Subduction legacies in the mantle transition zone modulate intraplate oceanic volcanism

How oceanic crust forms and intraplate volcanism arises remains central to resolving the mechanisms driving Earth's dynamic evolution. Anomalously thick oceanic crust is conventionally attributed to thermal mantle plumes, yet large igneous provinces such as the Azores Plateau, with its 8-30 km thick crust, dispersed volcanism, and distinctive water-rich geochemical signatures, challenge this paradigm. Here we use geodynamic numerical models to show that a migrating ridge over a locally hydrated layer (0.1-0.4 wt.% H₂O), generated by dehydration of the Mantle Transition Zone (MTZ), can trigger upwelling and melting sufficient to produce a 10-20 km-thick crust. This mechanism accounts for the plateau's anomalous crustal thickness, long-lived volcanism, and volatile-rich mantle source. We propose that recycled water, a subduction legacy stored in the MTZ, acts as a primary driver of intraplate volcanism, providing an alternative to the classical stationary mantle plume model. This mechanism may also help explain the widespread contamination of large-scale upper mantle domains by subduction-related fluid signatures, as in the DUPAL and South Atlantic domains.