Postdoctoral Fellowship: EAR-PF: Investigating spatiotemporal variability of forearc mantle wedge serpentinization and rheology during non-steady state subduction

Dr. Gabe Epstein has been awarded an NSF Earth Sciences Postdoctoral Fellowship to carry out research aimed at constraining the causes, variability, and consequences of forearc mantle wedge hydration under the mentorship of Dr. Adam Holt at the University of Miami.

At regions where two tectonic plates converge, subduction is the process by which the “subducting” plate thrusts beneath the “overriding” plate and then descends to great depths within the mantle of the Earth; this process occurs along ~50,000 km of coastline around the world including beneath the states of Washington and Alaska, the Island of Japan, and the west coast of South America. Mantle wedge hydration occurs when water is released from the subducting plate into the section of the Earth’s mantle that lies above the subducting plate (the forearc mantle wedge) and can only proceed if this forearc region of the mantle is cool enough for water-bearing minerals to be stable.

Release of water from the subducting plate, and the associated wedge hydration, may contribute to the localization and intensity of arc volcanoes (e.g., the “ring of fire” surrounding the Pacific Ocean) and large magnitude earthquakes. Despite this potential importance of mantle wedge hydration, the temporal variability and consequences of this hydration are poorly resolved, often because the dynamic and time-dependent nature of subduction systems is typically neglected in studies that target these processes.

This project will merge novel computational methods with laboratory analysis to investigate how fluid release and wedge hydration in forearcs contributes to subduction dynamics (e.g., the speed and shape of subduction zones), and the impacts such dynamics have on volcanism, seismicity, water storage in the mantle, and Earth’s long-term habitability. Utilizing the research findings, Dr.

Epstein will develop a series of accessible, open-source, online learning modules aimed at broadening understanding of geoscience problems pertinent to society, welfare, and equity. The design of the modules will allow for participants (the public and high school to undergraduate students) to interact with large datasets and manipulate software code.

Broadly, the learning outcomes will be to demonstrate connectivity between geoscience and other branches of research (natural and social sciences, engineering, art), and to highlight the myriad research endeavors of modern geoscientists (laboratory, computational, and field based). The research and broader impact goals will produce new avenues in interdisciplinary science and will contribute to the NSF goal of advancement of national welfare by enhancing both the scientific community’s and the general public’s understanding of natural disasters and the global, long-term water cycle.

Dr. Epstein will merge open-sourced software (Python, Perple_X, and ASPECT) with chemical/rheological insights from the exhumed rock record to develop dynamic models of fluid release and concomitant forearc mantle wedge hydration over a range of subduction conditions to better constrain the extents, spatiotemporal variability, and chemical/geodynamic consequences of mantle hydration.

The project goals are: (1) determine the relative timing and extents of wedge hydration during a subduction zone’s lifetime, and the associated spatio-temporal variability in mineralogy and viscosity, through geodynamic modeling; (2) perform microstructural observations of natural wedge serpentinites to characterize deformation mechanisms over a range of P-T conditions; and (3) use trace element ratios and stable isotope analysis to determine the relationship between fluid flow and deformation mechanism. These goals are iterative, with insights from (2) and (3) feeding back into the geodynamic model (1).

Laboratory chemical (major and trace element, stable N isotope analysis) and crystallographic/rheologic data (in-situ characterization of deformation mechanisms, EBSD, and Raman spectroscopy) will be performed on samples representing mantle wedge serpentinites or close analogs from New Idria (California, USA; blueschist facies), Cemetery Ridge (Arizona, USA; amphibolite facies), and the Western Alps (Italy/France; greenschist through eclogite). It is expected that earliest phase(s) of the thermal evolution of subduction zones is responsible for stabilization of large volumes of hydrous material in the mantle wedge, and that the rheologic behavior of this material varies depending on the extents and distribution of hydration combined with the temperatures and rates of viscous deformation.

Dissemination of the research will include development of a series of accessible, experiential, online learning modules hosted as Jupyter Notebooks (via Binder) and geared towards high school and undergraduate students with the aim of demonstrating the interplay among geoscience, chemistry, physics, and coding. The Universal Design for Learning (UDL) educational framework will be utilized to create an accessible and equitable learning environment throughout creation of the modules.

This EAR-PF award was co-funded by the EAR Tectonics Program.

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.