Pore-water geochemistry of Kumano Basin mud volcanoes reveals multi-source fluids from sedimentary and crustal dehydration

Mud volcanoes in subduction-zone forearcs provide direct access to fluids generated at depth, yet their occurrence tens of kilometres landward of the deformation front poses several questions about the origin and nature of such fluids, since most of the fluid sources from sediment compaction and clay mineral dehydration are exhausted within a few 10s of km into the subduction system. Here we present a comprehensive pore-water geochemical and isotopic dataset from 12 submarine mud volcanoes in the Kumano Basin (SW Japan), located 42–85 km landward of the trench. More than 460 pore-fluid samples from gravity cores and seafloor drill cores were analysed for major and minor elements and for Li, B, and Sr isotopic compositions.

Mud volcano fluids are strongly depleted in Cl, Na, Mg, and K and enriched in B and Li relative to seawater, indicating substantial freshening and deep fluid input. Elemental and isotopic systematics define a ternary mixing between seawater, a pristine deep fluid, and a shallowly overprinted deep fluid affected by volcanic ash alteration and ion exchange. Lithium and boron isotopes constrain fluid sources to a combination of sedimentary clay mineral dehydration and a higher-temperature component inconsistent with purely sedimentary origins. Inferred Li isotope compositions of the fluids sources indicate fluid–rock interaction temperatures of ~150–290 °C, exceeding the smectite-to-illite reaction window and pointing to dehydration of altered oceanic crust (saponite) beneath the forearc. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr ≈ 0.708) further support mixing between sedimentary, volcanic, and crustal sources.

Our results demonstrate that Kumano Basin mud volcanoes are fueled by multiple fluid sources spanning shallow diagenesis, deep accretionary prism dehydration, and subducting oceanic crust. These findings imply that crustal dehydration fluids can migrate into the overriding plate along inherited fault systems and play a major role in forearc hydrogeology. Mud volcanoes thus represent key natural observatories for integrating deep subduction-zone fluid processes beyond the reach of scientific drilling.