ALBERT

Description: Volcanism following the initiation of subduction is vital to our understanding of this specific magma-generation environment. This setting is represented by the first development of the Izu–Bonin–Mariana arc system as subduction commenced along the Western Pacific margin in the Eocene. A new collection of volcanic rocks recovered from the islands and exposed crustal sections of the Bonin Ridge spans the first 10 Myr of arc evolution. An elemental and radiogenic isotope dataset from this material is presented in conjuction with new 40Ar/39Ar ages and a stratigraphic framework developed by a detailed mapping campaign through the volcanic sections of the Bonin Islands. The dating results reveal that both the locus and type of magmatism systematically changed with time in response to the progressive sinking of the slab until the establishment of steady-state subduction at around 7–8 Ma. Following initial mid-ocean ridge basalt (MORB)-like spreading-related basalt magmatism, volcanic centres migrated away from the trench and changed from high-Si boninite to low-Si boninite or high-Mg andesite, then finally tholeiitic or calcalkaline arc magma. Subducting pelagic sediment combined with Pacific-type igneous ocean crust dominates the slab input to the shallow source of high-Si boninites at 49 Ma, but high-precision Pb isotope data show that this sediment varies in composition along the subducting plate. At around 45 Ma, volcanism switched to low-Si boninite and the pelagic sediment signature was almost entirely replaced by volcanic or volcaniclastic material originating from a HIMU ocean island source. These low-Si boninites are isotopically consistent with a slab component comprising variable proportions of HIMU volcaniclastic rocks and Pacific MORB. In turn, this signature was replaced by a Pacific MORB-dominated flux in the post 45 Ma tholeiite and calcalkaline volcanic rocks. Notably, each change in slab-derived flux coincided with a change in the magma type. Fluctuations in the slab-derived geochemical signature were superimposed on a change in the mantle wedge source from highly depleted harzburgite to a depleted MORB-type mantle-type source. In turn, this may correspond to the increasing depth of the leading edge of the slab through this 5 Myr period.