16:15 〜 16:30
[PPS08-10] Mild shock metamorphism experienced by surface particles of asteroid Ryugu
キーワード：はやぶさ２、衝撃変成、透過電子顕微鏡、断層、高圧相
Shock effects related to hydrated asteroids are of particular interest in planetary sciences as this type of asteroid is thought to be one of the major sources of extraterrestrial dust particles. Hydrated materials are likely to be explosively pulverized to become dust particles by the vaporization of volatile components like H2O during shock heating, and therefore are unlikely to survive as meteorites [1]. This hypothesis was further confirmed by comparing the petrology and mineralogy of recovered samples of anhydrous CV and hydrated CM chondrites using laboratory shock experiments [2,3]. JAXA's Hayabusa2 mission has provided the first opportunity to directly evaluate the shock metamorphism of hydrated asteroidal materials [4]. The present study aims to evaluate the degree of shock metamorphism of Ryugu's surface materials by SEM and TEM, and to assess the hypothesis that massive dust production is driven by dehydration during impact processing on small hydrated asteroids.
Five Ryugu particles examined in this study initially appeared unshocked; however, we found some characteristic features related to shock metamorphism. Particle C0014 exhibits rare, thin straight veins less than 70 µm in length and less than 5 µm in width in SEM observation. In one of these veins, an aggregate of spherical magnetite particles has a thin lens shape, and the aggregate is terminated on both sides by elongated phyllosilicates along the veins. Along the same direction as the vein, another framboidal magnetite aggregate is deformed in simple shear. This feature is likely a fine-grained lithic vein formed by brittle cataclastic deformation: the vein is likely a micro-fault produced by shock metamorphism. This study attempts to evaluate shock-induced faulting as an analogous process to the faulting that causes earthquakes on Earth. By a fault mechanics analysis, the upper bound of the mean stress, which is approximated as the peak pressure, is estimated to be ~2 GPa.
In TEM observation, all the observed particles are dominated by phyllosilicates but lack dehydration textures. This suggests that the shock temperature was below ~500 °C. In the fine-grained phyllosilicate matrix, a unique Fe-sulfide grain was identified in particle A0002. The grain is euhedral and 1.2 µm in size with a pure FeCr2S4 composition. SAED patterns from the grain are indexed with a dense monoclinic FeCr2S4 mineral known as zolenskyite. Based on a phase equilibrium study of FeCr2S4 [5], the low pressure phase daubréelite would have transformed into zolenskyite above ~2 GPa in A0002.
Considering the above mineralogical and petrological features, we conclude the average peak pressure of the Ryugu particles is only ~2 GPa. The complete lack of dehydration textures and mineral features in Ryugu particles observed in the present study demonstrates that Ryugu's surface materials preserved their water as hydroxyl in phyllosilicates throughout the mild impact events that they experienced. This study also suggests that the production of dust particles on Ryugu-like asteroids, due to shock-heating-induced volatilization during such break-up events, is much smaller than previously expected.
[1] Scott, E. R. D., Keil, K. & Stöffler, D. (1992) Geochim. Cosmochim. Acta 56, 4281–4293.
[2] Tomeoka, K., Kiriyama, K., Nakamura, K., Yamahana, Y. & Sekine, T. (2003) Nature 423, 60–62.
[3] Tomioka, N., Tomeoka, K., Nakamura-Messenger, K. & Sekine, T. (2007) Meteorit. Planet. Sci. 42, 19–30.
[4] Yada, T., Abe, M., Okada, T., et al. (2021) Nat. Astron. 6, 214–220.
[5] Tressler, R. E., Hummel, F. A. & Stubican, V. S. (1968) J. Amer. Ceram. Soc. 51, 648–651.
Five Ryugu particles examined in this study initially appeared unshocked; however, we found some characteristic features related to shock metamorphism. Particle C0014 exhibits rare, thin straight veins less than 70 µm in length and less than 5 µm in width in SEM observation. In one of these veins, an aggregate of spherical magnetite particles has a thin lens shape, and the aggregate is terminated on both sides by elongated phyllosilicates along the veins. Along the same direction as the vein, another framboidal magnetite aggregate is deformed in simple shear. This feature is likely a fine-grained lithic vein formed by brittle cataclastic deformation: the vein is likely a micro-fault produced by shock metamorphism. This study attempts to evaluate shock-induced faulting as an analogous process to the faulting that causes earthquakes on Earth. By a fault mechanics analysis, the upper bound of the mean stress, which is approximated as the peak pressure, is estimated to be ~2 GPa.
In TEM observation, all the observed particles are dominated by phyllosilicates but lack dehydration textures. This suggests that the shock temperature was below ~500 °C. In the fine-grained phyllosilicate matrix, a unique Fe-sulfide grain was identified in particle A0002. The grain is euhedral and 1.2 µm in size with a pure FeCr2S4 composition. SAED patterns from the grain are indexed with a dense monoclinic FeCr2S4 mineral known as zolenskyite. Based on a phase equilibrium study of FeCr2S4 [5], the low pressure phase daubréelite would have transformed into zolenskyite above ~2 GPa in A0002.
Considering the above mineralogical and petrological features, we conclude the average peak pressure of the Ryugu particles is only ~2 GPa. The complete lack of dehydration textures and mineral features in Ryugu particles observed in the present study demonstrates that Ryugu's surface materials preserved their water as hydroxyl in phyllosilicates throughout the mild impact events that they experienced. This study also suggests that the production of dust particles on Ryugu-like asteroids, due to shock-heating-induced volatilization during such break-up events, is much smaller than previously expected.
[1] Scott, E. R. D., Keil, K. & Stöffler, D. (1992) Geochim. Cosmochim. Acta 56, 4281–4293.
[2] Tomeoka, K., Kiriyama, K., Nakamura, K., Yamahana, Y. & Sekine, T. (2003) Nature 423, 60–62.
[3] Tomioka, N., Tomeoka, K., Nakamura-Messenger, K. & Sekine, T. (2007) Meteorit. Planet. Sci. 42, 19–30.
[4] Yada, T., Abe, M., Okada, T., et al. (2021) Nat. Astron. 6, 214–220.
[5] Tressler, R. E., Hummel, F. A. & Stubican, V. S. (1968) J. Amer. Ceram. Soc. 51, 648–651.