S2: 岩石-水相互作用 （スペシャルセッション）

14:45 〜 15:00

[S2-09] Hydrogen generation and element transfer during the alteration of the crust–mantle transition zone in the Bayankhongor Ophiolite, Mongolia.

*Nomin Tumurkhuu¹, Otgonbayar Dandar¹, Manzshir Bayarbold¹, Kazuki Yoshida², Yasuhiro Niwa^2,3, Masao Kimura^2,3, Atsushi Okamoto¹ (1. Tohoku University, 2. High Energy Accelerator Research Organization (KEK), 3. The Graduate University for Advanced Studies)

キーワード：

Element transport、Iron Partitioning、Hydrogen

Serpentinization in the oceanic lithosphere is a key water–rock reaction linking the hydrosphere and lithosphere, influencing global geochemical cycles, the water cycle, deep-sea microbial life, and hydrogen production. This process redistributes elements between the mantle and crust, mainly through the oxidation of Fe2+ in olivine and pyroxene to Fe3+ in magnetite and serpentine, releasing H2. Accurate hydrogen estimation depends on understanding iron partitioning during serpentinization. The crust–mantle transition zone is complex, featuring various mafic dykes and fluid pathways, but element transfer, iron partitioning, and hydrogen production remain poorly understood. This study uses samples from the Bayankhongor ophiolite (~650 Ma). Two serpentinite types (serpentinized dunite and harzburgite) show multi-stage serpentinization, indicating a mid-ocean ridge environment. Altered mafic dyke (Chlorite) at crust-mantle transition zone in the ophiolite altered by Mg/Fe-rich fluids show that gabbroic dyke alteration releases Si and Al, while the reaction zone absorbs Si and Al and releases Fe. Chloritization retains iron mainly as Fe2+ and dissolves magnetite. Iron oxidation via bulk-rock and 2D X-ray Absorption Fine Structure spectroscopy reveals the highest Fe3+/Fe_total ratio in foliated serpentinized dunite (0.77), lower in massive dunite (0.64), and harzburgite (0.49). Mesh core textures exhibit the highest ferric iron ratio (0.56), while chrysotile has a lower ratio (0.22). In the altered gabbroic dyke, both the reaction zone and adjacent serpentinite have similar ferric ratios (~0.65), despite magnetite loss. In contrast, chlorite has a much lower ratio (0.2). Hydrogen production estimates: serpentine produces 30–50 mM H2/kg rock, magnetite 60–190 mM/kg, and serpentinized dunite up to 230 mM/kg. Overall, serpentinites in the transition zone generate 98–230 mM H2/kg. Thus, the transition zone hosts significant hydrogen production linked to Fe oxidation, but mafic dykes altered by Mg/Fe-rich fluids contribute to magnetite dissolution.

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