Quantifying Geochemical Exchange at the Slab-Wedge Interface with Experiments and Natural Samples

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Abstract for Proposal 1447342 Quantifying Geochemical Exchange at the Slab-Wedge Interface with Experiments and Natural SamplesPI: C. TillThe goal of this project is to better understand the chemical reactions that result when one tectonic plate sinks below another, bringing water and other chemical constituents into the Earth's interior. The project includes collaboration with Japanese colleagues and is partly supported by International Science and Engineering. This type of plate-subduction produces volcanic arcs like those in the Ring of Fire, as well as some of the largest earthquakes on Earth every year. Understanding the chemical reactions that occur as a result of subduction is ultimately important to understand why our planet has a different type of crust than the other planets in our solar system, and how the gases and magmas emitted from volcanic arcs affect the composition of the Earth's atmosphere. Although we have long understood that subduction contributes to these planet-wide processes, many questions remain about the specific chemical reactions in which the plate-derived elements react with the Earth's mantle. This lack of knowledge is largely due to the limitations of studying subduction systems from the top down and can, in part, be resolved by studying rare samples from the mantle in conjunction with high-pressure experiments as proposed here.Part 2:This project addresses three specific questions about geochemical exchange at the subducted plate- mantle interface: 1) Do hydrous minerals, in particular chlorite, form at the base of the mantle wedge and play a key role in the formation of arc magmas as hypothesized by high-pressure experiments and studies of boron isotope fractionation?, 2) Does the composition of subducted sediment melt resemble experimental predictions and subsequently react with the mantle to produce andesitic arc magmas as predicted by geochemical models?, and 3) Do subducted plate fluxes oxidize the mantle as debated by geochemical studies of arc magmas and melt inclusions? To answer these questions we propose in-situ analyses of the major and trace element, Fe3+/FeT, and boron isotopic composition of rare peridotite and eclogite minerals from the Higashi-akaishi ultramafic complex, Japan, which represent restites and trapped melts exhumed from the base of the mantle wedge. The whole rock and mineral chemistry data will also be used to constrain starting compositions for high-pressure experiments on sediment melt-mantle wedge reactions in a complimentary effort to the in-situ chemical and isotopic analyses.