Hydration and Sulfur Metasomatism of Forearc Mantle

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During subduction, altered ocean crust and sediments dehydrate and these fluids react with the overlying sub-arc mantle. Knowledge of such fluids and reactions is critical to understanding chemical cycling in subduction zones, and what is ultimately recycled into the mantle. Conical and Torishima Seamounts in the Mariana and Izu-Bonin forearcs are serpentinite protrusions that result from large scale hydration of the forearc mantle wedge by dewatering of the underlying subducting slab. The proposed project will analyze the stable isotopic (O,H,S,C) compositions, opaque mineralogy, and sulfur contents of serpentinized mantle peridotites from these two seamounts. The objectives are 1. to determine the conditions of serpentinization at depth, 2. constrain the compositions of serpentinizing fluids and how they relate to those sampled at the surface, and 3. test the prediction from the sulfur isotope ratios of arc volcanics that a seawater sulfur component is present in slab-derived fluids. Results will provide a better understanding of metamorphic processes, conditions of hydration reactions, and fluid compositions at depth, as well as constraints on the thermal structure of the forearc mantle wedge. This study will provide fundamental data necessary for the evaluation of models for the thermal structure and hydration reactions in subduction zones.