EAR-PF: A new oxybarometer to quantify spatial and temporal scales of redox variation in subducting slabs

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Dr. Megan Holycross has been awarded an NSF EAR Postdoctoral Fellowship to investigate how subducting slabs record the Earth's deep oxygen cycle. Her research and public service will take place at the Smithsonian Institution and at Yale University. The availability of oxygen in the solid Earth influences the structure of the planet, the chemistry of rocks and ores as well as the composition of the atmosphere. Mass transfer of oxygen between the Earth's oxidized surface and its reduced interior takes place at subduction zones, but the oxidation state of the subducting slab itself, and its consequences for petrological, geochemical and geodynamic processes in subduction zones, is still poorly understood. Dr. Holycross will perform experiments to calibrate a new proxy for oxygen in eclogites, metamorphic rocks formed on subducting slabs that have made their way back to the surface. Following experimental calibration, she will apply the new proxy to determine how ancient and modern suites of natural eclogites track spatial and temporal variations in the oxidation state of subduction zones. During her tenure as an NSF Postdoctoral Fellow, Dr. Holycross will host educational events at both the National Museum of Natural History and the Yale Peabody Museum of Natural History to share the excitement of geoscience with the public. She will also develop a virtual peer-to-peer mentoring and support group to connect early career scientists at 8+ U.S. institutions. Eclogites form on subducting slabs during high-pressure and high-temperature alteration of oceanic basalts. Therefore they may retain important information about the oxidation state (or oxygen fugacity, fO2) of their basalt precursor, or alternately, their oxidation state may be a result of the mass transfer of oxidized materials throughout the subduction system. Comprehensive interrogation of the eclogite fO2 record, and understanding the geological significance of these values, requires an experimentally calibrated proxy that can be applied to natural samples using commonly available analytical tools. Eclogitic mineral assemblages (here, rutile, garnet and pyroxene) are excellent candidates for the development of a new fO2 proxy that will utilize the sensitivity of the multivalent trace element vanadium (V) to changes in redox environment. Vanadium shifts speciation (V2+, V3+, V4+, V5+) over Earth-relevant fO2s, and thus the incorporation of Vanadium in different phases should change as function of oxygen fugacity. Eclogite-melting piston-cylinder experiments will be run to calibrate the partitioning of Vanadium between rutile, pyroxene, garnet and silicate melt as a function of fO2, pressure and temperature. The new fO2 proxy will be applied to Archean eclogite xenoliths from West Africa and an exhumed Eocene terrane from the Cyclades subduction complex in Greece to probe redox changes in subducting slabs over time (scale of billions of years) and space (scale of a subduction zone).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.