Massive rare earth element storage in sub-continental lithospheric mantle initiated by diapirism, not by melting

Rare earth elements (REEs) are essential metals for modern technologies. Recent studies suggest that subcontinental lithospheric mantle (SCLM) remelting, previously fertilized by subducted marine sediments, leads to formation of REE-bearing rocks. However, the transfer mechanism of REE-rich sediments from the subducted slab to the overlying mantle wedge is unclear. We present high-pressure experiments on natural REE-rich marine sediments at 3–4 GPa and 800–1000 °C to constrain the phase relations, sediment melting behavior, and REE migration during subduction. Our results show recrystallization into an eclogite-like assemblage, with melting only occurring at 4 GPa, 1000 °C, experiments. Regardless of melting behavior, REE are refractory and mostly hosted by apatite. Buoyancy calculations suggest that most of the eclogite-like residues would form solid-state diapirs, ascending to the SCLM, resulting in the REE-fertilized source. Such flux may be required for substantial REE transport during subduction, as a foundation for economic-grade mineralization.