Elevated concentrations of certain large ion lithophile elements (LILE; e.g., Ba, K, Rb, Cs, Ca, Sr), U, and Pb in arc magmas relative to high field strength elements (HFSE; e.g., Ti, Th, Hf, Nb, Zr) are considered key indicators of fluid addition to arc magma source regions worldwide, but the fluid sources and processes of mass transfer are controversial. Dehydration of downgoing slabs releases fluids that can flow through and react with metamorphosed ultramafic-mafic rock packages in mélange zones near slab-mantle interfaces. New geochemical data from Syros, Greece, reveal that these fluids preferentially leach LILEs, U, and Pb when they infiltrate and react with subducted metasedimentary rocks. Transfer of these LILE-, U-, and Pb-enriched fluids to the mantle wedge at subarc depths could directly trigger partial melting and generate magmas with elevated Ba/Th, Sr/Th, Pb/Th, and U/Th, as well as radiogenic Sr. Alternatively, if fluid transfer occurs at shallower depths (e.g., Syros), the metasomatized mantle could be carried deeper by wedge corner flow to ultimately undergo partial melting in subarc regions.

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