Comparison of reduction power between serpentine and saponite based on oxidation analysis of Fe in Ryugu

<p>In mineral alteration processes in reducing hydrothermal environments such as the Central Ridge and subduction zones, much work has been done on the generation of hydrogen associated with the oxidation of divalent iron (Fe(II)) (e.g., McCollom and Bach, 2009), and the Fischer-Tropsch type reaction (carbon dioxide and hydrogen to produce aliphatic organic carbons such as methane) as a nonbiological organic matter formation process (e.g., Sforna et al., 2018). Similar arguments have been made for carbonaceous chondrites with similar mineral compositions, for example (e.g., Glein and Zolotov). As for the Fe(II)-associated reactions, the formation of magnetite from olivine has been considered important from the beginning, but it has been noted that oxidation reactions that Fe(II) undergoes at lower temperatures in the structure of layered silicates (serpentinite, saponite), once formed, contribute to the same extent (Andreani et al. 2013). On the other hand, the Fe(II)/Fetotal ratio in the structure of 2:1-type layered silicates (e.g., smectite) decreases with increasing Eh over a wide Eh range, but the changing Eh range has been shown to be mineral dependent (Gorski et al., 2013). Samples from the C-type asteroid Ryuguu brought back by Hayabusa2 indicate that Ryuguu experienced water alteration in the early solar system and that there was coevolution of minerals and organic matter associated with water alteration. Quantifying the Fe(II)/Fe(III) ratio in layered silicates from Ryuguu, which, unlike carbonaceous chondrites, are unaffected by terrestrial weathering, will provide important information for studying the reducing effects of layered silicates within the asteroid. In this study, the Fe(II)/Fe(III) ratios of layered silicates in Ryuguu were investigated by analyzing them with a scanning transmission X-ray microscope (STXM) without the influence of the oxidative Earth atmosphere.</p>