Collaborative Research: Behavior of Boron During Prograde Diagenesis and Metamorphism of Pelagic Sediments from the Nankai Trough

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Subduction zones, where one tectonic plate slides beneath another, are places where water, carbon dioxide and other chemical components are transferred between the Earth’s surface and interior. Some chemical components that enter into subduction zones are released back to the oceans (as water or gas); others contribute to magmas in volcanoes (where they become part of the crust as igneous rocks or are released to the atmosphere as volcanic gases) and still others are transported all the way into the deep interior of the Earth. Understanding how elements and compounds are divided between the surface and the interior of Earth can provide important information about the regulation of carbon dioxide levels in the atmosphere, the concentration of radioactive elements near the surface of the Earth, and the tendency of some volcanoes to erupt violently. This project uses the element boron, which is highly concentrated in seawater and ocean sediments compared to the interior of the Earth, to track the recycling of surface components through subduction zones. If we can measure the amount of boron entering into a subduction zone on the sea floor and how much boron is released by associated volcanoes and into the ocean, then we can calculate how much boron is recycled into the deep Earth. The workplan includes opportunities for intellectual exchange between collaborators in the United States, Italy, and Japan. Students at both the graduate and undergraduate levels will participate in and benefit from the collaborative exchange.Subduction zones are the primary locus for recycling of crustal materials into the Earth’s mantle, with important implications for mantle and continental crustal evolution throughout Earth history. Subducted sediments in particular contribute volatiles, trace elements, and unique isotopic signatures to the oceans, the mantle, and arc magmas. Boron is both highly fluid mobile and isotopically fractionated during thermally-driven desorption and metamorphic dehydration, making it an excellent tracer for both sediment and fluids. In contrast, Rare Earth Elements (HREE) and High Field Strength Elements (HFSE) are generally immobile, making them good tracers of sediment and basalt provenance. By observing the co-variation of B, d11B, and HREE/HFSE concentrations in progressively dehydrated marine clays and subsidiary basalts within a unique sample suite from the Nankai margin, we propose to constrain 1) the initial variability in slab inputs to the trench within a well-constrained segment of a single subduction zone; and 2) the evolution of sediment and released fluid composition through progressive desorption and dehydration prior to entering the sub-arc region. The workplan includes opportunities for intellectual exchange between collaborators in the United States, Italy, and Japan. Students at both the graduate and undergraduate levels will participate in and benefit from the international collaboration.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.