Structural and chemical modifications of oxides and OH generation by space weathering: Electron microscopic/spectroscopic study of hydrogen-ion-irradiated Al2O3

1,2Yohei Igami,1Shunsuke Muto,3Aki Takigawa,1Masahiro Ohtsuka,2Akira Miyake,4Kohtaku Suzuki,5Keisuke Yasuda,6,7,8Akira Tsuchiyama
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.09.031]
1Institute of Materials and Systems for Sustainability, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
2Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan
3Department of Earth and Planetary Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
4The Wakasa Wan Energy Research Center, Tsuruga, Fukui 914-0192, Japan
5Graduate School of Life and Environmental Science, Kyoto Prefectural University, Shimogamo-Hangicho, Sakyo-ku, Kyoto 606-8522, Japan
6Research Organization of Science and Technology, Ritsumeikan University, Kusatsu 525-8577, Japan
7CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China
8CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
Copyright Elsevier

Minerals on airless bodies exhibit characteristic spectral features such as darkening and reddening. Such space weathering is mainly due to hydrogen-ion irradiation by the solar wind and to micrometeorite impacts. Because of the reactivity of hydrogen, the associated H-implantation into O-bearing minerals can lead to the formation of new chemical bonds and may contribute to formation of water. However, laboratory studies still conflict about production efficiency of water and relevant H-bearing molecules such as OH formed by the H-ion irradiation. The production efficiency of the molecules within minerals may be influenced by short-range structural order of the host minerals. It is thus important to clarify how the implanted H interacts with various irradiation defects produced by H-ion bombardment. Here, we investigated H-ion-irradiated alumina (Al2O3), one of the most basic oxides, using scanning/transmission electron microscopy (S/TEM) and electron energy-loss spectroscopy (EELS). The TEM images revealed dense dislocations, nanoscale voids and nanoscale cracks—instead of amorphization—in the region subject to high energy deposition. Our analyses by STEM–EELS hyperspectral imaging (HSI) isolated a few essential spectral components, suggesting that chemical interactions between the implanted H and the host alumina resulted in local generation of OH species rather than amorphization. We also found a spectral feature which may be explained by H2 gas, presumably remaining in the nanovoids, most of which escaped through fractures formed by the coalescence of the high-pressure H2 nanobubbles. Such fractures/crack surfaces can act as additional reactive sites for the formation of the OH species. The present results strongly imply that H+ irradiation can be a source of water in minerals in various astrophysical conditions. The present methodology can be applied to a wide range of extraterrestrial materials, such as regolith grains, interplanetary-dust particles, and/or presolar grains in primitive meteorites.

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