[M-ZZ42] Frontiers in geochemistry: new challenges and future prospects

[MZZ42-01] Cation partitioning during aqueous alteration of asteroid Ryugu recorded in carbonate minerals and Mg isotopes

★Invited Papers

*Toshihiro Yoshimura¹, Daisuke Araoka², Hiroshi Naraoka³, Saburo Sakai¹, Nanako O. Ogawa¹, Hisayoshi Yurimoto⁴, Mayu Morita⁵, Morihiko Onose⁵, Tetsuya Yokoyama⁶, Martin Bizzarro⁷, Satoru Tanaka⁵, Naohiko Ohkouchi¹, Toshiki Koga¹, Jason P. Dworkin⁸, Tomoki Nakamura⁹, Takaaki Noguchi¹⁰, Ryuji Okazaki³, Hikaru Yabuta¹¹, Kanako Sakamoto¹², Toru Yada¹², Masahiro Nishimura¹², Aiko Nakato¹², Akiko Miyazaki¹², Kasumi Yogata¹², Masanao Abe¹², Tatsuaki Okada¹², Tomohiro Usui¹², Makoto Yoshikawa¹², Takanao Saiki¹², Satoshi Tanaka¹², Fuyuto Terui¹³, Satoru Nakazawa¹², Sei-ichiro WATANABE¹⁴, Yuichi Tsuda¹², Shogo Tachibana^15,12, Yoshinori Takano¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.National Institute of Advanced Industrial Science and Technology, 3.Kyushu University, 4.Hokkaido University, 5.HORIBA Techno Service Co., Ltd, 6.Institute of Science Tokyo, 7.University of Copenhagen, 8.NASA Goddard Space Flight Center, 9.Tohoku University, 10.Kyoto University, 11.Hiroshima University, 12.Japan Aerospace Exploration Agency, 13.Kanagawa Institute of Technology, 14.Nagoya University, 15.University of Tokyo)

Keywords:Ryugu, aqueous alteration, Sodium, Mg stable isotope ratio

Samples of the carbonaceous asteroid (162173) Ryugu, collected by the Hayabusa2 spacecraft, provide information on their similarities to CI chondrites and evidence for extensive aqueous alteration (e.g. Yokoyama et al., 2023; Nakamura et al., 2023; Naraoka et al., 2023). Secondary precipitates (e.g., carbonates and phyllosilicates) reveal elemental partitioning of the major component ions linked to the primordial brine composition of the asteroid. In this presentation, we report on the cation partitioning and Mg stable isotopic composition (‰ deviation of ²⁵Mg/²⁴Mg, δ²⁵Mg) for the single grain of carbonates and aggregate samples by sequential leaching extraction of salts, exchangeable ions, and silicates (Yoshimura et al., 2024). Carbonate minerals were enriched in ²⁴Mg, and the δ²⁵Mg value of the fluid had shifted lower by ~0.38‰ than the initial value (set to 0‰) before dolomite precipitation, the main Mg-bearing carbonates. As a simple isotope model, the Mg²⁺ first precipitated in phyllosilicates, followed by dolomite precipitation, at which time ~76−87% of Mg²⁺ had been removed from the primordial brine. A minor amount of phyllosilicate precipitation continued after dolomite precipitation. The element composition profiles of the latest solution that interacted with the cation exchange pool of Ryugu were predominantly Na-rich. This is also consistent with the recent discovery of highly soluble Na carbonate in Ryugu samples (Matsumoto et al., 2024). Presumably, Na⁺ acts as a bulk electrolyte and contributes to the stabilization of the negative surface charge of phyllosilicates and organic matter on Ryugu. The recent astrochemical observation of primordial brine signatures and chemical evolution revealed by the asteroid Bennu (McCoy et al., 2025; Glavin et al., 2025) will lead to a comparative examination in the aqueous alteration history of the two carbonaceous asteroids.

References:
Glavin et al. (2025) Abundant ammonia and nitrogen-rich soluble organic matter in samples from asteroid (101955) Bennu. Nature Astronomy, doi.org/10.1038/s41550-024-02472-9.
Mastumoto et al. (2024) Sodium carbonates on Ryugu as evidence of highly saline water in the outer Solar System. Nature Astronomy, 8, 1536–1543.
McCoy et al. (2025) An evaporite sequence from ancient brine recorded in Bennu samples. Nature, 637, 1072-1077.
Nakamura et al. (2023) Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples. Science, 379, eabn8671.
Naraoka et al. (2023) Soluble organic molecules in samples of the carbonaceous asteroid (162173) Ryugu. Science, 379, eabn9033.
Yokoyama et al. (2023) Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites. Science, 379, eabn7850.
Yoshimura et al. (2024) Breunnerite grain and magnesium isotope chemistry reveal cation partitioning during aqueous alteration of asteroid Ryugu. Nature Communications, 15, 6809.