[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-P04] 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、岡本 敦² (1.静岡大学理学部地球科学科、2.東北大学大学院環境科学研究科)

キーワード：炭酸塩化、交代作用、マントルウェッジ、蛇紋岩

Knowing the extent of CO₂ transfer from the subducting oceanic crust to the overlying mantle wedge is crucial to the quantification of carbon fluxes in subduction zones. In order to understand carbonation and metasomatic processes occurring in the shallow forearc mantle wedge, we conducted structural analysis of the Higuchi serpentinite body (Okamoto et al., 2021, Commun. Earth Environ.) in the Sanbagawa belt, Kanto Mountains, Japan. Based on field and microstructural observations together with Raman spectroscopic analysis, we found that the mantle wedge-derived peridotite (i.e., Higuchi body) first experienced complete antigoritization to form massive serpentinite, which then underwent a network of extensional and extensional-shear fracturing (i.e., fault-fracture meshes) under near-lithostatic pore fluid pressures. Just after the fracuring events, CO₂- and SiO₂-rich fluids that were possibly derived from the surrounding pelitic schists infiltrated the fractures, and then talc, magnesite, and chrysotile were precipiated via carbonation and metasomatism to form foliated serpentinite. Finally, faulting occurred under a differential stress field, and shear veins composed mainly of dolomite were formed. In the dolomite veins, fine- and coarse-grained dolomite layers are alternated with slicken-fibers of antigorite and talc, indicating repeated fault-slip events along these veins. These results suggest that carbonation and metasomatism in the shallow fore-arc mantle wedge is promoted by extensive fracturing and localized faulting that may occur under high pore fluid pressure, which promotes brittle faiure rather than viscous creep.