The First Two Million Years of Subduction Beneath the Izu-Bonin-Mariana System as Recorded in IODP Expedition 352 Cores

Abstract:

IODP Expedition 352 explored the upper crust generated in the aftermath of subduction beginning at c. 52 Ma beneath the Izu-Bonin-Mariana (IBM) system. Drilling confirmed the transition from of basalt to boninite with distance from the trench and showed that both lava types are underlain by compositionally related dolerites. This geology was attributed to generation of the basalt to low-Si boninite sequence during seafloor spreading before magmatism began to focus as a boninite arc (Reagan et al., 2017, Int. Geol. Rev.). Seafloor spreading lasted <1.2 Myr after subduction began (Reagan et al., 2019, Earth Planet. Sci. Lett.), and boninitic volcanism in the nascent arc lasted until about 45 Ma (e.g. Ishizuka et al., 2020, J. Pet.). The Philippine Sea and Pacific plates are isotopically distinct, allowing unusually tight constraints on the mass transfer between the subducting lithosphere and the sources for magmas in the mantle wedge (Li et al., 2019, Earth Planet. Sci, Lett.). The sources for the basalts were strongly depleted, and melting was shallow compared with mid-ocean ridge settings (Shervais et al., 2019, G-cubed). The compositions of basalts do not require the transfer of water soluble cations from the subducting plate, although high H₂O/Ce and Cl/K, and low δD values in fresh basalt glasses suggest that dilute aqueous solutions, perhaps derived from serpentinites, were involved in their genesis (Coulthard et al., submitted). Boninites were generated from the harzburgitic mantle left after basalt genesis at pressures of <1 GPa as a result of flux melting involving water-rich melts of subducted materials: altered basaltic crust in the case of LSB, and subducted sediment for HSB. The length scale of this short-lived subduction initiation event was >1700 km. Rare similarly large events are preserved in ophiolite belts with ages up to 2 Ga, suggesting that time marks the onset of IBM-style subduction initiation (Pearce and Reagan, 2019, Geosphere).