9:00 AM - 9:15 AM
[PPS07-19] Shock recovery with decaying compressive pulses: Shock effects in marble, granite, and basalt
Keywords:Hypervelocity impacts, Meteorites, Shock metamorphism, Shock recovery, Two-stage hydrogen gas guns, Shock physics modeling
Most of the meteorites found on the Earth have recorded metamorphic features caused by mutual collisions between planetesimals in the past Solar System. The shock-induced metamorphism in minerals, such as quartz, feldspars, pyroxenes, and olivines, has been widely investigated by previous shock recovery experiments and summarized as progressive shock stages [e.g., Stöffler et al. 2018]. This shock classification, which is known as the Stöffler table, has been used as a “dictionary” to decipher impact conditions producing shock metamorphisms in meteorites and to reconstruct past impact events in the Solar System. Most of the past shock recovery experiments were performed by uniaxial compression using metal plates. Although this method has the great advantage that the shock pressure can be calculated accurately, it has the problems that it is difficult to remove the effect of reflected shock waves and that it ignores the effect of shear stress due to extensional deformation in the direction perpendicular to the direction of shock wave propagation.
In order to overcome these problems, we have constructed a new experimental system and procedure. We used a sufficiently small projectile to produce a decaying compressive pulse into a rocky target with the size of about 3 cm [Kurosawa et al., In revision]. Since the compressive pulse propagates in a hemispherical shape in the target, the geometrical conditions are similar to those of natural impacts. Although the weak point of this method is that the pressure and temperature in the sample cannot be calculated analytically, shock physics modelling can provide accurate estimates. We have conducted experiments using terrestrial rock samples, including, marble, basalt, and granite. In this presentation, we introduce the experimental method in detail and discuss the results obtained.
The experimental and analytical procedures are briefly described as follows. The shock recovery experiments were carried out using a two-stage hydrogen gas gun installed at the Planetary Exploration Research Center of the Chiba Institute of Technology, Japan. Rock samples were processed into cylinders of 30 mm in diameter and 24 mm in thickness. Each sample was enclosed in a titanium container with a detachable front plate made of metals, such as aluminum and titanium. A polycarbonate sphere with a diameter of 4.8 mm or a titanium sphere with a diameter of 2 mm was used as a projectile. A decaying compressive pulse was generated in the rock sample by the hypervelocity impact of the projectile on the front plate. After the experiment, a crater is formed on the front plate, but the rock sample can be recovered keeping with the pre-impact stratigraphy. The recovered samples were cut into thick slices parallel to the projectile trajectory, and polished thin sections were made from the thick slices. Finally, we analyzed the polished thin sections with a variety of techniques, including three-dimensional CT imaging, optical/electron microscopy, electron microprobe analysis, X-ray diffraction analysis and Raman spectroscopy.
The results are briefly described as follows: (1) calcite shows undulatory extinction under >3 GPa compression [Kurosawa et al., In revision], (2) the shock metamorphic features in granite and basalt are generally consistent with the Stöffler table classification [Ono et al., 2021, LPSC, #1810; Hamann et al., 2022, LPSC, #2020], (3) the formation of feather features at 8–18 GPa in granite [Tada et al., 2022, LPSC, #1733], (4) the formation of vein-like structures corresponding to high-pressure polymorphs and quenched glass at 10–20 GPa in granite [Hamann et al., 2022, LPSC, #2020], and (5) in basalt, veins of quenched glass form at ~10 GPa [Ono et al., to be submitted]. The detected pressure effects in the shocked samples were consistent with the classification by the Stöffler table. However, the observed microstructures due to melting even under low pressures of about 10–20 GPa suggest that the conventional classification needs to be revised in terms of experiencing temperatures in meteorites.
In order to overcome these problems, we have constructed a new experimental system and procedure. We used a sufficiently small projectile to produce a decaying compressive pulse into a rocky target with the size of about 3 cm [Kurosawa et al., In revision]. Since the compressive pulse propagates in a hemispherical shape in the target, the geometrical conditions are similar to those of natural impacts. Although the weak point of this method is that the pressure and temperature in the sample cannot be calculated analytically, shock physics modelling can provide accurate estimates. We have conducted experiments using terrestrial rock samples, including, marble, basalt, and granite. In this presentation, we introduce the experimental method in detail and discuss the results obtained.
The experimental and analytical procedures are briefly described as follows. The shock recovery experiments were carried out using a two-stage hydrogen gas gun installed at the Planetary Exploration Research Center of the Chiba Institute of Technology, Japan. Rock samples were processed into cylinders of 30 mm in diameter and 24 mm in thickness. Each sample was enclosed in a titanium container with a detachable front plate made of metals, such as aluminum and titanium. A polycarbonate sphere with a diameter of 4.8 mm or a titanium sphere with a diameter of 2 mm was used as a projectile. A decaying compressive pulse was generated in the rock sample by the hypervelocity impact of the projectile on the front plate. After the experiment, a crater is formed on the front plate, but the rock sample can be recovered keeping with the pre-impact stratigraphy. The recovered samples were cut into thick slices parallel to the projectile trajectory, and polished thin sections were made from the thick slices. Finally, we analyzed the polished thin sections with a variety of techniques, including three-dimensional CT imaging, optical/electron microscopy, electron microprobe analysis, X-ray diffraction analysis and Raman spectroscopy.
The results are briefly described as follows: (1) calcite shows undulatory extinction under >3 GPa compression [Kurosawa et al., In revision], (2) the shock metamorphic features in granite and basalt are generally consistent with the Stöffler table classification [Ono et al., 2021, LPSC, #1810; Hamann et al., 2022, LPSC, #2020], (3) the formation of feather features at 8–18 GPa in granite [Tada et al., 2022, LPSC, #1733], (4) the formation of vein-like structures corresponding to high-pressure polymorphs and quenched glass at 10–20 GPa in granite [Hamann et al., 2022, LPSC, #2020], and (5) in basalt, veins of quenched glass form at ~10 GPa [Ono et al., to be submitted]. The detected pressure effects in the shocked samples were consistent with the classification by the Stöffler table. However, the observed microstructures due to melting even under low pressures of about 10–20 GPa suggest that the conventional classification needs to be revised in terms of experiencing temperatures in meteorites.