Unshocked Rocks Research Articles

Clast size distribution and quantitative petrography of shocked and unshocked rocks from the El'gygytgyn impact structure

AbstractA variety of quantitative petrographic methods, such as the study of clast size distribution (CSD), have been used in the study of melt rocks and have, for example, led to validation of the melting origin of pseudotachylites in paleoseismic faults, the estimate of the energy involved in melting and crushing processes, the distinction between seismic and aseismic faults, and understanding the crystallization process in volcanic rocks. Recently, quantitative petrography was applied to distinguish impact melts from pristine mare basalts on the Moon. Here, we apply this approach to impact lithologies formed in a volcanic target. The El'gygytgyn structure, an 18 km‐diameter and 3.58 Ma old impact crater in north‐eastern Chukotka, Arctic Russia, represents the only known impact crater on Earth mainly excavated in siliceous volcanic rocks. The structure was recently drilled in the framework of an ICDP project, providing fresh samples of suevite and other impact breccias, which can be compared with samples from the unshocked target. As the target is mostly composed of rhyolitic‐dacitic ignimbrites and tuffs, impact melt clasts are almost indistinguishable from the unshocked volcanic clasts in the absence of shock evidence. We show here that geometric characterization provides a reproducible technique for quantitative description of impact lithologies. Although the studied suevite reveals a high local variability in the evaluated geometric parameters, an overall homogenization of these parameters occurs. Furthermore, quantitative petrography allows the classification of unshocked to slightly shocked volcanic clasts included in the suevite.

Thermal infrared spectroscopy and modeling of experimentally shocked basalts

New measurements of thermal infrared emission spectra (250.1400 cm-1; ~7.40 μm) of experimentally shocked basalt and basaltic andesite (17.56 GPa) exhibit changes in spectral features with increasing pressure consistent with changes in the structure of plagioclase feldspars. Major spectral absorptions in unshocked rocks between 350.700 cm-1 (due to Si-O-Si octahedral bending vibrations) and between 1000.1250 cm-1 (due to Si-O antisymmetric stretch motions of the silica tetrahedra) transform at pressures >20.25 GPa to two broad spectral features centered near 950.1050 and 400.450 cm-1. Linear deconvolution models using spectral libraries composed of common mineral and glass spectra replicate the spectra of shocked basalt relatively well up to shock pressures of 20.25 GPa, above which model errors increase substantially, coincident with the onset of diaplectic glass formation in plagioclase. Inclusion of shocked feldspar spectra in the libraries improves fits for more highly shocked basalt. However, deconvolution models of the basaltic andesite select shocked feldspar end-members even for unshocked samples, likely caused by the higher primary glass content in the basaltic andesite sample.

Impact-shocked rocks – insights into archean and extraterrestrial microbial habitats (and sites for prebiotic chemistry?)

Impact-shocked gneiss shocked to greater than 10 GPa in the Haughton impact structure in the Canadian High Arctic has an approximately 25-times greater pore surface area than unshocked rocks. These pore spaces provide microhabitats for a diversity of heterotrophic microorganisms and in the near-surface environment of the rocks, where light levels are sufficient, cyanobacteria. Shocked rocks provide a moisture retaining, UV protected microenvironment. During the Archean, when impact fluxes were more than two orders of magnitude higher than today, the shocked-rock habitat was one of the most common terrestrial habitats and might have provided a UV-shielded refugium for primitive life. These potential habitats are in high abundance on Mars where impact crater habitats could have existed over geologic time periods of billions of years, suggesting that impact-shocked rocks are important sites to search for biomolecules in extraterrestrial life detection strategies. In addition to being favourable sites for life, during the prebiotic period of planetary history impact-shocked rocks might have acted as a site for the concentration of reactants for prebiotic syntheses.

Post-shock annealing of Miller Range 99301 (LL6): Implications for impact heating of ordinary chondrites

MIL 99301 is an LL chondrite that has experienced successive episodes of thermal metamorphism, shock metamorphism and annealing. The first recognizable petrogenetic episode resulted in thermal metamorphism of the rock to petrologic type 6 (as indicated by homogeneous olivine compositions, significant textural recrystallization, and the presence of coarse grains of plagioclase, metallic Fe-Ni and troilite). The source of heat for this thermal episode is not identified. The rock also experienced shock metamorphism to shock stage ∼S4 as indicated by extensive silicate darkening (caused by the dispersion within silicate grains of thin chromite melt veins and trails of metallic Fe-Ni and troilite blebs), polycrystalline troilite, myrmekitic plessite, a relatively high occurrence abundance (OA) of metallic Cu (3.6), the presence of numerous chromite-plagioclase assemblages, and coarse grains of low-Ca clinopyroxene with polysynthetic twinning. The shock event responsible for these effects must have occurred after the epoch of thermal metamorphism to type-6 levels; otherwise the polycrystallinity of the troilite would have disappeared and the low-Ca clinopyroxene would have transformed into orthopyroxene. Despite abundant evidence of strong shock, olivine and plagioclase in MIL 99301 exhibit sharp optical extinction, consistent with shock stage S1 and characteristic of an unshocked rock. This implies that an episode of post-shock annealing healed the damaged olivine and plagioclase crystal lattices and thereby changed undulose extinction into sharp extinction. The rock was probably annealed to metamorphic levels approximating petrologic type 4; more significant heating would have transformed the low-Ca clinopyroxene into orthopyroxene. It is not plausible that an episode of annealing occurring after the epoch of thermal metamorphism could have been caused by the decay of 26Al because this isotope would have decayed away by that time. Impact heating is a more plausible source of post-metamorphic annealing of rocks in the vicinity of impact craters on low-density, high-porosity asteroids with rubble-pile structures.

Experimental and Simulation Analysis of Jet-Perforated Rock Damage

Summary The finite-element method (FEM) is employed to evaluate stress distribution around a perforation tunnel with a sandstone formation. Experimental data are used to confirm the results of theoretical study. In the model, the effect of jet pressure, jet temperature, and the combination of both on formation are studied separately. The results indicated that a 3.5-g RDX charge is capable of disturbing rock formation up to 5 in. longitudinally and 2 in. radially. The extent of this damage zone for a 10.5-g RDX charge is 8 in. longitudinally and 4 in. radially. The linear effect is within the first few inches of the perforation tunnel entrance. In this region, rock grains are under an intensive sudden shock-pressure wave of 2 to 4 million psi for a fraction of a microsecond. As a result, the rock's physical structure will change to metamorphous with a lower bulk density than that of unshocked rock, causing a sudden decrease in formation porosity and, hence, formation permeability.

Impact‐induced tensional failure in rock

Planar impact experiments were employed to induce dynamic tensile failure in Bedford limestone. Rock discs were impacted with aluminum and polymethyl methacralate flyer plates at velocities of 10 to 25 m/s. This resulted in tensile stresses in the range of ∼11 to 160 MPa. Tensile stress durations of 0.5 and 1.3 μs induced microcrack growth which in many experiments were insufficient to cause complete spalling of the samples. Ultrasonic P and S wave velocities of recovered targets were compared to the velocities prior to impact. Velocity reduction, and by inference microcrack production, occurred in samples subjected to stresses above 35 MPa in the 1.3‐μs PMMA experiments and 60 MPa in the 0.5‐μs aluminum experiments. Apparent fracture toughnesses of 2.4 and 2.5 MPa m1/2 are computed for the 1.3‐ and 0.5‐μs experiments. These are a factor of ∼2 to 6 greater than quasi‐static determinations. Three‐dimensional impact experiments were conducted on 20 cm‐sized blocks of Bedford limestone and San Marcos gabbro. Compressional wave velocity deficits up to 50–60% were observed in the vicinity of the crater. These damage levels correspond to O'Connell and Budiansky damage parameters of 0.4 as compared to the unshocked rock. The damage decreases as ∼r−1.5 from the crater indicating a dependence on the magnitude and duration of the tensile pulse. Using the observed variation in damage with tensile stress from the one‐dimensional experiments, and estimates of the variation of peak dynamic tensile stress and tensile stress duration with distance from an impact on an elastic half‐space, the observed dependence of damage with radius in the three‐dimensional experiments are theoretically predicted and compare favorably to experimental data.

The Allochthonous Polymict Breccia Layer of the Haughton Impact Crater, Devon Island, Canada

Abstract— The central allochthonous polymict breccia of the Haughton impact structure is up to about 90 m thick and as much as 7.3 km in radial extent. It has been analyzed with respect to modal composition, grain‐size characteristics, and degree of shock metamorphism for the grain‐size ranges 10–∼ 50, 1–10, 0.03–1, and <0.03 mm. The mineralogy of the breccia matrix is dominated by dolomite and calcite, with minor amounts of quartz, other silicate minerals, and rare melt particles. The following lithic clasts have been identified in the 1–10 mm size fraction (averages of vol.% given in parentheses): dolomitic rocks (51), limestones (29), crystalline rocks (10), sandstones and siltstones (3.7), chert (0.7), melt particles (1.9). The mineral clasts (1–0.03 mm) comprise (with decreasing frequency) dolomite, quartz, calcite, feldspar, biotite, amphibole, garnet, opaques, rounded quartz derived from sandstones and accessory minerals. Lithic and mineral clasts display various degrees of shock. Fragments of crystalline rocks are shocked in the 0–60 GPa range; whole rock melts from the crystalline basement are lacking and unshocked rocks are very rare. In contrast, shock‐melted sandstones, shales, and chert were found in most samples. Large clasts of these melt rocks are highly concentrated near the center of the crater. Otherwise, no distinct change of the modal composition with radial range has been observed except that the frequency of limestone clasts increases slightly with radial range. The breccia near the center is more fine‐grained than that beyond about 1 km radius and the sorting parameter increases somewhat with radial range. Except for the high concentration of shock‐melted sedimentary rocks and highly shocked crystalline rocks near the center of the crater, the distribution of shock stages within the lithic clast population is quite uniform throughout the breccia formation. We conclude that the breccia constituents are derived from the lower part of the target stratigraphy (deeper than about 800 m) and that the total depth of excavation at Haughton is in the order of 2000 m. The mixing of sedimentary rocks of the Eleanor River Formation, Lower Ordovician, and Cambrian (∼850 m thickness) with crystalline basement rocks is quite thorough and homogeneous throughout the breccia lens, at least for the analyzed part. This may require an air‐borne mode of emplacement for the upper section of the breccia in analogy to the fall‐back suevite in the Ries crater. A calculation of the excavation (Z‐model) and of the shock pressure attenuation based on reasonable estimates of the energy and crater geometry of the Haughton impact confirms the observed maximum depth of excavation of about 2 km. Shock‐melted crystalline basement rocks, if present at all, must be confined to the very center of the structure below the excavation cavity.

Composition of the Crystalline Basement and Shock Metamorphism of Crystalline and Sedimentary Target Rocks at the Haughton Impact Crater, Devon Island, Canada

Abstract— About 100 cobble‐sized samples collected from the surface of the central polymict breccia formation of Haughton impact crater, Canada, have been studied microscopically and chemically. Breccia clasts derived from the 1700 m deep Precambian basement consist of 13 rock types which can be grouped into sillimanite‐ and garnet‐bearing gneiss; alkali feldspar‐rich aplitic or biotite‐hornblende‐bearing gneiss; biotite and hornblende gneiss; apatite‐rich biotite and biotite‐hornblende gneiss; calcitediopside gneiss; amphibolite; tonalitic orthogneiss; and basalts. The range of chemical compositions of these rocks is wide: e.g., SiO2 ranges from 40–85 wt.%; Al2O3 from 7–20 wt.%; CaO from 0.01–25 wt.%; or P2Os from <0.01–5 wt.%. Nearly all samples of crystalline rocks are shock metamorphosed up to about 60 GPa. Most conspicuous is the absence of whole‐rock melts and the very rare occurrence of unshocked rocks. The 45 samples examined can be classified into the following shock stages: stage 0 (<5 GPa): 4.5%, stage Ia (10–20 GPa): 9.0%, stage Ib (20–35 GPa): 33%, stage II (35–45 GPa): 29%, stage III (45–55 GPa): 18%, stage III–IV (55–60 GPa): 6.5%. Among Paleozoic sedimentary rock clasts higher degrees of shock than within crystalline rocks were observed such as highly vesiculated, whole‐rock melts of sandstones and shales. Within the northern and eastern sectors of the allochthonous breccia no distinct radial variation of the cobble‐sized lithic clasts regarding abundance, rock type, and degree of shock was observed, with the exception that clasts of shock‐melted sedimentary rocks and of highly shocked basement rocks (stage III–IV) are strongly concentrated near the center of the crater. Based on our field and laboratory investigations we conclude that vaporization and melting due to the Haughton impact affected the lower section of the sedimentary strata from about 900 to 1700 m depth (Eleanor River limestones and dolomites, Lower Ordovician and Cambrian limestones, dolomites, shales, and sandstones). The 60‐GPa shock pressure isobar reached only the uppermost basement rocks so that whole rock melting of the crystalline rocks was not possible.

Triaxial compression data on nuclear explosion shocked, mechanically shocked, and normal granodiorite from the Nevada Test Site

Compression tests were performed at room temperature on solid circular cylinders of medium-grained granodiorite from the Hardhat Event conducted at the Nevada Test Site. Confining pressures ranged from 1 to 7000 bars. A piston-cylinder apparatus equipped with an internal axial load cell and manganin coil was used. The confining fluid was a low-viscosity oil. Pre-detonation specimens were cut 0°, 45°, and 90° to the drill core axis and were taken from 195-, 225-, and 255-meter depths. Post-detonation rock was sampled at 23 and 27.5 meters from ground zero, which was located at a depth of 290 meters and experienced a calculated peak shock pressure of approximately 20 kb. Specimens mechanically shocked in the laboratory (35 to 40 kb) were from the 195-meter depth pre-detonation sample. No orientational effects were observed. The failure of granodiorite under compressional stresses was found to be adequately described by a modified Coulomb maximum shear stress theory. Under uniaxial compression, the mode of failure was brittle shear. Under triaxial compression, a quiet slip failure was observed at low confining pressures and brittle shear at high confining pressures. A possible explanation for the occurrence of slip failure may be the presence of a fluid pore pressure within the specimen. Strain measurements were made on several specimens. At atmospheric pressure, the stress histories of the rock were clearly apparent. Under confining pressure, shock effects rapidly disappeared, and, at 4.5 kb, the behavior was essentially that of normal granodiorite. Young's modulus at atmospheric confining pressure for unshocked rock was found to be 0.64 ± 0.26 × 106 bars, which in the upper limit agrees with the reported dynamic value. The average value for rock shocked by nuclear blast is 0.18 × 106 bars and that for mechanically shocked rock is of the order 0.01 × 106 bars.