The mantle source of REE-rich alkaline silicate magmas can be enriched by continent-derived sediment subduction.

Alkaline silicate intrusions host large rare-earth-element (REE) resources, yet the precise origin of the mantle enrichment that generates these magmas remains unresolved. Here, we use a combination of Li-Ba-Sr-Nd isotopic data to clarify the processes responsible for the metasomatic enrichment of the mantle source. We focus on alkaline complexes in the North China Craton that host giant REE deposits, which exhibit broadly similar Li-Ba-Sr-Nd isotopic signatures, despite forming in different tectonic settings. Neither recycling of altered oceanic crust nor serpentinite-related metasomatism can reproduce these coupled geochemical signatures; instead, they require the addition of subducted continent-derived sediment derived from weathered continental crust. Storage of these REE-rich sediments within the lithospheric mantle lowers its solidus and creates chemically fertile domains. Lithospheric extension subsequently triggers selective melting of these domains, generating REE-rich alkaline magmas. Our findings suggest that continent-derived sediment recycling plays an important role in shaping the critical-metal budget of the Earth.