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10Primitive melt inclusions in chrome spinel from the Ogasawara Archipelago comprise 11 two discrete groups of high-SiO2, MgO (high-Si) and low-SiO2, MgO (low-Si) boninitic suites 12 with ultra-depleted dish-and V-shaped, and less depleted flat rare earth element (REE) patterns. 13The most magnesian melt inclusions of each geochemical type were used to estimate the T-P 14 conditions for primary boninites, that range from 1345°C-0.56 GPa to 1421°C-0.85 GPa for the 15 48-46 Ma high-Si and low-Si boninites, and 1381°C-0.85 GPa for the 45 Ma low-Si boninite. 16

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 volcanics recovered from the islands and exposed crustal sections of the Bonin Ridge span 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 Myr. Following initial MORB-like spreading-related basalt magmatism, volcanic centres migrated away from the trench and changed from high-Si boninite to low-Si boninite/high-Mg andesite, then finally tholeiitic/calcalkaline arc magma. Subducting pelagic sediment combined with Pacific-type igneous ocean crust dominate 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 volcaniclastics and Pacific MORB. In turn, this signature was replaced by a Pacific MORB-dominated flux in the post 45 Ma tholeiite and calcalkaline volcanics. 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.

The Ogasawara Islands mainly comprise Eocene volcanic strata formed when the Izu–Ogasawara–Mariana Arc began. We present the first detailed volcanic geology, petrography and geochemistry of the Mukojima Island Group, northernmost of the Ogasawara Islands, and show that the volcanic stratigraphy consists of arc tholeiitic rocks, ultra‐depleted boninite‐series rocks, and less‐depleted boninitic andesites, which are correlatable to the Maruberiwan, Asahiyama and Mikazukiyama Formations on the Chichijima Island Group to the south. On Chichijima, a short hiatus is identified between the Maruberiwan (boninite, bronzite andesite, and dacite) and Asahiyama Formation (quartz dacite and rhyolite). In contrast, these lithologies are interbedded on Nakodojima of the Mukojima Island Group. The stratigraphically lower portion of Mukojima is mainly composed of pillow lava, which is overlain by reworked volcaniclastic rocks in the middle, whereas the upper portion is dominated by pyroclastic rocks. This suggests that volcanic activity now preserved in the Mukojima Island Group records growth of one or more volcanoes, beginning with quiet extrusion of lava under relatively deep water followed by volcaniclastic deposition. These then changed into moderately explosive eruptions that took place in shallow water or above sea level. This is consistent with the uplift of the entire Ogasawara Ridge during the Eocene. Boninites from the Mukojima Island Group are divided into three types on the basis of geochemistry. Type 1 boninites have high SiO2 (>57.0 wt.%) and Zr/Ti (>0.022) and are the most abundant type in both Mukojima and Chichijima Island Groups. Type 2 boninites have low SiO2 (<57.1 wt.%) and Zr/Ti (<0.014). Type 3 boninites have 57.6–60.7 wt.% SiO2 and are characterized by high CaO/Al2O3 (0.9–1.1). Both type 2 and 3 boninites are common on Mukojima but are rare in the Chichijima Island Group.