11:30 〜 11:45
[BCG05-10] Ubiquitous methane-rich fluid inclusions in olivine-baring dolomitic marble: Significance of “crustal serpentinization”
キーワード：ドロマイト質大理石、飛騨帯、無機的メタン生成、かんらん石、流体包有物
Abiotic hydrocarbons and H2-bearing reduced fluids in the solid Earth are significantly important to provide energy and carbon to the deep subsurface biosphere. Hydration of olivine, i.e., serpentinization, causes the oxidation of iron from Fe2+ to Fe3+ and releases H2-bearing fluids. The generated H2-bearing fluids can reduce inorganic carbon to form abiotic methane (CH4). During the last two decades, aqueous alteration of ultramafic rocks has been studied as one of the major processes of abiotic CH4 synthesis. For example, relatively shallow low-temperature serpentinizations of peridotite generate reduced fluids, including H2 (e.g., [1],[2]). Recent studies have also reported that the high-temperature serpentinizations could generate reduced fluids based on the observation of natural antigorite serpentine samples ([3],[4]). In this contribution, we address the significance of “crustal serpentinization” with a particular focus on olivine-bearing dolomitic marbles as another key lithology to enable to generate abiotic CH4.
Dolomitic marble is a magnesium-rich metacarbonate rock that contains both calcite and dolomite as carbonate minerals. It occurs from Eoarchean (protolith age) to Phanerozoic and various orogenic belts with a wide range of P–T conditions. Metamorphic recrystallization of the dolomitic marble produces various olivine-bearing mineral assemblages. In the case of the Hida Belt, Japan, amphibolite-facies dolomitic marbles consist mainly of calcite, dolomite, and olivine; most olivine crystals have partially serpentinized along the rims and cracks. The olivine commonly contains CH4-rich fluid inclusions. Raman spectroscopic analyses found that fluid inclusions also contain serpentine (lizardite and/or chrysotile) and brucite. The occurrence of serpentine and brucite in olivine-hosted fluid inclusions indicates micrometer-scale serpentinization among H2O-rich fluid inclusions and the host olivine after the fluid infiltration. The micro-serpentinization of host olivine generated H2-bearing fluids, and consequently the fluids reduced inorganic carbon, including CO2, HCO3– and CO32–, and formed CH4. Although such abiotic CH4 synthesis in olivine-hosted fluid inclusions has been reported from abyssal and orogenic peridotites ([5],[6],[7]), we postulate that abiotic CH4 synthesis in dolomitic marble is a common crustal process. We propose that “crustal serpentinization” in dolomitic marble can provide abiotic CH4 synthesis as well as CH4 storage in the Earth's crust. Although the “crustal serpentinization” might have contributed to the evolution of life, quantitative estimations of CH4 production and total storage are still required.
Reference
[1] Etiope and Sherwood Lollar, 2013. Rev. Geophys. 51, 276–299. doi: 10.1002/rog.20011
[2] Kelley et al., 2005. Science 307, 1428–1434. doi: 10.1126/science.1102556
[3] Boutier et al., 2021. Lithos, 396-397, 106190. doi: 10.1016/j.lithos.2021.106190
[4] Vitale Brovarone et al., 2020. Nat. Commun. 11, 3880. doi: 10.1038/s41467-020-17342-x
[5] Grozeva et al., 2020. Phil. Trans. R. Soc. A 378, 20180431. doi: 10.1098/rsta.2018.0431
[6] Klein et al., 2019. Proc. Natl. Acad. Sci. 116, 17666–17672. doi: 10.1073/pnas.1907871116
[7] Zhang et al., 2021. Geochim. Cosmochim. Acta 296, 1–17. doi: 10.1016/j.gca.2020.12.016
Dolomitic marble is a magnesium-rich metacarbonate rock that contains both calcite and dolomite as carbonate minerals. It occurs from Eoarchean (protolith age) to Phanerozoic and various orogenic belts with a wide range of P–T conditions. Metamorphic recrystallization of the dolomitic marble produces various olivine-bearing mineral assemblages. In the case of the Hida Belt, Japan, amphibolite-facies dolomitic marbles consist mainly of calcite, dolomite, and olivine; most olivine crystals have partially serpentinized along the rims and cracks. The olivine commonly contains CH4-rich fluid inclusions. Raman spectroscopic analyses found that fluid inclusions also contain serpentine (lizardite and/or chrysotile) and brucite. The occurrence of serpentine and brucite in olivine-hosted fluid inclusions indicates micrometer-scale serpentinization among H2O-rich fluid inclusions and the host olivine after the fluid infiltration. The micro-serpentinization of host olivine generated H2-bearing fluids, and consequently the fluids reduced inorganic carbon, including CO2, HCO3– and CO32–, and formed CH4. Although such abiotic CH4 synthesis in olivine-hosted fluid inclusions has been reported from abyssal and orogenic peridotites ([5],[6],[7]), we postulate that abiotic CH4 synthesis in dolomitic marble is a common crustal process. We propose that “crustal serpentinization” in dolomitic marble can provide abiotic CH4 synthesis as well as CH4 storage in the Earth's crust. Although the “crustal serpentinization” might have contributed to the evolution of life, quantitative estimations of CH4 production and total storage are still required.
Reference
[1] Etiope and Sherwood Lollar, 2013. Rev. Geophys. 51, 276–299. doi: 10.1002/rog.20011
[2] Kelley et al., 2005. Science 307, 1428–1434. doi: 10.1126/science.1102556
[3] Boutier et al., 2021. Lithos, 396-397, 106190. doi: 10.1016/j.lithos.2021.106190
[4] Vitale Brovarone et al., 2020. Nat. Commun. 11, 3880. doi: 10.1038/s41467-020-17342-x
[5] Grozeva et al., 2020. Phil. Trans. R. Soc. A 378, 20180431. doi: 10.1098/rsta.2018.0431
[6] Klein et al., 2019. Proc. Natl. Acad. Sci. 116, 17666–17672. doi: 10.1073/pnas.1907871116
[7] Zhang et al., 2021. Geochim. Cosmochim. Acta 296, 1–17. doi: 10.1016/j.gca.2020.12.016