Contribution of slab melting and slab dehydration to magmatism in the NE Japan arc for the last 25 Myr: Constraints from geochemistry

Hf isotope compositions, coupled with Pb‐Sr‐Nd isotopes and trace element compositions, for the Miocene to the Quaternary volcanic rocks from the NE Japan arc have documented the geochemical evolution in the magma source compositions in association with back‐arc opening (23–14 Ma). Clear temporal variation in Hf isotope ratios is demonstrated, from the lowest ɛ Hf (+9) in the early Miocene to the highest ɛ Hf (+14) in the late Miocene and the Quaternary, whereas there are little changes in Sr and Nd isotope composition. Hf isotope ratios are clearly correlated with some trace elemental ratios. The observed temporal variation is attributed to a change in metasomatic agents derived from the subducted slab, which could have been caused by cooling of the mantle wedge during the syn and post stages of the back‐arc opening. In the early stage of the back‐arc opening, when injection of hot asthenospheric mantle induced high‐temperature conditions in the mantle wedge, slab melting contributed to low ɛ Hf coupled with low ɛ Nd , relatively low La/Nb, and high Th/Yb in volcanic rocks. On the other hand, the later stage volcanism, characterized by high ɛ Hf , decoupling of ɛ Hf and ɛ Nd , high and variable La/Nb and Ba/La, is best explained by addition of fluids from subducted sediments and oceanic crust to the mantle wedge.