Nitrogen Transiting the Subduction and Redox Barrier

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Earth's habitability depends on maintaining a high abundance of nitrogen in the atmosphere. Nitrogen is added to the atmosphere during volcanic eruptions and is removed from the atmosphere when it is incorporated into the crust. Crust is subducted into Earth's interior over geological time. During the subduction process some nitrogen is retained in crust and some is liberated into fluids and magmas that ultimately return nitrogen to the atmosphere. The proportions of nitrogen retained in and liberated from crust are expected to depend on the redox state of crust. A critical observation in geochemistry is that the ratio of nitrogen to potassium is constant between Earth's exterior (crust+atmosphere) and interior (mantle). Geochemical coupling of nitrogen and potassium is expected under reducing conditions. Therefore, the constancy of nitrogen/potassium ratios between Earth's exterior and interior suggests that reduced environments within crust are largely responsible for transporting nitrogen into Earth's interior. The specific redox boundary where nitrogen and potassium become geochemically coupled, however, remains poorly defined. Defining this boundary is the goal of this project. By defining this boundary, a constraint on the redox state of subducted materials will be derived that will yield further insight into how a nitrogen-rich atmosphere has been maintained on Earth and into redox-sensitive processes occurring in subduction zones, including volcanic outgassing, continental crust formation, and ore genesis. This project advances STEM fields by providing research opportunities for undergraduate students and by providing training to a young scientist.  A series of experiments will be conducted that simulate the conditions that nitrogen and potassium are subjected to during subduction to define the redox boundary of nitrogen-potassium coupling. The redox state of each experiment will be controlled by metal-oxide buffers and will be independently verified by XANES and/or Pt-in-Fe oxybarometers. Nitrogen speciation will be determined within the glasses and trapped fluids of each experiment. Experiments will also be chemically analyzed to determine how the relative affinity of nitrogen and potassium for minerals and magma/fluids changes as a function of pressure, temperature, and redox potential. These data will then be applied to geochemical models of slab melting/dehydration to constrain the pressure, temperature, and redox conditions that crust can be subjected to while maintaining nitrogen-potassium coupling.