Heterogeneous stagnant slab controls focused crustal recycling and volcanic clustering.

Stagnant slabs transport cold, hydrous, and compositionally distinct materials into the mantle transition zone (MTZ). Intraplate volcanism above these slabs documents the geochemical imprints of recycled subducted components. Yet the recycled mechanism remains elusive, largely because slab geometry and thermochemical state within the MTZ are difficult to constrain. Here we use receiver-function imaging to resolve the morphology and properties of seismic discontinuities within the MTZ beneath Northeast China. Integrating mineral physics modeling, we develop a joint inversion to quantify spatial variations in temperature, composition, and water content, enabling us to track the stagnant Pacific slab and associated thermochemical anomalies. We determine the westward extent and internal heterogeneity of the stagnant slab, identifying a drop-shaped penetration segment and a lens-shaped weakened zone without through-going tearing. The upper MTZ above the slab shows near-ambient temperatures, basalt enrichment ( ~ 0.4), and water enrichment ( ~ 0.38 wt%), reflecting an upward slab flux. Peak fluxes are concentrated above the slab lens and slab edge, spatially coinciding with two volcanic clusters. We propose that focused, water-driven upwelling induced by slab heterogeneity was responsible for intraplate volcanism in Northeast China, and that slab-scale heterogeneity may be widespread in stagnant slabs globally, driving focused crustal recycling and intraplate volcanic clustering.