Metasomatism at a metapelite–ultramafic rock contact at the subduction interface: Insights into mass transfer and fluid flow at the mantle wedge corner

Metasomatism of the subducting slab and mantle wedge influences the rheological and chemical properties of rocks at the subduction zone interface. We investigated a serpentinite body that originated from the mantle wedge in the Tomisato area of the Sanbagawa metamorphic belt, SW Japan. At the boundary between the serpentinite body and a pelitic schist, metasomatic reaction zones have developed, consisting of pelitic schist, albite schist, muscovite rock, chlorite rock, tremolite schist, talc schist, talc + serpentine rock, and serpentinite. Using petrological observations, we showed that metasomatism at the interface occurred around the peak P–T conditions (0.8–1.0 GPa and 530–570 °C), which correspond to those at the mantle wedge corner. Mass balance calculations revealed that the metasomatism of the pelitic schist was accompanied by the removal of Si and Ca and the addition of Mg, whereas the metasomatism of the serpentinite was accompanied by the addition of Ca and Al. The Ca was supplied externally, and Na, K, and C were released to an external system. Fluid-mediated mass transfer induces formation of tremolite-rich rocks after serpentinite and chlorite-rich rocks after metasediments, which could be widespread along the subduction interface and the mantle wedge corner. We propose that metasomatism at the mantle wedge corner redistributes elements between fluids and rocks, introduces spatial heterogeneities in the mineral assemblages in the mantle wedge and subducting slab, and that it could produce fluid overpressure. Consequently, such metasomatism would influence slab–mantle decoupling and seismicity.