11:15 AM - 11:30 AM
[SCG54-09] Contrasting evolution of the hydrogen generation between the lower crust gabbroic rocks and the mantle peridotite serpentinization
Keywords:Serpentinization, Hydrogen, Oman Ophiolite, XAFS
The oxidation of ferrous iron (Fe2+) in iron-bearing minerals during serpentinization reduce water, and supply hydrogen to microbial communities at seafloor (Kelley et al., 2005). In addition, hydrogen produced by geological processes including serpentinization is highlighted as a potential clean energy source for a carbon-free society (Zgonnik et al., 2020). While there are many studies on the serpentinization of peridotite, there are few studies on the serpentinization of gabbroic rocks. Thus, the effect of lithology on the amount of hydrogen produced during the alteration is not well understood. In this study, we investigated the effect of lithology on hydrogen generation during the water-rock reaction of the lower crust and upper mantle of the oceanic lithosphere using bulk rock chemical composition including determination of Fe3+/ΣFe by X-ray absorption fine structure spectroscopy (XAFS), thermogravimetric analysis, magnetic analysis, and two-dimensional (2D) imaging XAFS.
In this study, we used continuous drill core samples from the lower crust to the upper mantle from the Oman Drilling Project site CM1A and CM2B (Kelemen et al., 2020). The olivine of the gabbroic rocks is altered to serpentine + magnetite or chlorite. Magnetite content is high in samples with less plagioclase alteration. The dunite is completely serpentinized and is cut by later antigorite + chrysotile veins and magnetite veins. Harzburgite is serpentinized to varying degrees of 60-90%. Significant magnetite veins were commonly observed in samples that were particularly serpentinized. The Fe3+/ΣFe in all rocks shows no systematic variation with depth but varies mainly by lithology. The Fe3+/ΣFe ratios of dunite and harzburgite are Fe3+/ΣFe = 0.5±0.1, while the gabbroic rocks have lower Fe3+/ΣFe ratios of 0.2±0.1. 2D imaging XAFS revealed the distribution of the redox state of iron in each rock texture and the changes in water-to-rock ratio (W/R) recorded in the rock texture. The mesh texture serpentine shows Fe3+/ΣFe ratios of 0.3-0.5 regardless of the lithology. Serpentine around magnetite veins that cut the mesh texture shows higher Fe3+/ΣFe ratios of 0.5-0.6. These observations suggest a change from an early serpentinization with low W/R and more reducing conditions to a later serpentinization with high W/R and more oxidative conditions.
Based on the mass balance calculations, we estimated the amount of hydrogen produced by (i) early stage serpentinization with mesh texture, (ii) formation of late magnetite veins, and (iii) oxidation of brucite. During the early stages of serpentinization, the amount of hydrogen produced per unit rock in olivine gabbro, dunite, and harzburgite are 9-314, 159-268, , and 132-244 mM H2/kgrock, respectively. In the case of dunite and harzburgite, magnetite veins of later stage serpentinization may have generated an additional hydrogen of 36-220 mM H2/kgrock. In addition, the increase in W/R associated with fracturing may promote the magnetite formation by brucite consumption. The brucite in dunite and harzburgite has the potential to generate hydrogen up to 12-73 mM H2/kgrock. While gabbroic rocks have the potential to generate a significant amount of hydrogen in the olivine-dominated early stage serpentinization, the supply of silica from plagioclase suppresses formation of magnetite and hydrogen generation in later stages of serpentinization. These results suggest a contrasting evolution of the hydrogen production due to lithological difference: in the case of dunite and harzburgite, the generation of hydrogen is promoted by the fracturing and increase in W/R that accompanies the progress of the reaction, whereas in the case of gabbroic rocks, hydrogen generation is suppressed by the increase in the silica activity due to the progress of the plagioclase alteration.
In this study, we used continuous drill core samples from the lower crust to the upper mantle from the Oman Drilling Project site CM1A and CM2B (Kelemen et al., 2020). The olivine of the gabbroic rocks is altered to serpentine + magnetite or chlorite. Magnetite content is high in samples with less plagioclase alteration. The dunite is completely serpentinized and is cut by later antigorite + chrysotile veins and magnetite veins. Harzburgite is serpentinized to varying degrees of 60-90%. Significant magnetite veins were commonly observed in samples that were particularly serpentinized. The Fe3+/ΣFe in all rocks shows no systematic variation with depth but varies mainly by lithology. The Fe3+/ΣFe ratios of dunite and harzburgite are Fe3+/ΣFe = 0.5±0.1, while the gabbroic rocks have lower Fe3+/ΣFe ratios of 0.2±0.1. 2D imaging XAFS revealed the distribution of the redox state of iron in each rock texture and the changes in water-to-rock ratio (W/R) recorded in the rock texture. The mesh texture serpentine shows Fe3+/ΣFe ratios of 0.3-0.5 regardless of the lithology. Serpentine around magnetite veins that cut the mesh texture shows higher Fe3+/ΣFe ratios of 0.5-0.6. These observations suggest a change from an early serpentinization with low W/R and more reducing conditions to a later serpentinization with high W/R and more oxidative conditions.
Based on the mass balance calculations, we estimated the amount of hydrogen produced by (i) early stage serpentinization with mesh texture, (ii) formation of late magnetite veins, and (iii) oxidation of brucite. During the early stages of serpentinization, the amount of hydrogen produced per unit rock in olivine gabbro, dunite, and harzburgite are 9-314, 159-268, , and 132-244 mM H2/kgrock, respectively. In the case of dunite and harzburgite, magnetite veins of later stage serpentinization may have generated an additional hydrogen of 36-220 mM H2/kgrock. In addition, the increase in W/R associated with fracturing may promote the magnetite formation by brucite consumption. The brucite in dunite and harzburgite has the potential to generate hydrogen up to 12-73 mM H2/kgrock. While gabbroic rocks have the potential to generate a significant amount of hydrogen in the olivine-dominated early stage serpentinization, the supply of silica from plagioclase suppresses formation of magnetite and hydrogen generation in later stages of serpentinization. These results suggest a contrasting evolution of the hydrogen production due to lithological difference: in the case of dunite and harzburgite, the generation of hydrogen is promoted by the fracturing and increase in W/R that accompanies the progress of the reaction, whereas in the case of gabbroic rocks, hydrogen generation is suppressed by the increase in the silica activity due to the progress of the plagioclase alteration.