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 (AOC), but currently available Pb elemental and isotopic data for Izu-Bonin arc magmas and Pacific crust subducting at the adjacent trench are at odds with this model. The Izu-Bonin arc is one of the best-investigated volcanic arcs in the world and Pb is one of the foremost tracers of AOC recycling. Thus, the failure of the Pb tracer in the Izu-Bonin arc-trench system puts into question the concept that the AOC is an important source of arc magmas and our understanding of arc magmatism as a whole. The main objective of the funded project is to determine whether there is indeed a discrepancy between AOC input and arc output in the Izu-Bonin arc-trench system. We plan to analyze basaltic crust samples that will be collected through dredging of the active, vertical fault scarps along the Pacific Plate subducting into the Izu-Bonin Trench from 26.5°N to 34.5°N. This 'trench transect' of nearly ~900 km length encompasses a significant range of crustal ages, and includes a hypothesized change in the composition of the Mesozoic Pacific crust at ~125 Ma. Our comparative study between trench and arc chemistry along this transect will include mass balance calculations of slab input-arc output and 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 AOC plays a pivotal role. The main hypothesis to be tested is that the AOC being subducted along the Izu-Bonin trench, contrary to current belief, is compositionally variable due to the complex tectonic history of the Pacific Plate. The null hypothesis is that the subducted AOC is not the source of the compositional variation of arc magmas, and arc Pb and other elements are derived mainly from other sources such as the subarc mantle and/or the arc crustal basement. These science objectives are directly linked to the Integrated Ocean Drilling Program (IODP) 2013-2023 high-priority scientific Challenge 11 “How do subduction zones initiate, cycle volatiles, and generate continental crust?” (IODP Science Plan for 2013-2023 "Illuminating Earth's Past, Present, and Future"). Specifically, results will provide essential data input to four upcoming IODP drilling expeditions (three JOIDES Resolution legs scheduled for 2014; CHIKYU expedition pending) that will investigate the petrological and geochemical variations of the volcanic output and the mechanisms of crustal growth in the Izu-Bonin arc through time and space (proposals IODP-695, 696, 697 and 698).
The funded project will foster national and international collaboration among P.I.’s from the Scripps Institution of Oceanography (SIO), Lamont-Doherty Earth Observatory (L-DEO) and Institute for Frontier Research on Earth Evolution (IFREE). It will involve two female PIs on career tracks and will provide funds to support graduate and undergraduate students. The students will be heavily involved in the mapping survey and dredge sampling of the study area, geochemical and isotopic analyses and interpretation and modeling of the analytical results. All the analyses will be done using existing infrastructure at SIO, L-DEO and IFREE. Results will be of interest to the broad community of geochemists, geophysicists and geodynamicists working in convergent margins and in mid-ocean ridge environments. Some of the project results will also be used to sponsor a 1-day workshop for teachers from the San Diego Unified School District through the NSF-funded Center for Ocean Science Education Excellence - California (COSEE CA).