Leg 185 will drill two deep-water (5674 and 6000 m) sites, one, an existing Ocean Drilling Program hole (Hole 801C) seaward of the Mariana Trench, and the other, a new site (Site BON 8A), east of the Izu-Bonin Arc. 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) will help test whether geochemical differences in the volcanics are derived from contrasts in the crustal inputs to the two trenches. Previous drilling has already provided sections through the sedimentary layer approaching the Mariana Trench. Coring during Leg 185 will provide samples of the remaining input fluxes to the subduction zones: the altered basaltic crust at Hole 801C and the sediments and altered basaltic crust at Site BON-8A. The specific drilling plan is to deepen Hole 801C an additional 250 m (to ~400 m total basement depth) to penetrate the upper oxidative alteration zone, and to core the entire 470 m sedimentary section at BON-8A, along with as much basement as possible (to a maximum of 430 m) in the time allowed.

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. These data will permit a better understanding of the recycling of oceanic crust in the Earth's mantle, the formation of continental crust in arc systems and the role of recycled oceanic crust in the generation of mantle plume magmas. With Hole 801C, which penetrates the world's oldest in situ oceanic crust (~165 Ma), the science party will also provide the first reference site for the structure and composition of old, fast-spreading oceanic crust to compare with other crustal end-members. Based on the seismic structure of the East Pacific Rise crust formed at comparable spreading rates (160 mm/yr full rate), the upper 450 m at Hole 801C may include the entire extrusive section, or layer 2A, of the oceanic crust. Thus, it is possible that Leg 185 will provide the second example of in situ oceanic crust into the sheeted dike complex.

Leg 185 has been targeted to conduct contamination testing and development of a standard sampling procedure for deep biosphere research. The objective of the contamination test is to determine the amount of mixing created by the coring process by introducing a tracer (latex beads) at the mouth of the core barrel as the core is cut.

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