A contribution of slab-melts to the formation of high-Mg andesite magmas; Hf isotopic evidence from SW Japan

Hf isotope ratios were measured for high-Mg andesites (HMA) and basalts from the Setouchi volcanic belt, Southwest Japan, in order to examine whether fluid or melt from subducted slabs act as metasomatic agents for HMA magma genesis. Since Hf is one of the incompatible elements that may not be readily transported by aqueous fluids, Hf isotopes should provide strong constraints on the origin of such slab-derived components. Setouchi HMAs and basalts are distinct in their lower 176Hf/177Hf (εHf < +10.8) than MORBs and Izu-Bonin-Mariana arc basalts (>+13), suggesting significant contribution of subducted sediments to Setouchi magma genesis. It should be further stressed that Hf isotopes are well correlated with Sr-Nd isotopes and La/Nb ratios among the Setouchi magmas. Geochemical modeling suggests that such characteristics can be best explained by addition of slab-derived components with sediment melts rather than aqueous fluids to the HMA source.