Heterogeneous slab mantle hydration controls the chemical architecture of the Kamchatka arc.

Fluids released from subducting slabs play an important role in arc magmatism and global volatile cycles, yet resolving their lithological origins and spatial distribution remains a major challenge. Here, we integrate molybdenum (Mo) isotope systematics with improved P-wave tomography across the Kamchatka arc to trace the origins and transport pathways of slab-derived fluids. Arc lavas exhibit δ98/95Mo values systematically higher than the depleted mantle and correlate positively with fluid-mobile element enrichment. We identify a distinctive "dumbbell-shaped" isotopic pattern cospatial with prominent low-velocity zones in the mantle wedge. The anomalies further align with regions of marked velocity reductions in the incoming Pacific Plate, indicating that the fluid budget of the volcanic front is directly inherited from heterogeneous serpentinization of the subducting lithospheric mantle prior to subduction. Our results demonstrate that preexisting slab hydration exerts first-order control on subduction zone chemical architecture, providing a predictive framework for understanding global volatile cycling.