15:30 〜 15:45
[SCG47-07] Hydrogen Production in the Oceanic Lithosphere: Implications from the Oman Ophiolite
キーワード：蛇紋岩化反応、水素、地下生命圏
Hydrogen production during serpentinization of the oceanic lithosphere plays an important role in sustaining the deep underground biosphere (Schrenk et al., 2013). However, as the direct sampling of the upper mantle peridotite has yet to be difficult, the potential for hydrogen production in the deep oceanic lithosphere is still unclear. In this study, to understand the variation of hydrogen production along the crust-mantle section of the oceanic lithosphere, we analyzed the amount and partitioning of ferric irons of the core samples from the lower crust to the upper mantle of the Oman ophiolite obtained by the Oman Drilling project at the CM site (Kelemen et al., 2020)). The bulk rock total Fe is measured by X-ray fluorescence (XRF). Fe2O3 and FeO content was calculated by Fe(III)/ΣFe measured by X-ray absorption fine structure (XAFS) analyses performed at the BL-12C beamline of the Photon Factory (PF), High Energy Accelerator Research Organization (KEK). The mass ratio of hydrous minerals was determined using thermogravimetry analysis. The amount of magnetite is estimated based on saturation magnetization measured by a vibrating sample magnetometer.
Dunite in the crust-mantle transition zone and harzburgite in the upper mantle are extensively altered and characterized mainly by serpentinized olivine and orthopyroxene. Dunite is completely serpentinized and composed of lizardite, chrysotile, brucite, magnetite, and Cr-spinel. The dunite matrix is cut by later-stage antigorite veins. In the upper mantle, harzburgite is 70-90% serpentinized. The harzburgite is composed of lizardite, chrysotile, brucite, magnetite, olivine, and pyroxenes. The harzburgite matrix is cut by magnetite veins. In both dunite and harzburgite, brucite is partly weathered and changed to coalingite. Harzburgites have a higher mass ratio of serpentine/brucite than dunites because of their high silica activity due to the presence of pyroxene in harzburgite. The dunites contain more magnetite (3.5±1.9 wt%) than harzburgites (1.3±1.5 wt%). This is due to the thermodynamic instability of magnetite when silica activity is high (Frost and Beard, 2007). The ratio of Fe(III)/ΣFe estimated by XANES analysis of serpentinite agreed with the value obtained by titration to within ±10 mol% root mean square error (RMSE). Harzburgite, which contains less magnetite than dunite, was also found to contain more Fe2O3 (Dunite: Fe2O3 = 6.7±1.2 wt%, Harzburgite: Fe2O3 = 5.1±0.6 wt%). In order to clarify the distribution of Fe(III) in the minerals, the two-dimensional imaging XAFS was performed at the AR-NW2A beamline of PF, KEK. As a result, the spatial distribution of the redox state of iron was revealed at the micrometer scale, and Fe(III) was found to be distributed mainly in serpentinite, magnetite, and coalingite.
The bulk composition and mass fraction of each mineral was used to determine whether Fe(III) is preferentially distributed in serpentinite or magnetite. The results show a high ratio of Fe(III)/ΣFe=0.7±0.1 in serpentine, and the ratio are not significantly different between dunite and harzburgite. In both of dunite and harzburgite, about 70% of the total rock Fe(III) is in serpentine (67±20 wt%). These results suggest that even in harzburgites in the upper mantle, where magnetite is less likely to form, considerable hydrogen may be produced by the formation of Fe(III)-rich serpentinite.
References
1. Schrenk et al. (2013) Rev. Mineral Geochem. 75, 575-606. DOI: 10.2138/rmg.2013.75.18
2. Kelemen et al. (2020) Proceedings of the Oman Drilling Project. DOI: 10.14379/OmanDP.proc.2020
3. Frost and Beard (2007) J. Pet. 48, 7, 1351-1368. DOI: 10.1093/petrology/egm021
Dunite in the crust-mantle transition zone and harzburgite in the upper mantle are extensively altered and characterized mainly by serpentinized olivine and orthopyroxene. Dunite is completely serpentinized and composed of lizardite, chrysotile, brucite, magnetite, and Cr-spinel. The dunite matrix is cut by later-stage antigorite veins. In the upper mantle, harzburgite is 70-90% serpentinized. The harzburgite is composed of lizardite, chrysotile, brucite, magnetite, olivine, and pyroxenes. The harzburgite matrix is cut by magnetite veins. In both dunite and harzburgite, brucite is partly weathered and changed to coalingite. Harzburgites have a higher mass ratio of serpentine/brucite than dunites because of their high silica activity due to the presence of pyroxene in harzburgite. The dunites contain more magnetite (3.5±1.9 wt%) than harzburgites (1.3±1.5 wt%). This is due to the thermodynamic instability of magnetite when silica activity is high (Frost and Beard, 2007). The ratio of Fe(III)/ΣFe estimated by XANES analysis of serpentinite agreed with the value obtained by titration to within ±10 mol% root mean square error (RMSE). Harzburgite, which contains less magnetite than dunite, was also found to contain more Fe2O3 (Dunite: Fe2O3 = 6.7±1.2 wt%, Harzburgite: Fe2O3 = 5.1±0.6 wt%). In order to clarify the distribution of Fe(III) in the minerals, the two-dimensional imaging XAFS was performed at the AR-NW2A beamline of PF, KEK. As a result, the spatial distribution of the redox state of iron was revealed at the micrometer scale, and Fe(III) was found to be distributed mainly in serpentinite, magnetite, and coalingite.
The bulk composition and mass fraction of each mineral was used to determine whether Fe(III) is preferentially distributed in serpentinite or magnetite. The results show a high ratio of Fe(III)/ΣFe=0.7±0.1 in serpentine, and the ratio are not significantly different between dunite and harzburgite. In both of dunite and harzburgite, about 70% of the total rock Fe(III) is in serpentine (67±20 wt%). These results suggest that even in harzburgites in the upper mantle, where magnetite is less likely to form, considerable hydrogen may be produced by the formation of Fe(III)-rich serpentinite.
References
1. Schrenk et al. (2013) Rev. Mineral Geochem. 75, 575-606. DOI: 10.2138/rmg.2013.75.18
2. Kelemen et al. (2020) Proceedings of the Oman Drilling Project. DOI: 10.14379/OmanDP.proc.2020
3. Frost and Beard (2007) J. Pet. 48, 7, 1351-1368. DOI: 10.1093/petrology/egm021