[S-CG46] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG46-02] Subducting slab-derived fluid preserved in exhumed mantle wedge serpentinite: Example from the Kamabuseyama serpentinite body, Kanto Mountains, Japan.

*Koki Aida¹, Simon Richard Wallis¹, Hirochika Sumino² (1.Department of Earth and Planetary Science, The University of Tokyo, 2.Research Center for Advanced Science and Technology, the University of Tokyo)

Keywords:Noble gas, Halogen, Sanbagawa metamorphic belt, Metasomatism

Making the fluid behavior at subduction zone plate boundaries clear is important for understanding igneous and seismic activities, geochemical cycles, and other phenomena induced by aqueous fluids. Water at the surface is brought into the Earth's interior as pore fluid in oceanic sediments, altered oceanic crust, and serpentinized oceanic lithospheric mantle, and is progressively dehydrated with increasing temperature and pressure due to subduction, infiltrating into the mantle wedge and forming serpentinite (Hyndman & Peacock, 2003). Noble gases and halogens, whose elemental and isotopic ratios differ greatly between surface and mantle materials, have attracted attention as geochemical tracers of fluids in subduction zones (Nishiyama et al., 2020; Sumino et al., 2010). However, there are few examples of noble gas and halogen analysis applied to rocks in depth of metamorphism, and the 3D fluid behavior at the rock-subduction interface has not yet been constrained. Here we show the traces of dehydrating slabs in a series of rocks corresponding to a plate boundary surface 20-30 km below the surface by noble gas and halogen analysis.
We focused on the Kamabuseyama serpentinite body in the Kanto Mountains, which is considered to be one of the shallowest wedge mantle among the ultramafic rocks of the Sanbagawa metamorphic belt, which preserves the rocks of the plate boundary, and performed noble gas and halogen analyses on rocks in the metasomatic reaction zone (pelitic schist, talc/actinolite, and serpentinite) where fluids are thought to have passed through. The ³He/⁴He results indicate that the fluids in the pelitic schist have a mixed composition of oceanic sediments and altered oceanic crust, and the fluid in the talc/actinolite rock and serpentinite have a mixed composition of fluids from oceanic sediment, altered oceanic crust and the upper mantle. Non-radiogenic noble gas element ratios (¹³⁰Xe/³⁶Ar, ⁸⁴Kr/³⁶Ar) and ⁴⁰Ar/³⁶Ar are explained by a mixing of the mean composition of the altered oceanic crust, ocenic sediments, and seawater. Halogen element ratios (I/Cl, Br/Cl) have values that are explained by the composition of oceanic pore fluid for the pelitic schist and talc/actinolite rock, and by the mixing of altered oceanic crust and oceanic pore fluids for the serpentinite. These results suggest that the Kamabuseyama serpentinite body is a serpentinite formed by the infiltration of the fluid generated from dehydration of altered oceanic crust and compression of oceanic sediment. These results indicate that altered oceanic crust may have started dehydrating at the subduction plate boundary that formed the Sambagawa Belt 20-30 km below the surface.