Collaborative Research: Subduction Initiation and Development of the Izu-Bonin-Mariana arc: An Investigation of Samples from Cores from Recent Ocean Drilling

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The subduction of oceanic plates at deep-sea trenches around the globe leads to powerful earthquakes, dangerous volcanic eruptions, and economically important hydrothermal mineral deposits. This project will investigate some of the least understood aspects this globally important process: what causes it to start, what kind of crust is produced after subduction starts, and what materials are released from the subducting slab as it initially sinks. The International Ocean Discovery Program Expedition 352 drilled 1.22 kilometers of volcanic rocks at water depths of 3.1-4.8 kilometers in the Bonin fore-arc in the western Pacific Ocean to investigate the start of subduction. This research performs a complete geochemical and petrological characterization of drilled fore-arc, seafloor, mafic, volcanic rocks, (i.e., basalts and boninites) recovered during the drilling expedition to better understand how subduction zone volcanism begins and evolves. The rock sequences encountered in the Bonin fore-arc are remarkably similar to those found in many ophiolites (ocean crust and upper mantle rocks found on the continents), so this project will also provide insights into the origin of ophiolites as well. This project supports and expands the extensive international scientific collaborations developed during Expedition 352 to include additional top-flight female scientists as well as undergraduate and graduate researchers. Results from this project will become part of classroom resources supported by an ongoing NSF-funded education project. It will also provide funding to researchers at institutions in three EPSCoR states (Iowa, Utah, and Rhode Island) and foster international collaboration with Australian scientists.The overarching goal of this research is to document the nature of the first crust produced after subduction initiation in the Izu-Bonin-Mariana arc system. Samples collected from four sites drilled during the International Ocean Discovery Program Expedition 352 to the Bonin fore-arc will be analyzed. Drill cores from two of the sites primarily recovered fore-arc basalts and two recovered boninites. This research, in collaboration with that being done by other members of the shipboard scientific party, will comprehensively analyze the recovered volcanic rocks, providing geochemical data on the major and trace element compositions of volcanic glasses. It will also complete analyses of oxygen isotopes, water and CO2 concentrations in the volcanic glasses; measurements of water concentrations in pyroxenes; Fe ratios in glasses; concentrations of fluid-mobile elements and B-Li isotopes in whole rocks and glasses; radiogenic Re-Os isotopes and PGE abundances in whole rocks and glasses; and major element and trace element compositions of whole rocks. Resulting data will be used to constrain melting processes and understand variations in the fluid, solid, and magma fluxes through the nascent mantle wedge from subduction initiation onward as the mode of magma generation changes from decompression melting to flux melting. The data will be used to address four principle scientific questions: (1) are fore arc basalts generated by decompression melting in a unique high temperature, low-pressure environment during rapid sea-floor spreading related to subduction initiation; (2) are the mantle sources of fore arc basalts and boninites unusually depleted and do they exhibit signatures of ancient recycled mantle or continental components; (3) were low- and high-Si boninites from drilling Sites U1439 and U1442 generated in succession from depleted mantle left after generation of fore arc basalts as spreading rates decreased and the involvement of subducted fluids increased; and (4) did magma generation migrate landward with time and to lower pressures. These questions align with the principal objectives of the NSF-funded International Ocean Discovery Program's Initial Science Plan, including testing the validity of the ophiolite model and understanding the initial origins of continental crust. Additionally the work will address questions in the NSF-funded GeoPRISMS science plan that focus on what the physical and chemical conditions are that control subduction zone initiation and the early development of arc systems.