Hypervelocity collisions of solid bodies occur frequently in the solar system and affect rocks by shock waves and dynamic loading. A range of shock metamorphic effects and high‐pressure polymorphs in rock‐forming minerals are known from meteorites and terrestrial impact craters. Here, we investigate the formation of high‐pressure polymorphs of α‐quartz under dynamic and nonhydrostatic conditions and compare these disequilibrium states with those predicted by phase diagrams derived from static experiments under equilibrium conditions. We create highly dynamic conditions utilizing a mDAC and study the phase transformations in α‐quartz in situ by synchrotron powder X‐ray diffraction. Phase transitions of α‐quartz are studied at pressures up to 66.1 and different loading rates. At compression rates between 0.14 and 1.96 GPa s−1, experiments reveal that α‐quartz is amorphized and partially converted to stishovite between 20.7 GPa and 28.0 GPa. Therefore, coesite is not formed as would be expected from equilibrium conditions. With the increasing compression rate, a slight increase in the transition pressure occurs. The experiments show that dynamic compression causes an instantaneous formation of structures consisting only of SiO6 octahedra rather than the rearrangement of the SiO4 tetrahedra to form a coesite. Although shock compression rates are orders of magnitude faster, a similar mechanism could operate in impact events.
In the basement fractured reservoirs, geometric parameters of fractures constitute the main properties for modeling and prediction of reservoir behavior and then fluid flow. This study aims to propose geometric description and quantify the multiscale network organization and its effect on connectivity using a wide-ranging scale analysis and orders scale classification. This work takes place in the Noble Hills (NH) range, located in the Death Valley (DV, USA). The statistical analyses were performed from regional maps to thin sections. The combination of the length datasets has led to compute a power law exponent around −2, meaning that the connectivity is ruled by the small and the large fractures. Three domains have been highlighted in the NH: (1) domain A is characterized by a dominance of the NW/SE direction at the fourth order scale; (2) domain B is characterized by a dominance of the E/W and the NW/SE directions at respectively the fourth and third order scales; (3) domain C is also marked by the E/W direction dominance followed by the NW/SE direction respectively at the fourth and third order scale. The numerical simulations should consider that the orientation depends on scale observation, while the length is independent of scale observation.
Abstract-We carried out hypervelocity cratering experiments with steel projectiles and sandstone targets to investigate the structural and mineralogical changes that occur upon impact in the projectile and target. The masses of coherent projectile relics that were recovered in different experiments ranged between 58% and 92% of their initial projectile masses. A significant trend between impact energy, the presence of water in the target, and the mass of projectile relics could not be found. However, projectile fragmentation seems to be enhanced if the target contains substantial amounts of water. Two experiments that were performed with 1 cm sized steel projectiles impacting at 3400 and 5300 m s )1 vertically onto dry Seeberger sandstone were investigated in detail. The recovered projectiles are intensely plastically deformed. Deformation mechanisms include dislocation glide and dislocation creep. The latter led to the formation of subgrains and micrometer-sized dynamically recrystallized grains. In case of the 5300 m s )1 impact experiment, this deformation is followed by grain annealing. In addition, brittle fracturing and friction-controlled melting at the surface along with melting and boiling of iron and silica were observed in both experiments. We estimated that heating and melting of the projectile impacting at 5300 m s )1 consumed 4.4% of the total impact energy and was converted into thermal energy and heat of fusion. Beside the formation of centimeter-sized projectile relics, projectile matter is distributed in the ejecta as spherules, unmelted fragments, and intermingled iron-silica aggregates.
SummaryLattice preferred orientations of serpentines induce a strong anisotropy of various properties in serpentine bearing-rocks. Lattice preferred orientations had so far been obtained only by X-ray diffraction techniques. We have applied electron back-scattering diffraction to the measurement of the lattice preferred orientations of antigorite in a naturally deformed high-pressure serpentinite. This technique is very sensitive to sample preparation that can lead to surface amorphization in the case of serpentine. A polishing procedure is described that avoids amorphization and allows accurate electron back-scattering diffraction measurements with optimized experimental conditions in a variable pressure scanning electron microscope. Results indicate that deformation leads to lattice preferred orientations characterized by extremely strong c-axis clustering perpendicular to the foliation, as expected for a layered silicate. In the foliation plane, a significant clustering of the a-axis is observed and tentatively attributed to intracrystalline deformation mechanisms. These data suggest that antigorite deforms mostly by gliding along the basal plane of the layered phyllosilicate structure, but that gliding may occur along directions favouring a-axis alignment. Electron back-scattering diffraction appears to be a reliable method for determining phyllosilicate lattice preferred orientations in deformed rocks, with potential applications for determining anisotropy of properties like seismic velocities or thermal and electrical conductivities.
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