講演情報
[S2-06]Influence of the degree of serpentinization on hydrogen production from ultramafic rocks.
*Nomin Tumurkhuu1, Otgonbayar Dandar1, Jiajie Wang1, Kazuki Yoshida2, Noriaki Watanabe1, Masao Kimura2,3, Atsushi Okamoto1 (1. Tohoku University, 2. High Energy Accelerator Research Organization (KEK), 3. The Graduate University for Advanced Studies)
キーワード：
Hydrogen generation
Serpentinization of ultramafic rocks at mid-ocean ridges generates significant amounts of hydrogen (H2), supporting specialized biological communities and highlighting its potential as a green energy resource. Hydrogen is produced abiotically through water reduction coupled with the oxidation of ferrous iron in primary minerals such as olivine and pyroxene. Its generation potential depends on the abundance and oxidation state of iron. Previous experimental and geochemical modeling studies have mainly focused on fresh peridotite and olivine to clarify the mechanisms and reaction pathways of H2 generation (Klein et al., 2009, 2013; Malvoisin et al., 2013; McCollom et al., 2020). However, despite the widespread occurrence of serpentinized peridotites, the influence of the degree of serpentinization on hydrogen generation remains poorly understood. In this study, we conducted hydrothermal experiments using natural ultramafic rocks with varying degrees of serpentinization. Hydrothermal experiments were performed in a stirring batch-type reactor at 300 deg C and 10 MPa for 72 h using six ultramafic rock samples from Hokkaido. The starting materials range from least serpentinized harzburgites (G1 and G5) and moderate serpentinized harzburgites (G2, G3, and G6), composed predominantly of olivine with minor orthopyroxene (Opx) and clinopyroxene (Cpx), to a fully serpentinized sample (G4). Two types of brucite were identified in G4: Fe-rich brucite (#Mg = 0.81) occurring as large matrix lenses and Fe-poor brucite (#Mg = 0.94) occurring in small veins. Sample G1 produced the highest H2 yield (15.85 mmol/kg rock H2), followed by G6 (11.7 mmol/kg rock H2) and G5 (10.3 mmol/kg rock H2). Samples G2 and G3 generated similar amounts (8-9 mmol/kg rock H2), whereas G4 produced the lowest yield (5.5 mmol/kg rock H2). An additional experiment using pure olivine powder yielded the highest H2 production (21.9 mmol/kg rock H2). These results demonstrate a negative linear relationship between H2 production and the degree of serpentinization, indicating that progressive serpentinization reduces the hydrogen-generation potential of harzburgite. Nevertheless, the fully serpentinized sample G4 still generated measurable H2, likely through the oxidation of Fe-rich brucite during magnetite formation. Bulk-rock Fe3+/ΣFe ratios were determined by Fe K-edge X-ray absorption near-edge structure (XANES) spectroscopy to evaluate changes in iron oxidation during the experiments. All samples exhibited increased Fe3+/ΣFe ratios after reaction, indicating progressive oxidation of Fe2+. H2 yields estimated from the XANES-derived Fe3+/ΣFe changes agreed well with the measured H2 production, with the highest yield in G1 (15.86 mmol/kg rock H2), followed by G2 (14.63), G5 (10.72), G3 (7.32), G6 (7.17), and G4 (5.02 mmol/kg rock H2). Although bulk XANES cannot resolve mineral-scale Fe redistribution, the results indicate that H2 generation is primarily controlled by the abundance of remaining olivine and decreases with increasing serpentinization. However, the measurable H2 production from G4 suggests that Fe-rich brucite provides an additional Fe2+ source for magnetite formation, sustaining H2 generation even after olivine has been consumed.
