How does water move through the subducting slab? Slab-scale fluid pathways and deformation-fluid flow feedbacks at eclogite facies

University Of Washington, Seattle WA

Investigators

Abstract

Subduction zones, where one tectonic plate dives beneath another, are the location of damaging geologic hazards including explosive volcanic eruptions and the largest earthquakes on Earth. These hazards are catalyzed by water that is released by metamorphism of hydrous minerals and must travel to the source of earthquakes or melting along pathways such as veins and zones of ductile deformation. These pathways are too small to resolve with remote sensing of modern subduction zones, but their spatial and temporal distribution exercises a first-order control on subduction zone hazards. Subducted rocks that have been exhumed and exposed at Earth’s surface provide an opportunity to study the pathways transporting water from the micro- to plate-scale. However, previous studies of such rocks have focused at the small scale hindering a plate-scale view of where, when and how water moves to the source of geologic hazards. Using a complete cross-section of a subducting plate exposed in the Italian Alps, this project will combine field mapping of water pathways with chemical and structural characterization of rocks and minerals that interacted with passing water to construct a map of how water is transported in the subducting plate. This map will inform conceptual models of how water can initiate subduction zone hazards and be used to interpret remote observations of modern subduction zones. The broader impacts of this work focus on engaging a diverse group of undergraduate students in hands-on research. Four undergraduate students will have the opportunity to map and collect samples in the field, operate scientific instruments, collect data, and present their findings at conferences. To decrease barriers to participation in research among low-income students, students will be paid for their research time, and all travel expenses associated with field work, data collection and conferences will be covered. Benefits from this research will be returned to the communities around the field area in Italy through public lectures on the local geology. Aqueous fluids modulate key subduction zone processes like seismicity and arc magma production, and geochemical cycling. These fluids are produced by metamorphic reactions in the subducting slab and transported to the locations of these processes along a network of pathways that influences the location, timing, and magnitude of hazards and geochemical cycling. Geologic studies of exhumed subduction zone rocks are the only way to resolve fluid transport networks at the scale of individual pathways, and they suggest that both veins and shear zones can transport fluid through the slab. However, previous studies have largely focused on single outcrops or subordinate lithologies, impeding a cohesive view of slab-scale fluid transport and the relative importance of veins versus shear zones. The proposed project will use the exceptionally well-exposed eclogite-facies Monviso Ophiolite (W. Alps, Italy) to determine the dominant pathways and mechanisms for transporting fluid at the scale of the subducting slab. Detailed field mapping of veins and shear zones in the complete slab cross-section of the Monviso Ophiolite will be used to constrain the structure and abundance of fluid pathways in the eclogite-facies lower oceanic crust. Integrated with this macro-scale mapping, micro-scale petrological-geochemical-microstructural analysis of representative fluid pathways will reconstruct their fluid-deformation-time histories. Major and trace element (EPMA and laser ablation ICPMS), and in situ clinozoisite oxygen isotope geochemistry (SIMS) will be used to track the source of fluids transported along fluid pathways. These fluid sources will be linked to dehydration reactions throughout the slab using thermodynamic modeling. Textural analysis and in situ measurements of mineral zoning will constrain the relative timing of fluid transport along different fluid pathways, and the temporal distribution of fluid sources within a single pathway. Feedbacks between fluid transport and ductile deformation will be constrained using co-located in situ microstructural (EBSD) and geochemical measurements that will define deformation mechanisms and directly link them to fluid infiltration. The final product will be a temporally-resolved map of slab hydrology at eclogite facies that will inform conceptual models of fluid transport through the subducting slab and its relationship to hazards and geochemical cycling, and provide key inputs for interpreting geophysical observations of modern subduction zones. 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.