Lithium isotope perspective for mélange diapir melting beneath the Ryukyu arc

Abstract

A long-standing debate concerns whether arc magmas are generated by melting of a slab- metasomatized mantle wedge or by direct melting and dehydration of mélange diapirs. This study focused on the Ryukyu subduction zone, where new Li isotope measurements of subducting sediments reveal a transition from lighter Li isotope values outboard of the northern segment to heavier values outboard of the southern segment. The results show that δ7Li values of volcanic rocks are lower in the northern segment and higher in the southern segment than in the mantle. These volcanic rocks preserve the Li isotopic signatures of their subducted sediment inputs, indicating limited Li isotope diffusion. Our diffusion modeling confirms that sediment-derived Li could be transported to arc magmas if sediments migrate through the mantle as mélange diapirs. These findings support mélange diapirs as the mechanism for subducted material transfer beneath the Ryukyu arc, highlighting the critical role of mélange diapirs in material recycling within subduction zones.