[S-CG54] 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

[SCG54-03] 関東山地三波川帯の樋口蛇紋岩体にみられる破壊に誘発された炭酸塩化と交代作用

*平内 健一¹、平 芽依¹、若松 優希¹、大坪 誠²、岡本 敦³ (1.静岡大学理学部地球科学科、2.産業技術総合研究所 活断層・火山研究部門、3.東北大学大学院環境科学研究科)

キーワード：炭酸塩化作用、マントルウェッジ、古応力解析、三波川帯、蛇紋岩化作用

Understanding the extent of CO₂ transfer from the subducting oceanic crust to the overlying mantle wedge is crucial for quantifying carbon fluxes in subduction zones (e.g., Lara and Dasgupta, 2022). To better understand serpentinization and carbonation processes in the shallow forearc mantle wedge, we conducted a structural analysis of the Higuchi serpentinite body (Okamoto et al., 2021) in the Sanbagawa belt, Kanto Mountains, Japan. Field and microstructural observations suggest that the mantle wedge-derived peridotite body experienced complete antigoritization to form massive serpentinite. The massive serpentinite subsequently developed a network of conjugate extensional and extensional-shear fractures (i.e., fault-fracture meshes; Sibson, 2017) under supra-lithostatic pore fluid pressures. These fractures were then infiltrated by SiO₂- and CO₂-rich fluids, possibly derived from subducting metasediments (Okamoto et al., 2021), leading to the formation of elongate magnesite grains enclosed in a foliated talc- or chrysotile-rich matrix. The foliated serpentinite was later crosscut by coeval dolomite-rich shear or extensional veins. The dolomite-rich veins contain fibrous or elongate aggregates, whose long axes are oriented parallel or perpendicular to the vein walls, suggesting multiple crack-seal events during vein formation. Paleostress analysis shows that shear veins developed in a compressional stress regime with a horizontal maximum compressional (σ₁) axis and a vertical least compressional stress (σ₃) axis, while extensional veins formed under an extensional stress regime with a vertical σ₁ axis and a horizontal σ₃ axis. Since the extensional and shear veins mutually crosscut each other, these results indicate that the σ₁ and σ₃ axes switched cyclically. This local principal stress switch may have occurred in an environment where pore fluid pressure remained very high near the base of the shallow mantle wedge corner.

References: Sibson, 2017, Earth Planets Space, 69, 113. Okamoto et al., 2021, Commun. Earth Environ., 2, 151. Lara & Dasgupta, 2022, Earth Planet. Sci. Lett., 588, 117578.