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.
