Collaborative Research: Geochemical variation of the Pacific crust subducting beneath the Izu-Bonin arc and its implications for the generation of arc magmas

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The Izu-Bonin arc in the western Pacific Ocean is one of the best-investigated volcanic arcs in the world and lead (Pb) isotopes are one of the foremost isotopic tracers of the recycling of altered ocean crust. It is widely accepted that there is a strong compositional link between magmas erupted along volcanic arcs above subduction zones and subducted altered oceanic crust that is cycling back down into the mantle via deep ocean trenches. That said, currently available Pb elemental and isotopic data for Izu-Bonin arc magmas and the associated subducting altered ocean crust are at odds with this model, raising the spector that altered ocean crust may not, in fact, be an important source of arc magmas, which throws into question our understanding of arc magmatism as a whole. This research will use geochemical analyses (elemental and isotopic) of new and already collected samples of ocean crust to determine whether there is a discrepancy between subduction zone input and arc output in the Izu-Bonin arc-trench system. Using a UNOLS ship, samples of basaltic crust will be dredged from the seafloor along a ~900 km transect targeting active, vertical fault scarps along the Pacific Plate which is subducting into the Izu-Bonin Trench. This transect allows sampling of rocks of a range of crustal ages, which will include samples from the area where there is a hypothesized change in the composition of Mesozoic Pacific crust at ~125 Ma. Samples will be analyzed for chemical composition and the results will be used to determine mass balances of slab input/arc output. The work will also entail a calibration of the magnetic timescale. Results of the investigation will provide a crucial test of current concepts on crustal recycling at subduction zones and modes of crustal extraction from the mantle in which the subduction processing of carbon plays a pivotal role. Goals of this research are linked directly to the science plan of the NSF-funded Integrated Ocean Drilling Program (IODP) and will provide essential data input to four upcoming IODP drilling expeditions (three JOIDES Resolution legs scheduled for 2014 and one CHIKYU expedition that is pending). Broader impacts of the project will foster national and international collaboration among the investigators and will support two investigators from groups under-represented in the sciences. The project also will support graduate and undergraduate students who will be fully integrated into the science discovery aspect of the project. Project results will be incorporated into a 1-day workshop for teachers from the San Diego Unified School District through the NSF-funded Center for Ocean Science Education Excellence in California.