Testing Models of Magma Generation in Warm-slab Subduction Zones: A Case Study of Volatiles in the Cascade Arc

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The goal of this project is to better understand how magma is produced inside the Earth beneath the volcanoes in the Cascade mountains of Washington, Oregon, and northern California. Volcanism in this region is caused by the process of subduction, in which oceanic crust offshore of the Pacific northwest sinks back into the Earth?s mantle along the continental margin. Subduction is also responsible for the very large earthquakes that can occur in this region. As the sinking plate heats up, it releases H2O and other ?volatile? elements (Cl, S, F, C), which in turn cause the overlying mantle rocks to melt, producing magma that feeds the Cascade volcanoes. While the process of subduction occurs in many regions around the world (e.g., the Andes), a distinctive feature of the Cascades system is that the subducted oceanic crust is geologically young when it starts to subduct. This causes the crust to be hotter and should result in release of H2O and other volatiles when the subducted crust is still quite shallow, before it lies beneath the volcanoes. The main challenge, then, is to understand what role H2O and other volatiles play in creating magma beneath the Cascade volcanoes, and if volatiles are important, how they move from the subducted oceanic crust back into the mantle to produce magma.This project will test some fundamental ideas about volatile recycling and magma generation in ?warm-slab? subduction zones such as the Cascades arc. The project will involve new measurements of H2O, other volatiles, fluid mobile elements, and stable and radiogenic isotope tracers in primitive Cascade magmas. Many of the measurements will be done on melt inclusions, which are small blobs of silicate melt trapped inside of growing phenocrysts at depth. Such inclusions are not affected by low pressure degassing as magma moves towards the Earth?s surface, and thus provide a record of the pre-eruptive dissolved volatile contents in magma. The data obtained in this project will provide new information on mantle and slab temperatures, magma production processes, and the sources of volatiles beneath the Cascades arc. This project involves collaborations with other scientists in the U.S., Canada, and Japan for some of the specialized chemical analyses that will be done. Another collaborative effort will involve geodynamic modeling of mantle wedge flow patterns beneath the Cascades so the new geochemical data can be integrated with geophysical results.