9:00 AM - 9:20 AM
[MIS07-01] Estimation of aqueous environment of the Ryugu parent body and the relationship between solubility of elements and their bioessentiality based on molecular geochemistry
★Invited Papers
Keywords:Speciation, Ryugu parent body, Adsorption reaction, Essential elements
The parent body of the C-type asteroid Ryuguu is thought to have been rich in volatile elements and abundant in liquid water, making it an important research target for the origin of water and organic matter on Earth. The water caused various alteration 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 reactions of 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 the aqueous 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 are adsorbed 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 the selectivity coefficient K. Therefore, the cation composition in the water layer at the time of aqueous alteration can be estimated from the interlayer cation ratio determined by various analytical methods. In this processes, speciation of Na, K, and Ca were conduted 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 an Eh-pH diagram of Mg. We have estimated pH range that can explain the presence of both saponite and dolomite, which are widely 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 an alkaline condition is suitable for the polymerisation of amino acids (Kitadai and Maruyama, 2018).
In the presentiion, I will also dicusss the relationship between solubility of elements and their bioessentiality based on molecular geochemical analysis, which is related to the adsorption/desorption behaviors of trace but essential elements at the solid-water interface in aqueous environment (mainly in seawater).
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 are adsorbed 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 the selectivity coefficient K. Therefore, the cation composition in the water layer at the time of aqueous alteration can be estimated from the interlayer cation ratio determined by various analytical methods. In this processes, speciation of Na, K, and Ca were conduted 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 an Eh-pH diagram of Mg. We have estimated pH range that can explain the presence of both saponite and dolomite, which are widely 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 an alkaline condition is suitable for the polymerisation of amino acids (Kitadai and Maruyama, 2018).
In the presentiion, I will also dicusss the relationship between solubility of elements and their bioessentiality based on molecular geochemical analysis, which is related to the adsorption/desorption behaviors of trace but essential elements at the solid-water interface in aqueous environment (mainly in seawater).