Isotopic evidence for slab-derived magmatic water beneath the Kussharo Caldera, Japan

It is well known that water transported by a subducting oceanic plate (slab) plays a key role in the formation of island-arc magmas. However, direct evidence demonstrating that magmatic water is derived from the slab remains limited. To address this, we conducted isotope and dissolved ion analyses of hot and cold spring waters, river waters, fumarolic vapor, and hydrothermal waters at and around Kussharo Caldera in northern Japan. In this region, the Pacific Plate subducts beneath the North American Plate at the Kuril Trench, and more than a dozen Quaternary volcanoes form a volcanic front parallel to the trench. The results show that the Kawayu hot springs, located near the active lava dome within the caldera, are not derived solely from meteoric water. Strong linear correlations were observed among δ 2 H, δ 18 O, Cl − concentrations, and SO₄ 2− concentrations, suggesting mixing between meteoric water and magmatic water. The isotopic composition of the magmatic end-member coincides with the simulated value for Pacific slab-derived water at this location. This provides direct evidence that the magmatic water originates from the slab. In contrast, the isotopic compositions of fumarolic vapor and hydrothermal waters were partially modified by isotopic fractionation associated with vapor –liquid separation. This study highlights that the large flux of slab-derived water can preserve its original isotopic signature, even though that of the surrounding rocks may be substantially modified by isotopic exchange.