels, which is also observed for low Mach number. The rotational collision number Zr over the period of the rotational relaxation is found to show a step pattern with the plateaus corresponding to the rotational collision number Zjj′ of the two-level systems (jj′) = (02), (24), (46),....

Post-shock temperatures in minerals

An experimental technique was developed for measuring post-shock temperatures in a wide variety of materials, including those of geophysical interest such as silicates. The technique uses an infrared radiation detector to determine the brightness temperature of samples shocked to pressures in the range 5 to approximately 30 GPa; in these experiments measurements were made in two wavelength ranges (4.5 to 5.75 microns and 7 to 14 microns). Reproducible results, with the temperatures in the two wavelength bands generally in excellent agreement, were obtained for aluminum-2024 (10.5 to 33 GPa, 125 to 260 C), stainless steel-304 (11.5 to 50 GPa, 80 to 350 C), crystalline quartz (5.0 to 21.5 GPa, 80 to 250 C), forsterite (7.5 to 28.0 GPa, approximately 30 to 160 C) and Bamble bronzite (6.0 to 26.0 GPa, approximately 30 to 225 C). It is concluded that release adiabat data should be used, wherever available, for calculations of residual temperature, and that adequate descriptions of the shock and release processes in minerals are more complex than generally assumed.

Limited response of the K–Ar system to the Nordlinger Ries giant meteorite impact

FOR the proper interpretation of radiometric rock ages it is important to know which processes are able to reset radioactive clocks. In particular, the resetting by intense shock effects is significant in lunar chronology because most lunar highland rocks are impact breccias produced in the course of multiple large meteorite impacts. For the important K–Ar scheme, we have dated variously shocked samples from ejecta and from a drill core through one of the best documented terrestrial meteorite craters the Nordlinger Ries (Southern Germany)1. From K–Ar and fission track dating of thoroughly melted impact glasses the 24 km diameter Ries crater was found to have formed 14.7±0.7 Myrago by meteorite impact into the variscian bedrock (for summary of ages see ref. 2). The modern version of K–Ar dating (39Ar–40Ar technique) can reveal quantitative information about partial losses of radiogenic argon (fractional resetting) in addition to yielding ages eventually not affected by these gas losses3,4. We have applied this dating technique to mineral separates (hornblende, biotite, chlorite) from ejecta, fall-back breccias and underlying bedrock of the Ries Crater, including drill core samples taken at depths up to 1,201 m(ref. l).The samples exhibit various degrees of mechanical strain (from ∼ 10 kbar up to >400 kbar shock pressure). Apart from the thoroughly molten glasses which reproduced the impact age of ∼ 14.7 Myr, we found that all other rocks involved in the impact irrespective of the degree of shock metamorphism and within the limits of error, yield the same 39Ar–4oAr age of 313±3 Myr, probably the age of the bedrock. Apparently, rocks exposed to the very intense shock effects frequent on the Moon give reliable K–Ar ages when measured as K–Ar plateau ages. Age resetting, if present, probably results from heating associated with such impacts. In the case of the Ries, we found from the diffusion characteristics of argon that the post-shock temperature of the crystalline fragments within the suevite layer has not exceeded 600 °C.

The metallic microstructures and thermal histories of severely reheated chondrites

The metallographic structures of eight severely reheated chondrites (Farmington, Ramsdorf, Orvinio, Wickenburg, Lubbock, Rose City, Arapahoe and Tadjera) have been studied by optical, scanning electron microscope and electron microprobe techniques. Unreheated chondrites and experimentally heat treated chondritic material have also been examined. The following metallographic characteristics can be used to estimate the post-shock residual temperature of reheated chondrites: melted appearance of metal-troilite, presence of martensite, P enrichment of the metal, and the averaging of central metal grain compositions. Metallographic characteristics used to estimate the cooling rates of the severely reheated chondrites are the Ni content of troilite, the Ni gradients in metal grain rims, and the presence of secondary kamacite and phosphides. Farmington, Ramsdorf, Orvinio, Lubbock, Rose City and several of the heat treatment specimens have substantial P in solution in the metal grains ( > 0.1 wt%). P enrichment is apparently caused by reduction of phosphates upon severe reheating and partial melting of metal-troilite areas in chondritic meteorites. The eight severely reheated chondrites studied showed evidence of reheating to temperatures ranging from ∼ 950°C to ∼ 1250°C. Ramsdorf has the highest reheating temperature (1200–1250°C) and the fastest cooling rate ∼ 100°C/day. Wickenberg has the lowest reheating temperature (950–1000°C) and the slowest cooling rate, ∼ 1°C/100yrs. Cooling rate estimates correspond to post-reheating burial depths of less than 1 to ∼ 1000m.

Precursor photo-ionization model for magnetically-driven transverse shock waves

Shock waves produced by magnetic compression are called transverse when the magnetic field contains no component in the direction of the shock wave normal. It is experimentally well known that very strong shocks of this type show classical MHD behaviour in terms of both jump conditions and structure. It is also known that relatively slow transverse shocks can be of an ionizing gasdynamic type with no jump in the imbedded transverse field. Since there are fundamental differences in both the structure and the jump relationships of these two types of shock waves, it is of interest to investigate the transition behaviour in the intermediate shock speed regime. A previously widely accepted model due to Kulikovskii & Lyubimov and Chu assumes no precursor ionization and requires significantly large values of the magnetic Prandtl number, Pm, within the shock structure. That model is shown to be physically inappropriate because experimentally observed transition speeds and corresponding post-shock temperatures imply negligibly small shock Pm values. Also in that model, MHD conditions are approached only asymptotically at large shock speeds. The present precursor ionization model assumes effectively zero Pm values throughout transition and into the low-speed MHD regime. As distinct from the previous theory, this model predicts the attainment of full MHD conditions at and above a well-defined finite shock speed. The assumption of a characteristic post-shock temperature model for the ionization mechanism allows unique jump relations to be formulated independently of other transport mechanisms. Results include computed values of various jump ratios and the electric field as functions of shock speed throughout gasdynamic, transition, and MHD regimes. The solutions form a one-parameter family depending on the relative combination of upstream magnetic field and density (e.g. the Alfvén velocity). Shock structure phase plane trajectories are computed at a number of typical shock speeds throughout transition.

Natural shock behavior of almandite in metamorphic rocks from the ries crater, Germany

An extensive study of a big number of gneiss specimens with various shock features from the suevite allowed unravelling of the shock behavior of almandite garnets. Almandites in shocked metamorphic rocks show with increasing dynamic pressures strong irregular fracturing. differently oriented sets of planar fractures or elements, brown turbidity and nucleation of minute crystals of an unknown phase in solid garnets. At higher peak pressures garnet was found to break down to (1) orthopyroxene + spinel + glass, and to (2) spinel + glass due to fast shock-melting. Extensive quantitative electron microprobe studies of almandite garnets and their breakdown products were carried out. The breakdown products within the original grain boundaries of the garnets consist of an alumina-rich orthopyroxene (with up to 10 wt. % Al 2O 3), hercynite to pleonaste spinels and a silica and calcium-rich glass matrix. The chemical zonation of magnesium and manganese of the former garnets is inherited in the composition of the newly formed orthopyroxenes. Petrographic evidence and chemical composition suggest a fast breakdown of the almandite garnets after passing of shock waves at rapidly falling pressures and very high post-shock temperatures within the ejected gneissic rock material.

Subshock criterion for switch-on shock waves

Conditions are established for determining whether the downstream state of a switch-on hydromagnetic shock wave will be subsonic, sonic, or supersonic, thus giving an explicit criterion for the existence of a gasdynamic subshock within the overall structure. The analysis is made for the general case of an ionizing shock wave with nonzero electric field. The results include the well-known magnetohydrodynamic case in the limit of zero-electric field. Post-shock temperatures are investigated, and it is found that it may be possible for an ionizing switch-on shock to exist with a nonzero electric field without involving a gas-dynamic subshock. In this case, the structure consists of an initial photoionization layer followed by magnetohydrodynamic compression giving a high-temperature post-shock region, the radiation from which drives the upstream photoionization.

Shock-induced thermal transformations of ries-biotites

Thermal transformations in biotites depending on increasing postshock temperatures were studied. A decrease of the unit cell volume, caused by oxidation of octahedral Fe2+ to Fe3+ at temperature rangings of about 400–500° C was observed. In the same temperature range (∼500° C) the biotite lattice becomes thermodynamically unstable. Intersheet and octahedral layers are decomposed, whereas individual SiO4-tetrahedra are more resistant to thermal vibrations. The decomposition of the biotite leads to the formation of new minerals and amorphous phases. By X-ray analysesmagnetite, hercynite, pyroxene, feldspar, and quartz were identified. Thermal transformations of biotite induced by shock waves are characterized by states of distinct disequilibrium.

Production and Investigation of Very Strong Ionizing Shock Waves

Experiments are described in which shock speeds up to 1% of the speed of light were achieved in gaseous hydrogen or deuterium. Separation between the transverse shock wave and the drive current are clearly observed. The density and magnetic field shock compressions are measured and the post-shock electron temperature, determined from plasma x-ray bremsstrahlung spectra, is found to be about 107 °K. Neutron emission was detected from shocks in deuterium. The axial electric field in the shock structure is measured and interpreted as resulting from a combination of Hall effect and ion displacement current.

Formation of diaplectic glass by experimental shock loading of orthoclase

Orthoclase has been experimentally shock loaded in porous mixtures with copper powder and as porous layers. Pressures up to 345 kb were obtained by utilizing reverberations within orthoclase from the higher impedance copper matrix and encasing stainless steel holder. Consequently, postshock temperatures were controlled to rather lower values than if the same pressures were achieved by single shock pulse. It was found that orthoclase commences to form diaplectic glass at 240 kb, and at 320 kb almost the entire sample is transformed. The pressure at which this glass formation is initiated was not affected by varying the duration of the shock pulse from 1 to 4 μsec. Orthoclase in porous matrix of similar shock impedance, if singly shocked as in natural events, will probably transform at rather lower pressures than those found in this experiment, owing to much higher postshock temperatures.

Progressive metamorphism and classification of shocked and brecciated crystalline rocks at impact craters

The principle of progressive shock metamorphism in nonporous silicate rocks is discussed on the basis of petrographic observations and experimental data. The p-T conditions and the nature of the basic types of shock effects observed in rock-forming minerals are considered as far as they are indicative of the degree of shock metamorphism of the source rock. Particular shock effects that are related to the main regimes of the Hugoniot curves of quartz and feldspar, as well as the shock-melting and vaporization behavior of the whole rock, characterize and define six stages or zones of increasing shock metamorphism. Each stage or zone represents a certain range of peak pressure and temperature. Rocks of stage O are shocked to shock states below the Hugoniot elastic limit of quartz and feldspar that are irregularly fractured. Shock stage I is characterized by diaplectic quartz and feldspar that are released from shock states in the ‘two-phase regime’ of the Hugoniot. Diaplectic glass of quartz and/or feldspar composition occurs within stage II, which is related to the lowest part of the ‘high-pressure-phase regime’ of the Hugoniot. Stage III is characterized by selective melting of feldspar minerals at sufficiently high postshock temperature. Postshock temperatures exceeding the liquidus of the whole rock produce rock glasses on pressure and temperature release (stage IV). Stage V is represented by condensation products of shockvaporized rock material. On the basis of experimental data of several authors, the proposed stages of metamorphism are tentatively correlated with a pressure-temperature scale. It is concluded that the proposed classification is generally valid for shock processes in nonporous rocks and can well be applied to the recognition and distinction of the different kinds of impact breccias of terrestrial or extraterrestrial origin.

Shock metamorphism of silicate glasses

The changes in refractive index caused by shock compression have been determined for tektite, soda-lime, and silica glasses shocked to pressures up to 460 kb. For shock compression below 80 kb for fused silica and 40 kb for tektite and soda-lime glasses, compression is reversible as the refractive indices are within 0.0025 of the starting values. Index increases of 0.01, 0.04, and 0.06 are observed for soda-lime, tektite, and silica glasses shocked to pressures of 80, 130, and 140 kb respectively. For soda-lime glass subjected to shock pressures between 80 and 230 kb there is a decrease in the postshock refractive index to n=1.5211 at 230 kb. For fused silica shocked to pressures of 140 to 460 kb, refractive index drops from 1.52 to 1.47. The reasons for these decreases in index are not obvious. New values for postshock temperatures for fused silica based on release adiabat data, e.g. ∼1000°C for a shock state at 250 kb, suggest that the decreases in refractive index are caused by a combination of decompression along release adiabats and reconstructive transformation from a shock-induced stishovitelike phase to a low-density glass. Postshock densities calculated from the refractive index data agree closely with those calculated from the release adiabat data.

Investigation of Nitric Oxide Formation Kinetics in Combustion Processes: The Hydrogen-Oxygen-Nitrogen Reaction

Abstract An experimental and analytical investigation of the kinetics of formation of NO in shock-induced combustion of hydrogen-oxygen-nitrogen mixtures has been carried out. In the experimental study, concentration lime-histories of NO, OH and H20 were measured during reaction using spectroscopic techniques. Experimental concentration profiles were obtained for an oxidizer-rich and a fuel-rich mixture for initial post-shock temperatures in the range 2150−2800 °K and for an initial post-shock pressure of 2.2 ± 0.3 atm. In the analytical study, time rates of change of species concentrations and thermodynamic properties during reaction were calculated by numerically integrating the coupled reaction kinetic, state and energy equations. Calculated concentration profiles were compared with experimental profiles to obtain information on the reaction mechanism for formation of NO. Observed NO formation rates in both the oxidizer-rich and the fuel-rich mixture were consistent with a two-reaction mechanism for nitrogen chemistry, There was no evidence to suggest that reactions other than (1) and (2) were of importance in the NO formation mechanism. NO concentration profiles obtained from two simplified models for NO formation were compared with experimental profiles. It was found that under certain conditions these simplified models predicted NO formation rates substantially lower than observed experimentally.

Shock deformation of sillimanite from the Ries crater, Germany

Shocked sillimanite was found as a main constituent of fragmental garnet-sillimanite-schists which are included very rarely in the suevite breccia from the Ries crater, Germany. Sillimanite — probably shocked to a peak pressure of more than 300–400 kbar — reveals sets of planar deformation structures oriented parallel to bipyramids, prisms and pinacoids. Optical properties, X-ray diffraction and infra-red absorption of these crystals show marked changes with respect to unshocked sillimanite due to the extreme mosaicism and shock disordering. Partial transformation to mullite and cristobalite by very high post-shock temperatures is indicated by X-ray and microscopic investigations. No high-pressure polymorph of sillimanite composition nor stishovite + corundum could be detected.

Shock compression of feldspars

Hugoniot data for oligoclase and microcline to 670 and 580 kb and release adiabat data for oligoclase were obtained by means of the inclined mirror and immersed-foil-reflected-light techniques, respectively. Oligoclase and microcline have Hugoniot elastic limits in the range of 40–55 and 80–85 kb. These limits increase slightly with increasing driving shock pressure. Above the elastic limit, extending to ∼300 and ∼400 kb, transition regions of anomalously high compression are observed for microcline and oligoclase. These data and the data of McQueen, Marsh, and Fritz for albitite and anorthosite indicate that at successively higher shock pressures within this region, the feldspars gradually transform to a high-pressure, high-density polymorph. This polymorph probably corresponds to the rutile-like hollandite structure obtained in high-pressure quenching experiments by Kume, Matsumoto, and Koizumi (in KAlGe_3O_8) and by Ringwood, Reid, and Wadsley (in KAlSo_3O_8, microcline). In the hollandite structure germanium or silicon is in octahedral coordination with oxygen. The zero-pressure density and the Birch-Murnaghan equation of state parameters for the adiabat and isotherm are calculated for the high-pressure polymorphs of oligoclase, microcline, anorthosite, and albitite. The release adiabat centered at 180 kb indicates that at this shock pressure some (∼15%) of the hollandite phase forms but apparently reverts to a lower-density phase on pressure release. Release adiabat curves centered at 272 and 417 kb and calculated postshock temperatures indicate that material of feldspar composition recovered from meteorite and laboratory impacts is converted to the hollandite structure upon shock compression; upon pressure release this material probably reverts to the low-density maskelynite form.

Equilibrium properties of shock ionized xenon

The equilibrium shock wave parameters for xenon are calculated for several cases of fore-pressures between 0.5 and 760 torr and post-shock temperatures up to 20 000 °K. The problem of determining the electronic excitation partition function of neutral and ionized xenon is discussed with respect to both the presence of auto-ionized neutral xenon levels and the missing energy level data for both high principal quantum numbers and of intermediate azimuthal quantum numbers. The role of both the missing energy levels and also Debye-Huckel electrostatic effects on the shocked gas properties is discussed.