Intellectual Merit. Geochemical exchange between the earth's interior and exterior, via crust formation at ridges and crust destruction at subduction zones, has modified the composition of both the surface and interior reservoirs through time. The oceanic crust becomes progressively oxidized and hydrated as it ages, linking water and oxidation state in the subducting plate. This link could decouple within subduction zones due to the mantle's buffering capacity, or it may persist to depth. Oxidized or oxidizing components from the subducted slab may modify mantle and magmatic oxidation states, thereby influencing element partitioning, magmatic differentiation and degassing, and the long-term evolution of oxygen availability in the mantle. Several recent studies of bulk-rock and mineralogical proxies for oxidation state (e.g., redox-sensitive V partitioning, spinel composition, whole-rock Fe+3/_Fe) have yielded contradictory views of mantle oxidation state in modern tectonic settings and throughout earth history. Modern subduction zones provide an ideal setting to investigate if and how redox conditions of the mantle and hydrosphere have co-evolved. This study aims to develop a new method of measuring the oxidation state of basaltic magmas, using a synchrotron-based microbeam technique (micro-XANES) for non-destructive, in situ measurement of the redox-sensitive Fe+3/_Fe ratio in glasses. This technique will allow direct comparison of melt oxidation state to major, volatile, and trace element composition in primitive, undegassed magmatic liquids (i.e., natural pillow glasses and melt inclusions) and experimental glasses at a ~10 _m sampling scale. Both natural glasses from along and across the Mariana arc/trough system and synthetic glasses from analog piston-cylinder experiments will be analyzed, with the goals of (1) testing magmatic Fe+3/_Fe ratios against alternative proxies of both melt and mantle oxidation state (2) evaluating the role of magmatic processes, volatiles, and slab-derived components in influencing oxidation state, and (3) modeling the effects of the subduction cycle on the long-term evolution of redox conditions in the earth's interior. Mariana arc lavas are ideal for this study because they carry primitive, undegassed, basaltic melt inclusions that record a range of magmatic water contents and slab-derived chemical signatures. Also, samples of submarine glass from the full length of the Mariana trough back-arc basin, can be analyzed to provide information regarding spatial variations in volatile content and ozidation state.
Broader Impacts. This project will develop a potentially transformative new micro-analytical technique for the quantification of oxidation state in natural and experimental glasses. Results will be disseminated through peer-reviewed publications and on-line databases. These results will have broad-reaching results with direct relevance to the MARGINS, RIDGE, and deep earth science communities. This project also will foster the careers of two early-career female PIs who will bring their complementary skills to bear on a new problem requiring a highly collaborative, multidisciplinary team. The two PIs are deeply committed to mentoring under-represented minorities in the earth sciences and have established a track record with such students during the pilot work for this project. Finally, this award will fund a Ph.D. student to be jointly mentored by the two PIs, and therefore exposed to a broad palate of experimental and analytical techniques with which to launch a career in the earth sciences.
Most of the oxygen on planet Earth resides in the Earth's deep interior -- in the space between the crust and the core, the vast region of solid rock called the mantle. Oxygen leaves the mantle via volcanoes and returns to the mantle via the recycling of tectonic plates. This project contributed significantly to our understanding of planetary-scale oxygen cycling. Specific outcomes of our work include: (1) Creation and characterization of silicate reference glasses for use by the international community in the determination of iron oxidation state. The standards are available through the Smithsonian and have been applied in fields as diverse as the geosciences, chemical engineering, and nanomaterials. (2) Demonstration that the plate recycling process releases oxidizing fluids to the overlaying mantle and critically influences the nature of volcanoes around the Pacific Basin ("ring of fire"). (3) Quantification of the relationship between water, tracers of aqueous fluids, and iron oxidation state in arc basalts. (3) Demonstration that the Earth's mantle is approximately an order of magnitude more oxidized than previously believed. This in turn implies that the mantle may begin melting at much greater depths, which may explain slow seismic velocities and high electrical conductivity in the upper mantle. Moreover, the mantle's oxidation state may vary from place to place in a manner that reflects the tectonic history of a mantle parcel and its carbon content. (4) Training of two PhD students and 4 undergraduate students in advanced analytical techniques that are widely applied in science and industry. (5) Publication of 7 peer-reviewed publications in high-impact international journals during the award period, with more publications yet to come; presentation of results 49 times to international audiences; presentation of results in long-format hour long presentations to technical and public audiences to increase understanding of oxygen's role in Earth systems. (6) Production of videos, available free of charge to the general public on Smithsonian's iTunes University channel, for use in middle school classrooms aligned with standards of learning in national Earth Science curricula. (7) Participation in VANISHED, as a Featured Scientist. Produced by Smithsonian and MIT, "Vanished" was an online/offline mystery game for middle-school children, meant to inspire engagement and problem solving through science. It ran for 8 weeks in 2011.
