Distinguishing Sediment, Serpentinite, and Altered Oceanic Crust in the Source of Aleutian Volcanic Rocks Using Boron & Molybdenum Isotopes

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Subduction is the fundamental geologic process where one tectonic plate sinks beneath another into the hot interior of the Earth. Subduction has many important human impacts, especially hazards from seismic and volcanic activity that are a constant threat to population centers and commerce worldwide. Subduction also plays a key role in shaping the earth's surface and is important in the formation of continents and the mineral resources they contain. As a result, subduction-related processes are the subject of broad public and scientific interest. The key goal of this work is to understand how fluids (superheated water at high pressures) drive subduction magmatism, which is manifest in volcanoes at the earth’s surface. Under this project, chemical data will be collected on volcanic rocks of the Aleutian Island Arc. Data collection will focus on the elements boron (B) and molybdenum (Mo), which are relatively water-soluble in earth-surface systems, and as a result, are abundant in seawater and are easily transferred to rocks that underlie the ocean basins. When subduction pushes the hydrated oceanic crust back into the Earth’s interior, the water is driven out of the rocks by chemical reactions that produce fluids rich in B and Mo. Under the extreme pressures and temperatures of the Earth’s interior, fluids lower the melting point of the rocks that they encounter and trigger the melting processes that lead to volcanic eruptions at the Earth’s surface. In turn, the concentrations of B and Mo in these volcanic rocks reveal key information about the paths that the fluids follow and the rocks that are melted along the way. Immediate societal benefit of this work will be research training of the graduate students who will be supported under the project. The project also initiates an after-school intern program for students from Dreher High School, in Columbia, South Carolina. The student interns will assist with sample preparation, data reduction, and data management. The interns will also produce a small data set of their own as a final project. The goals of the program are to expose high-school students to areas of applied chemistry and to NSF-funded research in the earth sciences.The key technical goal of the project is to take advantage of the fluid-mobile behavior of boron and molybdenum in order to distinguish the competing roles of subduction inputs (sediments, serpentinite and altered basaltic crust) in the generation of the Aleutian Arc magmas. We will collect B and Mo isotope data for subduction inputs (sediment, altered oceanic crust, abyssal peridotite) and outputs (volcanic rocks) along the Aleutian arc and link the chemical results to physical parameters such as subduction rate, sediment flux, and thermal parameter, which change systematically along the arc. Boron isotopes are used as tracers of fluid pathways, which also respond to changes in the thermal state of the subducting plate. The importance of Mo isotopes stems from their value as a tracer of both fluids and sediment type. The combined B and Mo isotope tracers and Aleutian natural laboratory create opportunities to distinguish sources in subducted sediment, serpentinite, and altered oceanic crust, especially at the point along the arc where the Amlia Fracture Zone enters the trench, and in the western-most Aleutians, where hot-slab effects are uniquely well expressed.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.