Time-dependent changes in the origin of aqueous fluids in the shallow mantle wedge: Evidence from noble gas and halogen compositions of the Kamabuseyama serpentinite, Kanto Mnts., Japan

Blocks of serpentinite derived from the mantle wedge are widely distributed throughout the Sanbagawa metamorphic belt of SW Japan and the contact domains with surrounding schists represent the paleo subduction zone boundary. This paleo subduction boundary is well exposed in the Kamabuseyama area of the Kanto mountains and originally formed at relatively shallow mantle depths of ~25 km. The contact zone between the serpentinite and slab-derived schists is characterized by a domain of fluid-mediated metasomatism. Noble gas and halogen analyses of different lithologies from this domain suggest fluid influx occurred in two distinct stages represented by formation of (1) serpentinite and (2) boundary domain metasomatic rocks. Stage (2) metasomatism can be further divided into two substages: (2–1) associated with chlorite rock formation and (2–2) associated with talc rock formation. The origins of the dominant fluids involved in these different stages can be identified as: (1) altered oceanic crust, (2–1) altered oceanic crust + pore fluids in oceanic sediments, and (2–2) pore fluids in oceanic sediments + fluids derived from serpentinized domains of the slab mantle. These results indicate that the origin of the dominant fluid along the paleo subduction boundary changed with time reflecting the development of different fluid pathways. The identification of fluid derived from the dehydration of serpentinite of the slab mantle during the youngest fluid inflow event suggests changes in fluid pathways may be controlled by the progressive development of a boundary shear zone that introduces a permeability anisotropy that allows deep sourced fluids to flow laterally along the subduction boundary. • Geochemical characterization of fluids along the paleo plate interface. • Noble gas and halogen compositions to identify fluid sources for metasomatic rocks. • Early stage serpentinization of mantle wedge by fluid from altered oceanic crust. • Late-stage talc rock formation by fluid from deeper part of the subduction zone. • Fluid pathways along plate interface change with evolution of subduction zone.