Metasomatic dehydration of talc constrains the maximum depth of slab–mantle decoupling in subduction zones

Talc largely contributes to water transport into Earth’s interior in subduction zones and lubrication of the slab–mantle interface. However, its maximum stability limit has typically been evaluated under silica-saturated conditions. In-situ high-pressure, high-temperature experiments up to 33.8 kbar and 875 °C reveal that in water-rich, low-silica environments, geologically evidenced by talc coexisting with antigorite and olivine at natural subduction interfaces, talc undergoes a metasomatic dehydration reaction that produces forsterite and a silica-bearing aqueous fluid. This reaction shifts the dehydration boundary to ≈ 705 °C at 10 kbar and ≈ 650 °C at 30 kbar in the experimental low-silica condition, that is 90–170 °C lower than that under silica-saturated conditions. Critically, this metasomatic boundary intersects slab thermal profiles at a depth of ≈ 80 km across diverse global subduction zones, from cold (e.g., Mariana) to hot (e.g., Cascadia), matching the maximum depth of decoupling between the subducting slab and mantle wedge, which separates the stagnant, cold forearc mantle from the flowing, hot arc mantle.