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.

俯冲板片和地幔楔的交代作用影响俯冲带界面岩石的流变学和化学性质。我们研究了起源于日本西南部 Sanbagawa 变质带 Tomisato 地区地幔楔的蛇纹岩体。蛇纹岩体与泥质片岩交界处发育交代反应带，由泥质片岩、钠长石片岩、白云母岩、绿泥石岩、透闪石片岩、滑石片岩、滑石+蛇纹岩、蛇纹岩组成。使用岩石学观察，我们发现界面处的交代作用发生在峰值P-T附近条件（0.8-1.0 GPa 和 530-570 °C），对应于地幔楔角处的条件。质量平衡计算表明，泥质片岩的交代作用伴随着Si、Ca的去除和Mg的加入，而蛇纹岩的交代作用则伴随着Ca、Al的加入。Ca 由外部供应，Na、K 和 C 被释放到外部系统。流体介导的传质导致在蛇纹岩之后形成富含透闪石的岩石，在变质沉积物之后形成富含绿泥石的岩石，这可能沿着俯冲界面和地幔楔角广泛存在。我们提出地幔楔角处的交代作用重新分配了流体和岩石之间的元素，在地幔楔和俯冲板片中的矿物组合中引入了空间非均质性，并且它会产生流体超压。因此，这种交代作用会影响板块-地幔解耦和地震活动。