MARGINS: Collaborative Research: Experimental Investigation of Water Solubility in Arc Magmas at Upper Mantle Conditions

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Intellectual Merit. It is generally accepted that partial melting of peridotite is initiated at the base of the mantle wedge by an influx of volatiles (primarily H2O) from the subducted oceanic lithosphere. However, it is unclear whether these volatiles are transported into the mantle wedge by an aqueous fluid, a hydrous silicate melt, a supercritical phase, or some combination thereof. Further, the mechanism by which partial melting proceeds in the mantle wedge remains a matter of debate. Potential modes for melt generation at convergent margins include the development of small, low-density diapirs that rise through the wedge and partially melt due to a combination of decompression and heating or reaction between relatively cool, H2O-rich magma and hotter peridotite as the melt percolates upward through the mantle wedge. The concentration of H2O dissolved in partial melts, and its influence on liquidus temperatures, densities, viscosities and transport of melts in the mantle are critical to understanding arc magmatism. While there is an extensive body of literature relating to the solubility of H2O in silicate melts at crustal P-T conditions, such experimental data pertaining to upper mantle conditions do not exist. This reflects the difficulty of doing the relevant experiments in a solid-medium piston cylinder device. This proposal describes a new experimental approach in which the concentration of hydroxyl in a solid mineral container will be measured by ion microprobe and used as a proxy for the concentration of water in the coexisting melt. This approach obviates the need to analyze the silicate melt directly, which historically has been precluded due to rapid formation of quench crystals and volatile loss during cooling of experimental run products. Results from these experiments will provide data necessary for quantifying the flux of material from the subducted oceanic lithosphere to the mantle wedge as well as the processes involved in melt generation and transport within the mantle wedge - both of which are goals of the MARGINS Subduction Factory initiative. Further, these data will help to provide a framework for interpreting geochemical and seismic results from MARGINS Focus Sites in the Izu-Bonin-Mariana and Central American subduction systems.Broader Impacts. The proposed research will (1) advance discovery and understanding while promoting teaching, training and learning, (2) broaden participation of underrepresented groups, and (3) enhance infrastructure for research and education through the involvement of graduate students in the MIT/WHOI Joint Program, undergraduate students participating in the WHOI Summer and Minority Student Fellow programs, and a female Postdoctoral Fellow at DTM. The post-doc will spend time at WHOI learning experimental techniques and will help WHOI implement SIMS analytical techniques that have been developed at DTM.