[S-CG58] 岩石―流体相互作用の新展開：表層から沈み込み帯深部まで

[SCG58-P22] Abiotic methane synthesis within olivine-hosted fluid inclusions in dolomitic marble

*原田 浩伸¹、辻森 樹¹ (1.東北大学)

キーワード：かんらん石、流体包有物、蛇紋石化、メタン、ドロマイト質大理石

Abiotic synthesis of hydrocarbon-bearing fluids during geological processes has a significant impact on the evolution of both the Earth's biosphere and the solid Earth. Aqueous alteration of ultramafic rocks, i.e., serpentinization, which forms serpentinite, is one of the geological processes generating abiotic methane (CH₄). Experimental efforts have shown the abiotic CH₄ generation associated with serpentinization at various temperature^1,2. However, it has been pointed out that there are substantial kinetic barriers inhibiting abiotic CH₄ synthesis from the reduction of CO₂ unless certain catalysts^3,4. In the natural environment, although it has been known that shallow low-temperature serpentinizations generate reduced fluids, including H₂, the deep high-temperature and high-temperature serpentinization forming antigorite would not generate H₂-bearing reduced fluids⁵. However, recent studies reported that the high-temperature serpentinizations could generate reduced fluids based on the observation of natural antigorite serpentine samples^6,7. In any case, the nature and mechanism of abiotic CH₄ synthesis during serpentinization have been unclear. In this contribution, we address the olivine-bearing dolomitic marbles as a key lithology to enable the generation of abiotic CH₄.

Dolomitic marble is a magnesium-rich metacarbonate rock that contains both calcite and dolomite as carbonate minerals.Metamorphic recrystallization of the dolomitic marble produces various olivine-bearing mineral assemblages. The amphibolite-facies dolomitic marbles collected from the Hida Belt, Japan, consist mainly of calcite, dolomite, and olivine; most olivine crystals have partially serpentinized along the rims and cracks. The C–O isotope compositions of carbonate minerals in the dolomitic marble are δ¹³C = –3.3 to +2.8‰ and δ¹⁸O = +8.6 to +17.3‰ for calcite and δ¹³C = +0.2 and +0.8‰ and δ¹⁸O = +17.9 and +20.0‰ for dolomite. Raman spectroscopic analyses found that the olivine commonly contains CH₄-rich fluid inclusions. The fluid inclusions also contain hydrous minerals, such as serpentine (lizardite and/or chrysotile) and brucite. These observations indicate micrometer-scale serpentinization among H₂O-rich fluid inclusions and the host olivine after the fluid infiltration. The serpentinization of host olivine generated H₂-bearing fluids, and consequently the fluids reduced inorganic carbon, including CO₂, HCO₃^–, and CO₃^2–, and formed CH₄. Such abiotic CH₄synthesis in olivine-hosted fluid inclusions has been reported from abyssal and orogenic peridotites^8–10. We postulate that abiotic CH₄ synthesis in dolomitic marble is a common process. To further our understanding of the contributions to the impact of abiotic CH₄ storage in the solid Earth, quantitative estimation of CH₄ production is still required.



Reference
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