R5: Extraterrestrial materials

[R5-16] Aqueous alteration experiments using GEMS simulant particles

*Akira TSUCHIYAMA^1,2, Satomi Enju³, Haiyang Xian¹, Mingqi Sun¹, Ding Xing¹, Yuanyuan Wang⁴, Zhaolu He¹, Jing Du¹, Shan Li¹, Jiaxin Xi¹, Xiaoju Lin¹, Jian Di¹, Yuto Imura⁵ (1. Guangzhou Inst. Geochem., 2. Ritsumeikan Univ., 3. Ehime Univ. Sci, 4. Tokyo Univ. Sci., 5. Kyoto Univ. Sci.)

Keywords:Ryugu samples, Mg-S-H, carbonaceous chondrites, GEMS-like material, induction thermal plasma

Samples from the asteroid Ryugu by the Hayabusa2 spacecraft correspond to CI chondrites (e.g., [1]). In this study, to reproduce the initial stages of aqueous alteration of Ryugu parent body [2], we performed an aqueous alteration experiment using GEMS simulant particles, prepared by the induction thermal plasma (ITP) [3] with CI or GEMS average composition. Aqueous solutions of formic acid, hexamethylenetetramine, ammonium acetate, and/or ammonium hydrogen carbonate were heated at 200°C for 168~1550 hours in a sealed Teflon container with the GEMS simulant with the rock/water ratio of 5 (or 1). Run products were analyzed by XRD, SEM/EDS, TEM/EELS, and micro-Raman spectroscopy.In all runs, low-crystallinity hydrous layered silicates (Mg silicate hydrate: M-S-H) were formed. Hematite, maghemite or magnetite + hematite formed as Fe oxides consisted of fine particles (~a few 10 nm) and occasionally dendrites within a M-S-H matrix (Fig. 1a). The run products may represent the very early state of aqueous alteration. In most runs, small amounts of anhydrite were formed, but no carbonate formed. Organic crystals and ammonium sulfates also formed. IOM with G and D bands were confirmed by Raman spectroscopy.The run products are aggregates of several 10 μm sized clasts with different porosities (Fig. 1b), which reflects the original texture of the starting material. Ryugu sample matrix of similar texture [2] may be inherited from that formed by accumulation in the parent body. The present experiment was not fully successful in reproducing the minerals and their morphologies as seen in the Ryugu samples. In particular, the formation of maghemite and anhydrite indicates that further experiments under more reducing conditions are necessary.[1] Nakamura T. et al. (2023) Science, 379, eabn867. [2] Tsuchiyama A. et al. (2024) GCA, 375, 146-172. [3] Enju S. et al. (2022) A&A, 661, A121.