5:15 PM - 6:45 PM
[SCG46-P21] Effect of lithology on hydrogen generation by serpentinization reaction in the oceanic lithosphere
Keywords:Oceanic lithosphere, Serpentinization, Oman Ophiolite, Hydrogen generation
Serpentinization reaction have been highlighted since the discovery of unique microbial communities in hydrothermal vents that use hydrogen as an energy source (Kelley et al., 2001). Magnetite and serpentine are the key minerals in hydrogen generation during serpentinization. Addition to temperature condition, the formation of magnetite and serpentinite is controlled by silica activity, thus hydrogen generation should be varied with lithologies. However, the hydrogen-generation mechanism during serpentinization reactions in different lithologies have not been well investigated from natural samples. The objective of this study is to clarify the effect of different lithologies on hydrogen generation during serpentinization in oceanic lithosphere.
In this study, we analysed of samples from the Oman ophiolite lower crust-upper mantle continuous drilling samples obtained from the Oman Drilling Project. We conducted bulk rock chemical composition analysis including redox state of iron, measurement of magnetic susceptibility, and observation of rock microstructures. The drilled samples consisted mainly of a lower crust composed of gabbroic rocks, a crust-mantle boundary composed of dunite, and an upper mantle composed of harzburgite. The gabbroic rocks were composed of serpentinized olivine with mesh texture, plagioclase and less altered orthopyroxene. The mesh texture consists mainly of serpentine and magnetite, with little brucite. Highly altered gabbroic rocks have less magnetite and the serpentine is enriched in iron. Ni-Fe alloy (Awaruite) is observed in the mesh texture of dunite and harzburgite, suggesting that the serpentinization reaction proceeded under highly reductive conditions. Magnetite content is highest in dunite, and the Fe3+ (Fe2O3) content of all rocks cannot be explained only by magnetite content. This suggests that minerals other than magnetite (serpentine and weathered minerals) may contribute to the total Fe2O3 content. To clarify the spatial distribution of the redox state of Fe in rocks, we performed 2D imaging XAFS measurements at the beamline of NW2A in the synchrotron radiation facility PF-AR. The results indicate that mesh texture serpentine and bastite (serpentine after orthopyroxene) contains much Fe3+ (i.e. 25-46% of the total iron), regardless of lithology.
The contributions of magnetite and serpentine to hydrogen generation in each lithology and the mechanism of hydrogen generation at each stage were investigated in terms of bulk rock compositions, iron content of each mineral, and detailed microstructures[KM1] . Estimation of hydrogen generation in serpentinization with mesh texture and subsequent magnetite veins, respectively, indicates that serpentinization with mesh texture is the main stage. In the serpentinization with mesh texture, gabbroic rocks, dunite, and harzburgite were estimated to have generated 84 mM/kg rock, 144 mM/kg rock, and 107 mM/kg rock of hydrogen, respectively. Calculations of the contribution of serpentine and magnetite to hydrogen generation indicate that magnetite is the major contributor to hydrogen generation in the gabbroic rocks, whereas serpentine is the major contributor in dunite and harzburgite. Even in gabbroic rock, which is rich in silica-rich minerals, olivine is considered to[KM2] react selectively and form magnetite, which is different from the reported result (Katayama et al., 2010). Cracks caused by thermal stress may have selectively destroyed olivine and promoted olivine-dominant reactions in the early stages of serpentinization reactions. Subsequently, the cracks or voids formed by the reaction-induced stress led to extensive serpentinization and increased silica activity (Yoshida et al., 2020). These results suggest that the amount of hydrogen generation and the minerals responsible for it vary with lithology. Hydrogen[KM3] is produced by the formation of Fe3+-bearing serpentine even when magnetite is difficult to form, and the microcrack formation may control the overall amount of hydrogen generation.
In this study, we analysed of samples from the Oman ophiolite lower crust-upper mantle continuous drilling samples obtained from the Oman Drilling Project. We conducted bulk rock chemical composition analysis including redox state of iron, measurement of magnetic susceptibility, and observation of rock microstructures. The drilled samples consisted mainly of a lower crust composed of gabbroic rocks, a crust-mantle boundary composed of dunite, and an upper mantle composed of harzburgite. The gabbroic rocks were composed of serpentinized olivine with mesh texture, plagioclase and less altered orthopyroxene. The mesh texture consists mainly of serpentine and magnetite, with little brucite. Highly altered gabbroic rocks have less magnetite and the serpentine is enriched in iron. Ni-Fe alloy (Awaruite) is observed in the mesh texture of dunite and harzburgite, suggesting that the serpentinization reaction proceeded under highly reductive conditions. Magnetite content is highest in dunite, and the Fe3+ (Fe2O3) content of all rocks cannot be explained only by magnetite content. This suggests that minerals other than magnetite (serpentine and weathered minerals) may contribute to the total Fe2O3 content. To clarify the spatial distribution of the redox state of Fe in rocks, we performed 2D imaging XAFS measurements at the beamline of NW2A in the synchrotron radiation facility PF-AR. The results indicate that mesh texture serpentine and bastite (serpentine after orthopyroxene) contains much Fe3+ (i.e. 25-46% of the total iron), regardless of lithology.
The contributions of magnetite and serpentine to hydrogen generation in each lithology and the mechanism of hydrogen generation at each stage were investigated in terms of bulk rock compositions, iron content of each mineral, and detailed microstructures[KM1] . Estimation of hydrogen generation in serpentinization with mesh texture and subsequent magnetite veins, respectively, indicates that serpentinization with mesh texture is the main stage. In the serpentinization with mesh texture, gabbroic rocks, dunite, and harzburgite were estimated to have generated 84 mM/kg rock, 144 mM/kg rock, and 107 mM/kg rock of hydrogen, respectively. Calculations of the contribution of serpentine and magnetite to hydrogen generation indicate that magnetite is the major contributor to hydrogen generation in the gabbroic rocks, whereas serpentine is the major contributor in dunite and harzburgite. Even in gabbroic rock, which is rich in silica-rich minerals, olivine is considered to[KM2] react selectively and form magnetite, which is different from the reported result (Katayama et al., 2010). Cracks caused by thermal stress may have selectively destroyed olivine and promoted olivine-dominant reactions in the early stages of serpentinization reactions. Subsequently, the cracks or voids formed by the reaction-induced stress led to extensive serpentinization and increased silica activity (Yoshida et al., 2020). These results suggest that the amount of hydrogen generation and the minerals responsible for it vary with lithology. Hydrogen[KM3] is produced by the formation of Fe3+-bearing serpentine even when magnetite is difficult to form, and the microcrack formation may control the overall amount of hydrogen generation.