Intellectual merit: For decades, subduction zones have been studied intensely to understand the physical and chemical mechanisms responsible for the surface volcanism expressed as arcs around the globe. However, there are still large uncertainties about how subducted slab material is discharged into the mantle wedge. For example, despite almost ubiquitous sediment subduction [Plank and Langmuir, 1998], many arc segments appear to have attenuated to negligible sediment signatures [Tera et al., 1986; Morris et al., 1990; Leeman et al., 2005]. Other are inferred to have carried strong chemical signatures from altered ocean crust, even though sediment subduction is clearly taking place [Regelous et al., 1997; Singer et al., 2007]. Are such discrepancies related to the thermal structure of subducting slabs, whereby hotter slabs lose sediment at shallower depths? Or related to sediment accretion in the forearc? Or do we simply not currently have the appropriate means of tracing the different slab fluxes in all arcs? It is proposed to evaluate the utility of novel thallium (Tl) stable isotope systematics in quantifying subduction contributions of pelagic sediments and altered oceanic crust from the Tonga-Kermadec, Aleutian and Central American arcs. The Tl isotope system is unique because the mantle is isotopically homogenous and depleted in Tl, whereas pelagic sediments and altered oceanic crust are enriched in Tl by several orders of magnitude and exhibit highly fractionated Tl isotope compositions. Thallium isotopes appear to be ideally suited to determine slab fluxes to arc lavas. In addition, the three targeted arcs represent different thermal regimes due to the large age range of the subducting oceanic crust. Tl isotopes will be used in conjunction with more conventional trace element and isotope ratios to quantify the role of sediments versus altered oceanic crust and test whether there is a relationship between the age of the subducting oceanic crust and the components incorporated into the arc volcanics.
Broader impacts: The results obtained in this study are expected to be of broad interest to all scientists working on subduction zones. Geodynamicists may, for example, be able to incorporate quantitative fluxes into their models on subduction zone melting processes, which could help resolve if melting is driven by fluid fluxing or anhydrous decompression. Quantification of fluid and/or melt fluxes from the slab in individual arcs may also be used to model the thermal state of the slab/mantle interface, where material is discharged to the mantle wedge. This project will provide funds for a WHOI/MIT Joint Program graduate student and a newly appointed WHOI scientist (PI Nielsen). An undergraduate student will also be involved through the WHOI Summer Student Fellowship Program (funded by the NSF REU program), which provides research funds for undergraduate students from across the country. Even though both the chemical separation and mass spectrometric techniques used to measure Tl isotopes are highly advanced, short projects carried out over a 10?12 week period are possible. The PI has previous experience from four undergraduate students in Oxford, UK, who with close supervision learnt the technique and produced publishable results.