Leg 185 Crustal Fluxes and Mass Balances at the Mariana-izu Convergent Margin

During Leg 185 two deep-water sites will be drilled, one seaward of the Mariana Trench (Ocean Drilling Program Hole 801C) and one seaward of the Izu-Bonin Trench (Site BON8A). The primary objectives are to investigate sediment subduction along this arc-trench system and to characterize the chemical fluxes during alteration of the oceanic crust. Despite the simple setting and shared subducting plate, there are still clear geochemical differences between the Marianas and Izu volcanic arc systems. Drilling the crustal inputs (sediments and basalts) can help test whether geochemical contrasts in the volcanics derive from contrasts in the crustal inputs to the two trenches. Previous drilling has already provided sections through the sedimentary layer approaching the Mariana Trench. Drilling during Leg 185 will provide samples of the remaining input fluxes to the subduction zones: the upper 300-500 m of altered basaltic crust at Hole 801C, and the sediments (500-600 m) and upper 300-400 m of basaltic crust at Site BON-8A. With Hole 801C, the science party will also provide the first reference site for the structure and composition of old Pacific (Jurassic), fast-spreading oceanic crust to compare with other crustal end-members (e.g., young-slow, young-fast, and old-slow). One outcome of Leg 185 will be the best existing mass balance of input and output fluxes for several key tracers (H2O, CO2, U, and Pb) cycled through the subduction factory.