Peak Shock Pressure Research Articles

Akaogiite: An ultra-dense polymorph of TiO2 with the baddeleyite-type structure, in shocked garnet gneiss from the Ries Crater, Germany

Akaogiite, an ultra-dense polymorph of TiO2 was encountered in heavily shocked garnet-cordieritesillimanite gneiss in the Suevite breccia of the Ries meteorite impact crater in Germany. The chemical formula of akaogiite is TiO2, containing (wt%) TiO2 97.6-97.7; Nb2O5 0.15-0.20, and FeO 0.11-0.14. The empirical formula is: Ti0.998Fe0.002Nb0.001O2. Akaogiite is optically distinguishable from rutile only in reflected light microscopy, through its slightly higher brightness and the intense royal blue color of its internal reflections in crossed nicols. The mineral grains consist of countless randomly oriented submicrometer particles. In situ micro-beam X-ray diffraction established akaogiite to be monoclinic with the baddeleyite-type structure. Cell parameters are a = 4.606(2) Å, b = 4.896(3) Å, c = 4.933(3) Å, and β = 99.17(6)°; space group P21/c; molar volume = 16.82(2) cm3/mol; density = 4.72 g/cm3. Akaogiite coexists in the shocked gneiss with graphite-diamond phase transition assemblage, a second dense TiO2 polymorph with the scrutinyite-type structure (TiO2-II), liquidus idiomorphic jadeite inclusions in melt pockets in garnet, and a new FeTiO3-polymorph with the Li-niobate structure. The high-pressure assemblage constrains the equilibrium peak-shock pressure to be ≤22 GPa and a post-shock temperature <500 °C, thus correcting previous estimates. The name was approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2007-058). The name is for Masaki Akaogi, Professor at the Department of Chemistry, Gakushuin University in Tokyo, Japan.

Impact-induced N 2 production from ammonium sulfate: Implications for the origin and evolution of N 2 in Titan’s atmosphere

Chemical reactions and volatile supply through hypervelocity impacts may have played a key role for the origin and evolution of both planetary and satellite atmospheres. In this study, we evaluate the role of impact-induced N 2 production from reduced nitrogen-bearing solids proposed to be contained in Titan’s crust, ammonium sulfate ((NH 4) 2SO 4), for the replenishment of N 2 to the atmosphere in Titan’s history. To investigate the conversion of (NH 4) 2SO 4 into N 2 by hypervelocity impacts, we measured gases released from (NH 4) 2SO 4 that was exposed to hypervelocity impacts created by a laser gun. The sensitivity and accuracy of the measurements were enhanced by using an isotope labeling technique for the target. We obtained the efficiency of N 2 production from (NH 4) 2SO 4 as a function of peak shock pressure ranging from ∼8 to ∼45 GPa. Our results indicate that the initial and complete shock pressures for N 2 degassing from (NH 4) 2SO 4 are ∼10 and ∼25 GPa, respectively. These results suggest that cometary impacts on Titan (i.e., impact velocity v i > ∼8 km/s) produce N 2 efficiently; whereas satellitesimal impacts during the accretion (i.e., v i < 4 km/s) produce N 2 only inefficiently. Even when using the proposed small amount of (NH 4) 2SO 4 content in the crust (∼4 wt.%) (Fortes, A.D. et al., 2007. Icarus 188, 139–153), the total amount of N 2 provided through cometary impacts over 4.5 Ga reaches ∼2–6 times the present atmospheric N 2 (i.e., ∼7 × 10 20–2 × 10 21 [mol]) based on the measured production efficiency and results of a hydrodynamic simulation of cometary impacts onto Titan. This implies that cometary impacts onto Titan’s crust have the potential to account for a large part of the present N 2 through the atmospheric replenishment after the accretion.

Models of high velocity impacts into dust-covered ice: Application to Martian northern lowlands

The detection of fresh impact craters with bright floors and ejecta (arising from fresh clean water ice) in the northern lowlands of Mars ( Byrne et al., 2009b, Science 325, 1674), together with observations of polygonal structures and evidence from the Phoenix probe, suggests that there are substantial water ice deposits just below the surface over large areas. Specifically in cases of the larger craters observed, the impacts themselves may have raised the temperature and the pressure of the water ice deposits locally to values which allow phase changes. In this paper, we use smoothed particle hydrodynamics to model hyper-velocity impacts. We estimate peak shock pressures in a solid water ice target covered by a layer of loose material, modeled by pre-damaged dunite. In addition, we account for the possibility of a thin layer of sub-surface water ice by using a three-layer model where the ice is surrounded by dunite. We find that the peak shock pressures reached in the simulated events are high enough to produce several 100–1000 kg of liquid water depending upon the impact parameters and the exact shock pressure needed for the phase change. A difficulty remains however in determining whether liquid is generated or whether a type of fluidized ice is produced (or indeed some combination of the two). We also note that the process can become rather complex as the number of layers increases because of reflections of the shock at sub-surface boundaries—a process which should lead to increased fluidization.

The Keurusselkä impact structure, Finland—Impact origin confirmed by characterization of planar deformation features in quartz grains

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.

Magnetization, magnetic transition and magnetic entropy changes of bulk MnAs1−xSbx fabricated by underwater shock compaction

MnAs 1− x Sb x ( x = 0.068, 0.073) materials were fabricated by means of underwater shock compaction technique. A peak shock pressure of water acting on the powders was about 16 GPa. It was confirmed that underwater shock compaction leads to the compositional inhomogeneity, broad magnetic transition and lower magnetization and magnetic entropy changes of MnAs 1− x Sb x . However, these problems were improved by a post-annealing. Consequently, the post-annealed samples after the shock compaction exhibit the sharp magnetic transitions and high magnetic entropy changes of in a field of 1 T.

Bacterial Spores in Granite Survive Hypervelocity Launch by Spallation: Implications for Lithopanspermia

Bacterial spores are considered good candidates for endolithic life-forms that could survive interplanetary transport by natural impact processes, i.e., lithopanspermia. Organisms within rock can only embark on an interplanetary journey if they survive ejection from the surface of the donor planet and the associated extremes of compressional shock, heating, and acceleration. Previous simulation experiments have measured each of these three stresses more or less in isolation of one another, and results to date indicate that spores of the model organism Bacillus subtilis can survive each stress applied singly. Few simulations, however, have combined all three stresses simultaneously. Because considerable experimental and theoretical evidence supports a spallation mechanism for launch, we devised an experimental simulation of launch by spallation using the Ames Vertical Gun Range (AVGR). B. subtilis spores were applied to the surface of a granite target that was impacted from above by an aluminum projectile fired at 5.4 km/s. Granite spall fragments were captured in a foam recovery fixture and then recovered and assayed for shock damage by transmission electron microscopy and for spore survival by viability assays. Peak shock pressure at the impact site was calculated to be 57.1 GPa, though recovered spall fragments were only very lightly shocked at pressures of 5-7 GPa. Spore survival was calculated to be on the order of 10(-5), which is in agreement with results of previous static compressional shock experiments. These results demonstrate that endolithic spores can survive launch by spallation from a hypervelocity impact, which lends further evidence in favor of lithopanspermia theory.

Systematic study of universal‐stage measurements of planar deformation features in shocked quartz: Implications for statistical significance and representation of results

Abstract— The presence of shocked quartz is one of the key lines of evidence for the impact origin of rocks. Crystallographic orientations of planar deformation feature (PDF) sets in shocked quartz have been used to constrain the peak shock pressure that these grains have experienced. So far no systematic and comparative studies of the various orientation measurement methods and their biases are available. Therefore, three shocked‐quartz‐bearing thin sections from a meta‐greywacke clast in breccia, a biotite‐gneiss, and a sandstone, respectively, were independently analyzed by three operators (two experienced and one inexperienced) using a four‐axis universal‐stage (U‐stage), in order to evaluate the quality, precision, repeatability, and representativeness of U‐stage measurements. Based on the indexing of PDF sets using a new version of the commonly used stereographic projection template, the study of 1751 PDF set orientations in 666 quartz grains in three different shocked rocks shows that differences in abundance and orientation of various PDF sets, as measured by the three separate operators, are rather limited.The precision of U‐stage measurements depends mainly on the number of PDF sets investigated, as the ability level of the operator (experienced versus inexperienced) is only responsible for minor deviations in the number of unindexed planes. The frequency percent of dominant PDF planes may vary by up to 20 percentage points (pp) or 81% for a given crystallographic orientation when only 25 sets are measured. When 100 PDF sets are measured, however, this deviation in dominant orientations is reduced to about 7 pp or 28%. We recommend the use of a new stereographic projection template, which plots the pole positions of five additional, commonly occurring PDF orientations, as it can allow indexing of up to 12 pp more PDF planes; these are planes that would previously be considered unindexed and potentially regarded as errors of measurement.Our results suggest that by following a strict measurement procedure, the reproducibility of U‐stage measurements is good and the results of different studies can be readily compared. However, it is critical that published PDF orientation histograms clearly define what type of frequency measurement is used, whether or not unindexed PDF sets are included in the frequency calculations, the numbers of grains and sets analyzed, and the relative proportions of each PDF set population that are combined in the histograms. This information appears to be essential for effectively comparing datasets from different studies.

Validation of numerical codes for impact and explosion cratering: Impacts on strengthless and metal targets

Abstract— Over the last few decades, rapid improvement of computer capabilities has allowed impact cratering to be modeled with increasing complexity and realism, and has paved the way for a new era of numerical modeling of the impact process, including full, three‐dimensional (3D) simulations. When properly benchmarked and validated against observation, computer models offer a powerful tool for understanding the mechanics of impact crater formation. This work presents results from the first phase of a project to benchmark and validate shock codes. A variety of 2D and 3D codes were used in this study, from commercial products like AUTODYN, to codes developed within the scientific community like SOVA, SPH, ZEUS‐MP, iSALE, and codes developed at U.S. National Laboratories like CTH, SAGE/RAGE, and ALE3D. Benchmark calculations of shock wave propagation in aluminum‐on‐aluminum impacts were performed to examine the agreement between codes for simple idealized problems. The benchmark simulations show that variability in code results is to be expected due to differences in the underlying solution algorithm of each code, artificial stability parameters, spatial and temporal resolution, and material models. Overall, the inter‐code variability in peak shock pressure as a function of distance is around 10 to 20%. In general, if the impactor is resolved by at least 20 cells across its radius, the underestimation of peak shock pressure due to spatial resolution is less than 10%. In addition to the benchmark tests, three validation tests were performed to examine the ability of the codes to reproduce the time evolution of crater radius and depth observed in vertical laboratory impacts in water and two well‐characterized aluminum alloys. Results from these calculations are in good agreement with experiments. There appears to be a general tendency of shock physics codes to underestimate the radius of the forming crater. Overall, the discrepancy between the model and experiment results is between 10 and 20%, similar to the inter‐code variability.

Three-dimensional petrography of metal phases in equilibrated L chondrites—Effects of shock loading and dynamic compaction

To examine the role of impacts in the evolution of asteroids as seen through their chondritic offspring, we have performed a quantitative three dimensional (3D) study of metal grains in a suite of increasingly shocked L chondrites with synchrotron X-ray microtomography (XMT). Our data allow rigorous quantification of size-number distributions and collective morphology of Fe(Ni) metal phases in chondritic meteorites. At the resolution of our XMT measurements (8.4–17.9 μm/voxel), the number of metal particles increase with higher degrees of petrographically identified shock loading, indicating a coalescing of Fe–Ni metal at or below this scale. Our results demonstrate that collective degrees of metal grain preferred orientation increase with greater degrees of impact-related compaction and shock loading. Ductile metal grains in L chondrites begin to show foliation at peak shock pressures < 5 GPa, pressures great enough to compact and indurate loosely bound chondritic material, and our results constitute evidence for multiple generations of impact events acting on the L parent body or bodies.

Impact melt rocks from the Paasselkä impact structure (SE Finland): Petrography and geochemistry

Abstract— Recently, samples of allochthonous melt rocks from the ˜10 km and ≤1.9 Ga Paasselkä impact structure, SE Finland, were obtained. In this study, we present a first detailed petrographic and geochemical description of clast‐rich Paasselkä impact melt rocks. Shock metamorphic features comprise shocked feldspar grains, intensely shocked and toasted quartz, marginally molten and recrystallized clasts thought to have been diaplectic quartz glass, largely fresh and recrystallized feldspar glasses, decomposed biotite flakes, recrystallized fluidal silica glass (originally probably lechatelierite) in partially molten sandstone clasts, all set into a glassy to cryptocrystalline melt matrix. The degree of shock metamorphism of clasts suggests initial whole‐rock melting at peak shock pressures of ≥35 GPa and post‐shock temperatures of up to ˜1500 °C. Glass components vary in geochemical composition corresponding to the mixed character of the crystalline target rock (i.e., representing different monomineralic and mixed‐mineral melts). Feldspar glasses and the fresh glassy to cryptocrystalline melt matrix indicate that the Paasselkä melt rocks are not intensely altered. The geochemical composition of the Paasselkä impact melt rocks is roughly consistent with the compositions of melt rocks from a number of impact structures located within the crystalline basement of the Baltic Shield.

Shatter cone and microscopic shock-alteration evidence for a post-Paleoproterozoic terrestrial impact structure near Santa Fe, New Mexico, USA

Field mapping, morphologic description, and petrographic analysis of recently discovered shatter cones within Paleoproterozoic crystalline rocks exposed over an area > 5 km 2, located ∼ 8 km northeast of Santa Fe, New Mexico, USA, give robust evidence of a previously unrecognized terrestrial impact structure. Herein, we provisionally name this the “Santa Fe impact structure”. The shatter cones are composed of nested sub-conical, curviplanar, and flat joint surfaces bearing abundant curved and bifurcating striations that strongly resemble the multiply striated joint surfaces (MSJS) documented from shatter cones at Vredefort dome. The cones occur as a penetrative feature in intrusive igneous and supracrustal metamorphic rocks, are unusually large (up to 2 m long and 0.5 m wide at the base), display upward-pointing apices, and have subvertical, northeastward-plunging axes that crosscut regional host-rock fabrics. Key characteristics of superficially similar, but non-shock-generated conical and striated features are inconsistent with the properties of the Santa Fe cones. In thin section, sub-millimeter-scale, dark, semi-opaque to isotropic veneers on cone surfaces and veinlets within cone interiors closely resemble previously described shock-induced melt features. Microscopic grain alteration, restricted generally to within 1 mm of the cone surfaces, includes random fractures, fluid micro-inclusions, sericite replacement in feldspar, rare kink bands in mica, optical mosaicism, and decorated planar fractures (PFs) and planar deformation features (PDFs) in quartz. The PFs and PDFs are dominated by a basal (0001) crystallographic orientation, which indicate a peak shock pressure of ∼ 5–10 GPa that is consistent with shatter cone formation. Regional structural and exhumation models, together with anomalous breccia units that overlie and crosscut the shatter cone-bearing rocks, may provide additional age constraints for the impact event. The observed shatter cone outcrop area suggests that the minimum final crater diameter of the Santa Fe impact structure was ∼ 6–13 km.

Hydrocode modeling of the Sierra Madera impact structure

Abstract— We present the first hydrocode simulations of the formation of the Sierra Madera structure (west Texas, USA), which was caused by an impact into a thick sedimentary target sequence. We modeled Sierra Madera using the iSALE hydrocode, and here we present two best-fit models: 1) a crater with a rim (final crater) diameter of ˜12 km, in agreement with previous authors' interpretations of the original structure, and 2) a crater ˜16 km in diameter with increased postimpact erosion. Both models fit some of the geologic observational data, but discrepancies with estimates of peak shock pressure, extent of deformation, and stratigraphic displacement remain. This study suggests that Sierra Madera may be a larger crater than previously reported and illustrates some of the challenges in simulating impact deformation of sedimentary lithologies. As many terrestrial craters possess some amount of sedimentary rocks in the target sequence, numerical models of impacts into sedimentary targets are essential to our understanding of target rock deformation and the mechanics of crater formation.

The role of ricochet impacts on impact vaporization

Vaporization of carbonate targets by hypervelocity impact increases with decreasing impact angle (from the horizontal), in contrast with expectations based only on peak shock pressures. Experiments at the NASA Ames Vertical Gun Range were designed to allow isolating the underlying controlling processes and probing the vapor composition using high-speed spectroscopy. Vaporization associated with the maximum peak pressures (first contact) was separated from vaporization generated only by downrange ricochet impacts through the use of split targets. Four telescopes isolated the vapor with different velocities and revealed that grazing ricochet debris downrange contributed a significant fraction to the overall vaporization process. These results can be understood by the high temperatures and low pressures created by high strain-rate shear.

Ambient Pressure Dependence of Laser-Induced Impulse onto Polyacetal

L ASER ablation can be usefully employed to generate a propulsive impulse on an object not only in the atmosphere but also in vacuum [1–3]. Larson et al. [4] proposed a launch vehicle propelled by repetitively pulsed laser ablation. Several authors [5–9] show that favorable propulsion performance in the atmosphere can be achieved with a polymer material, polyacetal, which is commercially named “Delrin” or is abbreviated as “POM.” The ablated gas from this material does not contain much air pollutant. Targeting space applications, the laser-ablative-propulsion performance of metals and polymers at low ambient pressures has been intensively investigated [10–17]. For many kinds of metals and polymers, Cm ranges from 10 to 100 N-s=J, and depends strongly on the characteristics of the laser pulse. According to measurements byGregg andThomas [10], metallic materials have an optimum laser intensity that maximizes Cm. Phipps et al. [12] formulated experimental Cm characteristics of aluminum alloys and several polymers in terms of the intensity, width, andwavelength of the laser pulse.Cm can be further increased by utilizing the so-called “volume absorber” [13] or layered target [14]. Recent measurements by D’Souza and Ketsdever [17] of Cm for polyacetal at low ambient pressure using a frequency-doubled Nd:YAG pulsed laser showed peak of 110 N-s=J at a laser intensity of the order of 10 W=cm. Several authors investigated the influence of ambient pressure on Cm [18,19]. Pakhomov et al. [18]measured the impulse generated on an aluminum surface irradiated by a CO2 laser pulse whose pulse width was 200 ns; Cm decreased monotonically with decreasing ambient pressure. In Dufresne et al.’s surface-pressure measurement [19], the ambient-pressure dependence of Cm for aluminum was quite sensitive to the laser pulse width. For polymer materials, Beverly and Walters [20] measured the CO2-laser-induced shock pressure in cellulose acetate and polymethylmethacrylate. The peak shock pressures exhibited the complex dependence on the ambient pressure. The ablative impulse dependence of materials other than aluminum warrants further investigations. To quantitatively evaluate the feasibility of laser-ablativepropulsion systems, the propulsive performance of polyacetal over a wide range of ambient pressure needs to be known. In the present paper, the influence of the ambient pressure on the impulse characteristics of polyacetal is investigated experimentally using a transversely excited atmospheric (TEA) CO2 pulse laser.

Shock-induced crystal-plastic deformation and post-shock annealing of quartz: microstructural evidence from crystalline target rocks of the Charlevoix impact structure, Canada

Two distinct types of shock-induced quartz microstructure in charnockitic target rocks and quartz veins of the Charlevoix impact structure are described. The dominant shock effects in the type 1 microstructure in charnockites at ∼2–4 km from the centre of the structure are planar deformation features (PDFs) parallel to rhombohedral planes of quartz. The abundance of different sets of these PDFs indicates a high hydrostatic component of the shock wave-associated stress (∼10–15 GPa). Evidence of crystal-plastic deformation due to high deviatoric stresses is absent. In contrast, PDFs parallel to the basal plane are predominant in the type 2 microstructure developed in rocks at ∼4–9 km from the centre of the structure, whereas rhombohedral PDFs are rare. This indicates a lower hydrostatic stress component (∼7–8 GPa), which correlates with a radial decrease in recorded peak shock pressure. The basal PDFs are revealed by transmission electron microscopy to represent mechanical Brazil twins, which record crystal-plastic deformation at high deviatoric stresses (McLaren et al. , 1967). These findings imply that the deviatoric component of the shock wave-associated stress increases relative to the hydrostatic component with increasing distance from the centre of the impact structure. In the type 2 microstructure, numerous deformation bands, strong undulose extinction and cataclastic zones at the optical scale, as well as glide-dislocations and microcracks at the TEM scale, occur in association with basal PDFs, and are therefore also interpreted to be shock-induced. This is consistent with the observation that quartz from the outer part of the impact structure is devoid of similar features. Thus, the highly heterogeneous and localised glide-controlled deformation accompanied by mechanical twinning and microcracking recorded by the type 2 microstructure is suspected to be induced by the high deviatoric stresses and high loading rates during shock. Post-shock recovery is indicated in the type 1 microstructure by the actual microstructure of rhombohedral PDFs, dislocations in climb configuration and well-ordered low angle grain boundaries, as well as in the type 2 microstructure by the occurrence of small elongate subgrains with low angle grain boundaries paralleling low-index planes. This has probably taken place during annealing shortly after the impact event at quasi-static conditions and still sufficiently high post-shock temperatures, rather than during a separate regional thermal event.

High-pressure phases in a shock-induced melt vein of the Tenham L6 chondrite: Constraints on shock pressure and duration

The microtexture and mineralogy of a 580-μm-wide melt vein in the Tenham L6 chondrite were investigated using field-emission scanning electron microscopy and transmission electron microscopy to better understand the shock conditions. The melt vein consists of a matrix of silicate plus metal-sulfide grains that crystallized from immiscible melts, and sub-rounded fragments of the host chondrite that have been entrained in the melt and transformed to polycrystalline high-pressure silicates. The melt-vein matrix contains two distinct textures and mineral assemblages corresponding to the vein edge and interior. The 30-μm-wide vein edge consists of vitrified silicate perovskite + ringwoodite + akimotoite + majorite with minor metal-sulfide. The 520-μm-wide vein interior consists of majorite + magnesiowüstite with irregular metal-sulfide blebs. Although these mineral assemblages are distinctly different, the pressure stabilities of both assemblages are consistent with crystallization from similar pressure conditions: the melt-vein edge crystallized at about 23–25 GPa and the vein interior crystallized at about 21–25 GPa. This relatively narrow pressure range suggests that the melt vein either crystallized at a constant equilibrium shock pressure of ∼25 GPa or during a relatively slow pressure release. Using a finite element heat transfer program to model the thermal history of this melt vein during shock, we estimate that the time required to quench this 580-μm-wide vein was ∼40 ms. Because the entire vein contains high-pressure minerals that crystallized from the melt, the shock-pressure duration was at least 40 ms. Using a synthetic Hugoniot for Tenham and assuming that the sample experienced a peak-shock pressure of 25 GPa near the impact site, we estimate that the Tenham parent body experienced an impact with collision velocity ∼2 km/s. Based on a one-dimensional planar impact model, we estimate that the projectile size was >150 m in thickness.

Shock-induced dehydrogenation of polycyclic aromatic hydrocarbons with or without serpentine: Implications for planetary accretion

Polycyclic aromatic hydrocarbons (PAHs) were shocked across a peak shock pressure range of 5.8 to 36.6 GPa with or without serpentine and were then evaluated with an elemental analyzer, an X-ray diffractometer, a gas chromatograph, and a gas chromatograph–mass spectrometer. On the basis of the results, we examined the shock-induced dehydrogenation of PAHs and the matrix's effects on it. Moreover, we discussed the hydrogen amounts supplied to the Earth's atmosphere and interior during its formation, as well as the δ D values of those amounts. We did not find any matrix effects on the dehydrogenation of PAHs in the pressure range used in this study. The incipient dehydrogenation of PAHs occurred at 10 GPa and dehydrogenation was almost completed at 36.5 GPa. Assuming that carbonaceous chondrites, such as the Murchison meteorite accreted on and formed the Earth, they should have started and completed dehydrogenation at a point where the growing Earth reached a radius of 1020 km and 3060 km, respectively. By the end of the accretion, most of the hydrogen delivered to the Earth had been discharged into the atmosphere. The cumulative amount of hydrogen in the Earth's interior at that point and its δ D value were calculated to be 3.3×10 21 kg and −47‰, respectively. These calculated data were comparable to those already established for the Earth's interior. However, to explain the isotopically light-juvenile hydrogen ( D=−125‰) in the deep Earth reported by Hauri, Chem. Geol. 183 (2002) 115–141, we should consider materials that have low δ D values (e.g., −850‰ of the presolar nebular) as sources of the Earth's hydrogen.

The influence of explosive-driven “taylor-wave” shock prestraining on the structure/property behavior of 304 stainless steel

The mechanical response of high-explosive (HE)-driven “Taylor-wave” (TW) shock-prestrained 304 stainless steel loaded to 35 to 45 GPa has been investigated in compression and tension at room temperature and compared to results from compressive testing of the annealed microstructure at a strain rate of 10−3/s. The microstructure and substructure evolution due to TW shock prestraining has been investigated using optical metallography and transmission electron microscopy. The mechanical behavior of 304 stainless steel following explosive shock prestraining is shown to exhibit a factor of 2 increase in yield stress and a significant reduction in ductility under tension. Microstructural analyses reveal a high density of twins after TW prestraining and very little production/retention of deformation-induced martensite. The current experimental results are shown to agree well with previous literature results on stainless steel samples shock-prestrained utilizing “square-topped” (ST) shock-pulse loading. This finding suggests that the peak shock pressure, which controls the total plastic strain that the sample must accommodate, is the dominant variable in shock-wave-loading defect generation and storage in 304 stainless steel.

Stishovite and post-stishovite polymorphs of silica in the shergotty meteorite: their nature, petrographic settings versus theoretical predictions and relevance to Earth's mantle

The Shergotty meteorite contains three dense silica polymorphs in distinct petrographic settings: (1) two post-stishovite SiO 2 polymorphs in individual multiphase grains coexisting with glass with nearly labradorite composition, and (2) large individual stishovite grains in shock-melt pockets which also contain the new CAS phase (Calcium-aluminosilicate; CaAl 4Si 2O 11; [Phys Earth Planet Interiors 97 (1996) 97; Geophys Abstract 5(2003)] and hollandite structured plagioclase composition. Prismatic and wedge-shaped grains of the original accessory tridymite (or cristobalite) in the Shergotty meteorite were densified during a major impact event on the Shergottite–Nakhlite–Chaissingite (SNC) parent body and inverted either to (1) multiphase assemblages of several post-stishovite polymorphs depicting prominent tweed pattern or to (2) Large homogeneous stishovite grains in melt pockets. In the first setting we identified an orthorhombic and a monoclinic post-stishovite silica polymorph, respectively. TEM investigations of a grain containing the orthorhombic polymorph revealed an α-PbO 2 like phase that could be assigned to either Pnc2 (with the cell parameters: a=4.55±0.01 Å, b=4.16±0.03 Å, c=5.11±0,04 Å), or Pbcn space group and dense SiO 2 glass. The X-ray diffraction pattern from a second grain revealed a polymorph with a monoclinic lattice with the space group P21/ c, that is related to the baddeleyite (ZrO 2) structure with the cell parameters: a=4.375(1) Å, b=4.584(1) Å, c=4.708(1) Å, β=99.97(3), ρ=4.30(2) g/cm 3. TEM-SAED pattern of this grain revealed the presence of the α-PbO 2-like SiO 2 polymorph, stishovite, secondary cristobalite, and dense silica glass. The coexistence of several high-density polymorphs and dense silica glass in the same grain suggests that several post-stishovite phases were formed during the shock event in Shergotty. Some of these polymorphs were highly unstable and vitrified, presumably in the decompression stage. Based on diamond anvil experiments on cristobalite a peak shock pressure in excess of 40 GPa could be deduced. The petrographic setting and texture of the single stishovite grains in the melt pockets is different. The mono-phase individual grains occur exclusively as large (>10 μm) rounded objects inside melt pockets together with hollandite structured plagioclase composition and the new CAS phase [1]. Stishovite in melt pockets is barren of any sign of a tweed pattern and contains no silica glass. This suggests that the mechanisms of phase transitions were different in the two lithologies. Stishovite in the melt pockets probably did not form by a retrograde transformation from a post-stishovite polymorph.

Visible/near‐infrared spectra of experimentally shocked plagioclase feldspars

High shock pressures cause structural changes in plagioclase feldspars such as mechanical fracturing and disaggregation of the crystal lattice at submicron scales, the formation of diaplectic glass (maskelynite), and genuine melting. Past studies of visible/near‐infrared spectra of shocked feldspars demonstrated few spectral variations with pressure except for a decrease in the depth of the absorption feature near 1250–1300 nm and an overall decrease in reflectance. New visible/near–infrared spectra (400–2500 nm) of experimentally shocked (17–56 GPa) albite‐ and anorthite‐rich rock powders demonstrate similar trends, including the loss of minor hydrated mineral bands near 1410, 1930, 2250, and 2350 nm. However, the most interesting new observations are increases in reflectance at intermediate pressures, followed by subsequent decreases in reflectance at higher pressures. The amount of internal scattering and overall sample reflectance is controlled by the relative proportions of micro‐fractures, submicron grains, diaplectic glass, and melts formed during shock metamorphism. We interpret the observed reflectance increases at intermediate pressures to result from progressively larger proportions of submicron feldspar grains and diaplectic glass. The ensuing decreases in reflectance occur after diaplectic glass formation is complete and the proportion of genuine melt inclusions increases. The pressure regimes over which these reflectance variations occur differ between albite and anorthite, consistent with thermal infrared spectra of these samples and previous studies of shocked feldspars. These types of spectral variations associated with different peak shock pressures should be considered during interpretation and modeling of visible/near‐infrared remotely sensed spectra of planetary and asteroidal surfaces.