10:45 AM - 12:15 PM
[SCG58-P04] Fracture-induced carbonation and metasomatism of the Higuchi serpentinite body in the Sanbagawa belt, Kanto Mountains, Japan
Keywords:carbonation, metasomatism, mantle wedge, serpentinite
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.