Speciation of various elements using scanning transmission/fluorescnece X-ray microscopy(STXM/SFXM) and bulk XANES analysis related to aqueous environment
in the Ryugu parent body
Takahiro Kawai1, Keisuke Fukushi2, Hiroki Suga3, Masayuki Uesugi3, Shohei Yamashita4, Ryoichi Nakada5, Mizuho Koike6,
Hideto Yoshida1, Masaki Ohura7, Megumi Matsumoto8, Tomoki Nakamura8, Yasuhito Sekine,9 and Yoshio Takahashi1*(*Presenting author)
1
Department of Earth and Planetary Science, Graduate School of Sceince, The University of Tokyo, 2Institute of Nature andEnvironmental Technology, Kanazawa University, 3Japan Synchrotron Radiation Research Institute (JASRI), 4photon Factory,
Institute of Materials Structure Science, High-Energy Accelerator Research Organization (PF, IMSS, KEK), 5Kochi Institutefor Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 6Department of Earth andPlanetary Systems Science, Hiroshima University, 7RIKEN SPring-8 Center, 8Dept. of Earth Sciences, Tohoku University,9Earth-Life Science Institute, Tokyo Institute of Technology
The parent body of the C-type asteroid Ryuguu is thought to have been rich in volatile elements and abundant in liquidwater, making it an important research target for the origin of water and organic matter on Earth. The water caused variousalteration in the surrounding minerals, resulting in layered silicates such as smectite (saponite) and serpentinite being the main
constituent minerals of the Ryugu (Nakamura et al., 2023). In this process, the organic matter of the Ryuguu parent body also
underwent chemical reactions and a wide variety of organic matter was synthesised (Naraoka et al., 2023). It is particularly
important to obtain information on the water environment during the aqueous alteration, as the products of chemical reactionsof organic matter in water depend on the pH and redox potential (Eh) of the water. In this study, speciation of various elements
including iron (Fe), magnesium (Mg), calcium (Ca), and carbon (C) was investigated using scanning transmission X-ray
microscopy (STXM) and bulk X-ray absorption near-edge structure (XANES) analysis in the soft X-ray region related to theaqueous environment in the Ryugu parent body. For the elements with conentrations below the detection limit of STXM,
scanning fluorescnece X-ray microscopy (SFXM), newly developed in SPring-8 and Photon Factory was applied such as
nitrogen (N) and sodium (Na). The speication of these elements in saponite is particularly important, since the Fe oxidaiton
state and chemical composition within the interlayer of the saponite can indciate Eh-pH condition of the Ryugu water.
The Fe(II)/Fe(III) ratio in the octahedral structure of saponite was conducted by STXM. Based on our laboratory
experiments, dithionite reduction of natural Fe-bering saponite corresponding to Eh range from -0.47 to -0.66 (V) at pH = 7
shows the Fe(II)/Fetotal ratio around 0.55. The incomplete reduction by dithionite reveals that Fe(II)-bering saponite is a strong
reductant. Assuming that the relationship between the Fe(II)/Fetotal ratio and Eh of nontronite in Gorski et al. (2013) is similar
to that of saponite, the Fe(II) /Fetotal ratio greater than 0.68 measured by STXM indicates that the Eh was less than -0.45 (V).
On the other hand, because saponite has a negative charge in its layered structure, cations from the aqueous phase areadsorbed between the saponite layers during aqueous alteration. In this case, as shown by Fukushi et al. (2019), which cations
are retained in the interlayer depends on the concentration of various cations in the coexisting aqueous phase and theselectivity coefficient K. Therefore, the cation composition in the water layer at the time of aqueous alteration can be estimatedfrom the interlayer cation ratio determined by various analytical methods. In this processes, speciation of Na, K, and Ca wereconduted to obtain their fractions retained in the interlayer. The comparison of the Al content in saponite enables us to estimate
the Mg content in the interlayer. Contents of these cations with assumptions of dissolved silica etc. were used to construct anEh-pH diagram of Mg. We have estimated pH range that can explain the presence of both saponite and dolomite, which arewidely distributed in the altered crust in the Ryugu particles. The pH range estimated by the method was pH from 7.8 to 10.
Although the range is somewhat wide, the range is consistent with the results suggested in Nakamura et al. (2022).
The cmbination of Eh around -0.45 (V) and the alkaline pH range estimated above showed that the water was highly
reducing, which reveals that H2 and CH4 were present in the Ryugu water if we assume the equilibrium condition. Such analkaline condition is suitable for the polymerisation of amino acids (Kitadai and Maruyama, 2018).
References: Fukushi K. et al. (2019) Semiarid climate and hyposaline lake on early Mars inferred from reconstructed waterchemistry at Gale. Nat. Commun. 10, 1–11.; Gorski C. A. et al. (2013) Redox properties of structural Fe in clay minerals: 3.
Relationships between smectite redox and structural properties. Environ. Sci. Technol. 47, 13477–13485.; Kitadai N. andMaruyama S. (2018) Origins of building blocks of life: A review. Geosci. Front. 9, 1117–1153.; Nakamura T. et al. (2023)
Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples. Science 379, eabn8671.;
Naraoka H. et al. (2023) Soluble organic molecules in samples of the carbonaceous asteroid (162173) Ryugu. Science 379,eabn9033.