[M-IS19] Aqua planetology

[MIS19-P05] Low-temperature mineral synthesis experiments in the Mg²⁺-SiO₂-H⁺-H₂O system:Implications for aqueous alteration of the Ryugu parent body

*Takuma Fujiya¹, Keisuke Fukushi², Takahiro Kawai³, Ryoichi Nakada, So Fukaya¹, Yumi Morita¹, Baasansuren Gankhurel¹, Tegla Elizabeth Jones⁴, Yasuhito Sekine⁴, Yoshio Takahashi⁵ (1.Kanazawa University, 2.Institute of Nature & Environmental Technology, Kanazawa University, 3.University of Tokyo, 4.Earth-Life Science Insitute, Tokyo Institute of Technology, 5.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo)

The "Hayabusa2" successfully collected surface materials from Ryugu and returned to Earth in 2020. As a result of sample analysis by many researchers, it was found that the matrix of the Ryugu is mainly composed of hydrous phyllosilicates formed by aqueous alteration. Ito et al. (2022 Nature Astronomy) reported that the matrix composition of Ryugu is intermediate between that of saponite and serpentinite, as determined by chemical analysis. Nakamura et al. (2023 Science) reported that the matrix in Ryugu is composed of saponite, serpentine, and saponite-serpentine mixed layer minerals. Further detailed electron microscopic analysis indicates that these mineral species intimately coexist as a nanoscale mixture.
A mixed-layer mineral is a mineral consisting of alternating layers of different layered phyllosilicate unit structures. The mixed-layer minerals composed of various phyllosilicates have been reported in nature (Shirozu, 2011 Nendo Kobutsugaku). On the other hand, the natural occurrence of the mixed layer mineral composed of serpentinite and saponite has not been reported on Earth (Torii et al., 1998 J. Am. Ceram. Soc.). The coexistence of saponite and serpentinite has also rarely been reported on Earth. The mineralogy of hydrous phyllosilicates in the Ryugu matrix must record the environmental conditions for the aqueous alteration that once occurred in the Ryugu parent body. However, the absence or rare occurrence of the nanoscale mixture of saponite and serpentine including the mixed layer mineral makes it difficult to constrain the environmental conditions.
Detailed sample analyses generally showed that temperature of the aqueous alteration was <50 ºC (Nakamura et al., 2023Science; Yokoyama et al., 2022Science), of which temperature is generally lower than that of terrestrial serpentine formation. In this study, we conducted a mineral synthesis experiment in the Mg²⁺-SiO₂-H⁺-H₂O system as function of pH under low-temperature conditions that simulate the aqueous alteration process in Ryugu parent body. The results showed that the mineralogical characteristics especially from Mg-XANES spectra of the products formed in the experiments under high pH condition is comparable to those of the hydrous phyllosilicates found in Ryugu samples.