Constraining the release of oxidizing fluids from the mafic slab during subduction

Boston College, Chestnut Hill MA

Investigators

Abstract

Subduction zones are key areas of geologic activity in which two plates collide and one plate is forced beneath the other and subducted deep into the Earth’s mantle. Subduction zones are responsible for some of the most important global geologic events and significantly impact humans and populated areas by causing earthquakes and creating large volcanic arcs such as the Pacific ‘Ring of Fire’. Uncertainty exists in the degree to which subduction zone volcanism is controlled by the exchange of material and fluids between the subducting plate and the overlying mantle rocks. One way to study such processes, is to analyze rocks that were previously subducted to great depths but have since been exhumed and returned to the Earth’s surface. The aim of this project is to investigate the record of the release and characteristics of fluids produced during subduction contained within individual garnet crystals in exhumed subduction zone rocks from the Western Alps (Italy/Switzerland). This study will further our understanding into the importance of subduction-derived fluids in controlling whether rocks within the subducting plate and the overlying mantle are more chemically oxidized or reduced, which will provide insights into their role in controlling volcanic arc magmatism. This project will support a post-doctoral researcher at Boston College, who will work with undergraduate students, providing them with training in analytical techniques and research experience. The research team from Boston College will work with a large network of collaborators from Europe and the US. An important component of this project will include undergraduate students travelling to Europe to conduct their own research with the help of European collaborators. Much debate exists around the role of metamorphic fluids, released during dehydration of the subducting slab, in controlling the redox conditions of the overlying sub-arc mantle and in explaining the oxidized and volatile-enriched geochemical signatures of volcanic arc magmas. The transfer of multi-valent, redox-sensitive elements such as S, C, and Fe from the subducting slab, represents a potential mechanism to produce oxidation of the sub-arc mantle. The proposed research will test the hypothesis that oxidizing species are released within fluids during dehydration of the mafic component of the slab during subduction. This study involves creating a field-based record of the release of fluids during subduction of the mafic crust, examining whether these fluids contain oxidizing species, and assessing the implications for changing redox conditions within the subducting mafic slab and overlying sub-arc mantle. To achieve this, the project will utilize the first coupled Fe and Zn isotopic analyses of growth zones within individual garnet crystals from exhumed high-pressure metamorphic rocks from the Western Alps. This will be combined with precise Sm/Nd zoned garnet geochronology to provide insights into the rates and timescales of redox-controlling metamorphic reactions. With the help of an international network of collaborators, this project aims to provide a uniquely high-resolution geochemical and geochronological dataset to assess processes occurring at the subduction zone interface and their relationship to oxidation of the sub-arc mantle and volcanic arc magmatism. 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.