Melting of old refractory mantle during subduction initiation of the western Pacific plate

Abstract The source characteristics and melting mechanisms for forearc basalts formed during subduction initiation in the western Pacific plate remain unclear. Here we present new Hf-Nd isotopes and trace element data for the latest erupted forearc basalts collected during oceanic drilling in the Bonin forearc. These basalts have εNd values comparable to mid-ocean ridge basalts but much higher εHf, plotting above the terrestrial array, and elevated Lu/Hf and Sm/Nd ratios that indicate an unusually depleted mantle source. Systematic forward modeling indicates that these basalts cannot have been generated solely from the melting of the asthenosphere; both ~160 Ma and Proterozoic refractory mantle are required to explain the observed Hf-Nd isotope decoupling. This study proposes that the Bonin forearc lithospheric mantle may have formed during ancient events. Rollback of the earliest subducting slab caused substantial thinning of the forearc lithosphere and triggered extensive melting in both the asthenospheric and lithospheric mantle.