Origin of ultra rear‐arc magmatism at R ishiri V olcano, K uril A rc

The Rishiri Volcano is located at the very rear of the Kuril Arc at its junction with the NE Japan Arc, and its 300 km depth to the slab surface is one of the deepest among the active arc volcanoes in the world. In this study, the origin of this ultra rear‐arc magmatism was investigated by analyzing the basaltic lavas from the volcano. The lavas consist of low‐K and high‐K groups, with the low‐K lavas predating the high‐K lavas. Since it is unlikely that the high‐K magmas are derivatives of the low‐K magmas, the two magmas are thought to be derived from different source mantle materials. Analyses using multicomponent thermodynamics suggest that these magmas were both generated through the ∼2% melting of a source mantle with 0.04–0.11 wt.% H 2 O at 1280–1340°C and ∼2.3 GPa. The temperatures at the surface of the subducting Pacific slab, from which the slab fluids were released, were estimated to be 860–960°C for the low‐K magmas and 930–1040°C for the high‐K magmas. These temperatures of the slab surface are remarkably higher than those predicted by thermal models. The estimated high temperatures of the slab surface and the latest detailed seismic tomography results suggest that the low‐K and high‐K magmatism resulted from the progressive production of fluids at the slab surface due to heating by the injection of hot mantle materials into a relatively large‐scale fracture in the distorted portion of the subducting Pacific plate.