Assessing the redox state of Mariana forearc

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Subduction zones ("arcs") are the geological setting where one tectonic plate ("slab") is dragged underneath another into the Earth's deep interior ("mantle"). They are sites of major chemical exchange between the Earth's surface and interior. Water and other volatiles (e.g. sulfur, chlorine, fluorine, carbon) are released from the subducting slab in the form of fluids and transported to the overlying plate ("fore-arc mantle"). These fluids are considered to play a critical role in the transport and concentration of economically-important elements like copper and gold in subduction zones and the chemistry of arc magmas and associated volcanic gases. Deciphering the chemistry of these fluids is also fundamental to understanding the Earth's carbon and sulfur cycles and the evolution of the Earth's atmosphere and oceans. These cycles rely on knowing both the efficiency of carbon and sulfur burial by subduction, and the loss of these elements from the subducting slab to the fore-arc mantle. However there are major gaps in our knowledge of slab fluid chemistry and especially in relation to the redox cycling of sulfur, iron and carbon. The goal of this project is to constrain the nature of slab fluids and quantify the budgets and oxidation states of sulfur, carbon and iron in these fluids using a combination of novel chemical fingerprinting tools that we have demonstrated to be highly effective for this purpose. In this project we will take advantage of the IODP expedition 366 to visit and sample the Mariana arc. The Marianas are one of the best-studied arcs, and one of the few places where we can directly access and sample the fore-arc mantle. This expedition presents a unique opportunity to study the zone where fluids are released from the subducting slab and react with the mantle of the overlying plate. From working on samples collected during this expedition we will be able to directly study the reactions between the fluids released from the subducting slab and the overlying fore-arc mantle and we will be able to use our results to determine the compositions and redox budgets of these fluids and to address their role in global carbon and sulfur cycles.