Fluid-rock interaction in subducted mélange and implications for fluid flow along the subduction plate boundary

The knowledge of fluid-rock interactions in exhumed subduction complexes provides a window for understanding fluid flow along the plate boundary in subduction zones. We present geological and geochemical observations of fluid-rock interactions in mélanges exhumed from subduction plate boundary: (1) the Makimine mélange in the Shimanto accretionary complex, in central Kyushu, Japan formed at 350-400 °C and 15 km depth and (2) the serpentine mélange in the Nishisonogi metamorphic rocks, in western Kyushu, Japan formed at 440 °C and 25 km depth. In the Makimine mélange, the pelitic schist is bleached along the contact with dolerite or basalt over 10-20 cm in width. The similar bleached zone is observed in the serpentine mélange in the Nishisonogi metamorphic rocks: pelitic schist is bleached along the contact with chlorite-actinolite schist over <10 cm in width. The X-ray fluorescence (XRF) and microstructural analyses show that both bleached zones in the mélanges are characterized by the depletion in muscovite and carbonaceous material and the enrichment in albite, which suggest the alteration of muscovite to albite through the fluid-rock interaction at temperatures of 350-440°C. XRF data show that the reaction zone in the serpentine mélange expels silica while that in the Makimine mélange absorbs silica. The formation of albite means that the fluid associated with the water-rock interaction was saturated with albite. Thus, We can estimate the fluid composition based on the solubility of albite at 350-440°C. Our estimation indicates that when the fluid with a neutral pH was saturated with quartz, fluid containing sodium of seawater-like concentrations (>0.23 mol/L) can infiltrate into pelitic schist. The isocon analysis using XRF data indicates that the fluid-rock interaction in the serpentine mélange accompanies with rock volume loss, whereas that in the Makimine mélange induces rock volume increase possibly because of silica absorption. Thus, the fluid-rock interaction at the deeper plate boundary produces porous reaction zone and thereby enhances fluid flow toward shallow depths. As fluid migrates into the shallower plate boundary, the ability of fluid flow may be reduced due to the rock volume increase. We suggest that reaction zone likely represents the conduit of fluid flow along the plate boundary.