Decoupling of fluids and fluid-mobile elements during shallow subduction: Evidence from halogen-rich andesite melt inclusions from the Izu arc volcanic front

摘要: [1] Very rare, halogen-rich andesite melt inclusions (HRA) in bytownitic plagioclase phenocrysts (An89–90) from tephra fallout of the Izu arc volcanic front (Izu VF) provide new insights into processes fluid release slab trenchward to a cool subduction zone. These HRA are markedly enriched Cl, F and Li - by factors up 8 (Cl, F) 1.5 (Li) but indistinguishable with respect fluid-mobile large-ion lithophile elements (LILE; K, Sr, Rb, Cs, Ba, Pb, U), rare earths (REE) or high field strength (HFSE) low-K tholeiitic magmas VF. We suggest that chemical signature reflects presence mantle source originated serpentinized peridotite above metacrust. This “wedge serpentinite” presumably formed infiltration beneath forearc was subsequently down-dragged depths. The combined evidence VF (∼110 km slab) outer serpentinite seamounts (∼25 30 suggests flux B Cl is highest forearc, decreases increasing In contrast, minor increases depth. Fluorine may behave similarly Li, whereas LILE appear be largely retained Consequently, signatures both trench sediments basaltic rocks preserved until