[S-MP21] Oceanic and Continental Subduction Processes: petrologic and geochemical perspective

[SMP21-01] Water transportation of subducting oceanic plate beneath the Noto peninsula.

*岡本 和明¹ (1.埼玉大学教育学部地学)

キーワード：intra-arc seismicity、origin of fluids in the seismogenic zone、subducting Pacific Plate、water transportation in deep mantle

Fluid injection into the intra-continent (intra-arc) seismogenic zone is a trigger to slip active fault back arc region. Crustal structure beneath earthquake swarm in the Noto peninsula, was already analyzed in detail (e.g. Nakajima et al., 2022), and concluded that large volume of fluids accumulated below the seismogenic zone beneath the Noto peninsula. It is inferred that the fluids have been supplied from the uppermost mantle (50 to 150km) to the lower crust over a geological time scale. Origin of the fluids are considered that water has been transported from subducting Pacific plate to the upper mantle beneath the central Japan. Subducting plate can transport water down to deep mantle because several hydrous minerals can be stable. Stability of such hydrous minerals are dependent on P, T, and chemical composition of the rock. Lawsonite is major hydrous mineral stable in subducting oceanic crust in deep mantle. It is considered that temperature condition of the subducting Pacific plate beneath the central Japan is cold enough to stabilize lawsonite in the oceanic crust portion of the plate. Slab surface depth beneath the Noto peninsula is deeper than 250 km. Ultrahigh pressure experiment in the MORB+H₂O system (e.g. Okamoto and Maruyama, 1999) reveals that 1wt% H₂O can be stored in the depth (250 km depth) of the subducting oceanic crust. Further deep (down to 300 km depth), the lawsonite is decomposed to release fluids by dehydration reaction. Thus, it is considered that large volume of the fluids are supplied beneath the Noto peninsula.