Shock recovery of rocks with a variety of shock-induced pressure at a single shot

Introduction: Many meteorites have shock metamorphic textures, such as deformation and melting features under polarizing microscope. These shock features have been used to explore a variety of shock histories on meteorite’s parent bodies. A number of shock experiments have been conducted to estimate the shock pressure, which the shock metamorphic textures formed (e.g.,[1]). The previous experiments have been conducted mainly with uniaxial shock-recovery techniques (e.g., [2]). However, the conventional method can only obtain one data set at specific pressure per shot and is difficult to conduct a number of shots due to the destruction of metal containers. There is another technique pertaining to shock recovery, that is, the use of decaying shock waves. This method was applied to shock recovery of single olivine crystals with a high-power laser [3]. In this study, we extend this technique to macro rocky materials with the size of >10 mm using a two-stage light gas gun. Since we used a projectile much smaller than a metal container, a decaying compressive pulse propagates into the target, resulting in shocked samples compressed at various pressures. Our method allows us to reduce the effect of reflected waves from the wall of the container, which is a well-known problem in conventional shock recovery experiments.

Methods: We conducted the shock recovery experiments with a two-stage light gas gun at the Planetary Exploration Research Center of Chiba Institute of Technology, Japan [4]. We used marble and basalt as experimental samples. The samples were shaped into cylinders with a diameter of 30 mm and a height of 24 mm, sealed in a titanium (Ti) container, and covered with a Ti or Al front plate. The impact velocity was 7 km/s on average. The samples were cut across the epicenter and made into thin sections. We used an optical microscope and a scanning electron microscope (SEM) to observe shocked samples in detail. We also conducted numerical calculations under the same conditions as the experiments with the iSALE shock physics code [5-7] to estimate the peak pressure distributions in the samples.

Results and Discussion: We numerically estimated an allowable range of peak pressure in a recovered sample to be ~20 GPa around the epicenter and ~1 GPa around the rear surface of the sample. In the recovered marble sample, we found that calcite exhibited undulatory extinction which is one of the known shock metamorphic textures. The total number of calcite grains showing undulatory extinction decreased with increasing distance from the epicenter. The threshold pressure required for the production of undulatory extinction was estimated to be about 2 GPa, which is close to the Hugoniot elastic limit of calcite [8]. This suggests that plastic deformation leads to the formation of undulatory extinction in calcite. In the case of the recovered basalt sample, plagioclase and pyroxene showed undulatory extinction, and plagioclase does not exhibit mosaicism and vitrification. These observation results indicate that an experienced pressure is up to 20 GPa. In addition, shock melt veins (<4 µm wide) were found to be formed at a distance of 1–2 mm from the epicenter. We estimated that the experienced pressure of the materials at the locations of shock melt veins is about 10 GPa with the iSALE calculation. Therefore, the shock textures, which are the co-existence of undulatory extinction in silicates and shock melt veins, are produced at 10 GPa compression. The shock pressures estimated by the iSALE calculation are consistent with those based on the petrological and mineralogical observations of the recovered samples in this study.

Conclusion: We developed a method to collect shocked samples with a variety of shock pressures (1–20 GPa). Additionally, the peak pressures at the location where the shock features are found are quantitatively estimated with a combination of observation and shock physics modeling. These shock indicators may be useful to estimate the shock histories of chondrite parent bodies.

Acknowledgments: We appreciate the developers of iSALE, including G. Collins, K. Wünnemann, B. Ivanov, J. Melosh, and D. Elbeshausen. We also thank T. Davison for the development of the pySALEPlot.

References: [1] Stöffler, D. et al. (2018) Meteoritics & Planet. Sci. (MaPS), 53, 5-49. [2] Yamaguchi, A. et al. (2003) Springer Science+Business Media, LLC. pp. 29–45. [3] Nagaki, K. et al. (2016) MaPS, 51, 1153-1162. [4] Kurosawa, K. et al. (2015) Journal of Geophysical Research Planets (JGR), 120, 1237-1251. [5] Amsden, A. A., et al. (1980) LANL Report LA-8095. 101 p. [6] Ivanov, B. A., et al. (1997), IJIE, 20, 411. [7] Wünnemann, K., et al. (2006) Icarus, 180, 514. [8] Ahrens, T. J. & Gregson, Jr, V. G. (1964) JGR, 69, 4839-4874