IODP ³ Expedition 502 “Impact of Petit-Spot Magmatism on Subduction Zone Seismicity and Global Geochemical Cycles” Scientific Prospectus

Abstract. The International Ocean Drilling Programme (IODP3) Expedition 502 will explore the nature of the acoustic basement in the outer-rise area of the NW Pacific subduction system, where the seismic-layer-1 pelagic sediment cover of the subducting old (120–130 Ma) Pacific Plate is expected to be anomalously thin. Drilling will test our hypothesis that layer 1 is due to the intrusion of basaltic sills or the eruption of sheet-flow lavas into and onto the pelagic sediments fed by petit-spot magmatism. If our hypothesis is correct, petit-spot magmatism at the outer rise may be more widely distributed than previously thought. More extensive petit-spot volcanism could strongly impact our understanding of subduction systems, including rupture nucleation and slip propagation of plate boundary megathrust earthquakes. It will also require adjustments to the geochemical budgets of arc magmatism and global volatile cycles due to changes in the inventory of materials associated with the subducting oceanic plates. Testing this hypothesis will shed light on the impacts of subduction inputs and help to determine the global role of petit-spot magmatism.