11:15 AM - 11:30 AM
[PPS07-13] The shock pressure threshold for producing shock melt veins in basaltic rocks
Keywords:shock metamorphism, mafic rocks, basalt, shock experiment
Introduction
Shock metamorphic textures record the impact history of meteorite parent bodies. Shock features in meteorites have been classified into progressive stages (shock stages) based on petrological and mineralogical textures (e.g., Stöffler et al., 2018), and the classification has been used to decode the impact conditions, including projectile diameter and impact velocity. Stöffler et al., 2018 summarized the classifications for a variety of minerals and rocks as the table, which is known as “the Stöffler table”, by using a number of previous results of uniaxial-shock recovery experiments. Recently, Kanemaru et al. 2020 independently constructed a detailed shock metamorphic classification pertaining to basaltic eucrites. They showed that the shock stages of the Stöffler table for mafic rocks (Table M) could be further divided into a few shock degrees. In this study, we performed a shock recovery experiment with terrestrial basalt samples using decaying shock waves. We recovered the shocked sample which has experienced a wide range of shock pressures and temperatures. The shock textures in the recovered sample were compared with the previous study (Kanemaru et al., 2020). The peak pressures and temperatures depending on the initial locations in the target were estimated using numerical simulations under the same impact conditions.
Sample and Methods
We used terrestrial basalt samples (Inner Mongolia), which are composed mainly of plagioclase, pyroxene, and olivine, as a target. The sample was shaped to a cylinder (30 mm in diameter, 24 mm in height), and it was set in titanium (Ti) container. A polycarbonate sphere was used as a projectile. The impact velocity was 7.3 km s-1. A thin section of the recovered sample was observed by optical microscopy and a Scanning Electron Microprobe (SEM). We estimated pressures and temperature in the sample based on petrological and mineralogical textures. In addition, peak pressures and temperatures in the target were also numerical estimated with the iSALE shock physics code (Amsden et al., 1980, Ivanov et al., 1997, Wünnemann et al., 2006). The results from the above two different estimations were compared with each other.
Results and Discussion
Plagioclase, pyroxene, and olivine in the shocked sample around the epicenter in the recovered sample showed a shock feature called undulatory extinction. Several shock melt veins (up to 4 µm in width) were identified around the epicenter. The presence of vesicles (~2 µm in diameter) in the melt veins would be evidence of volatile degassing due to melting. The plagioclase grains adjoining the shock melt veins exhibit undulatory extinction, but they do not exhibit planar deformation features (PDFs), mosaicism, and vitrification. According to the Stöffler table M, the recovered sample is classified into the shock stage S2 in terms of the shock features in plagioclase. The sample also belongs to the shock stage S3 based on the fact that shock melt veins are produced due to some local effects. In contrast, the textures observed in our shocked sample uniquely correspond to the shock degree C proposed by Kanemaru et al. (2020). The iSALE simulations showed that the pressures and temperatures around shock melt veins were estimated to be up to 10 GPa and 600 K, respectively. Therefore, we recognized that the shock degree C (Kanemaru et al., 2020) was formed due to a shock compression to ~10 GPa, the local temperature could be about 800 K higher than the calculated temperature (the temperature is estimated from the solidus of dry basalt), and the Stöffler table does not predict such the temperature excess.
Conclusion
We performed a shock recovery experiment and determined the required pressure for producing shock melt veins in basaltic rocks to be ~10 GPa. The shock features in the collected sample are similar to the textures in basaltic eucrites at the shock degree C. The peak temperatures in the sample around the shock melt veins were estimated to be ~600 K, which is much lower than the solidus at the reference pressure. Thus, the local temperature could be much higher than the expected one from the conventional shock stage (Stöffler table).
Shock metamorphic textures record the impact history of meteorite parent bodies. Shock features in meteorites have been classified into progressive stages (shock stages) based on petrological and mineralogical textures (e.g., Stöffler et al., 2018), and the classification has been used to decode the impact conditions, including projectile diameter and impact velocity. Stöffler et al., 2018 summarized the classifications for a variety of minerals and rocks as the table, which is known as “the Stöffler table”, by using a number of previous results of uniaxial-shock recovery experiments. Recently, Kanemaru et al. 2020 independently constructed a detailed shock metamorphic classification pertaining to basaltic eucrites. They showed that the shock stages of the Stöffler table for mafic rocks (Table M) could be further divided into a few shock degrees. In this study, we performed a shock recovery experiment with terrestrial basalt samples using decaying shock waves. We recovered the shocked sample which has experienced a wide range of shock pressures and temperatures. The shock textures in the recovered sample were compared with the previous study (Kanemaru et al., 2020). The peak pressures and temperatures depending on the initial locations in the target were estimated using numerical simulations under the same impact conditions.
Sample and Methods
We used terrestrial basalt samples (Inner Mongolia), which are composed mainly of plagioclase, pyroxene, and olivine, as a target. The sample was shaped to a cylinder (30 mm in diameter, 24 mm in height), and it was set in titanium (Ti) container. A polycarbonate sphere was used as a projectile. The impact velocity was 7.3 km s-1. A thin section of the recovered sample was observed by optical microscopy and a Scanning Electron Microprobe (SEM). We estimated pressures and temperature in the sample based on petrological and mineralogical textures. In addition, peak pressures and temperatures in the target were also numerical estimated with the iSALE shock physics code (Amsden et al., 1980, Ivanov et al., 1997, Wünnemann et al., 2006). The results from the above two different estimations were compared with each other.
Results and Discussion
Plagioclase, pyroxene, and olivine in the shocked sample around the epicenter in the recovered sample showed a shock feature called undulatory extinction. Several shock melt veins (up to 4 µm in width) were identified around the epicenter. The presence of vesicles (~2 µm in diameter) in the melt veins would be evidence of volatile degassing due to melting. The plagioclase grains adjoining the shock melt veins exhibit undulatory extinction, but they do not exhibit planar deformation features (PDFs), mosaicism, and vitrification. According to the Stöffler table M, the recovered sample is classified into the shock stage S2 in terms of the shock features in plagioclase. The sample also belongs to the shock stage S3 based on the fact that shock melt veins are produced due to some local effects. In contrast, the textures observed in our shocked sample uniquely correspond to the shock degree C proposed by Kanemaru et al. (2020). The iSALE simulations showed that the pressures and temperatures around shock melt veins were estimated to be up to 10 GPa and 600 K, respectively. Therefore, we recognized that the shock degree C (Kanemaru et al., 2020) was formed due to a shock compression to ~10 GPa, the local temperature could be about 800 K higher than the calculated temperature (the temperature is estimated from the solidus of dry basalt), and the Stöffler table does not predict such the temperature excess.
Conclusion
We performed a shock recovery experiment and determined the required pressure for producing shock melt veins in basaltic rocks to be ~10 GPa. The shock features in the collected sample are similar to the textures in basaltic eucrites at the shock degree C. The peak temperatures in the sample around the shock melt veins were estimated to be ~600 K, which is much lower than the solidus at the reference pressure. Thus, the local temperature could be much higher than the expected one from the conventional shock stage (Stöffler table).