"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."
Intellectual merit: One of the unsolved paradoxes in the earth sciences is how fluid-assisted mass transfer occurs at convergent plate boundaries. Subduction causes rapid P-T changes in the downgoing slab and juxtaposes rocks of very different geochemical characteristics. Many studies have documented small and heterogeneous fluid fluxes during slab subduction, yet arc magmas show evidence for effective mass transfer between the slab and the overlying mantle wedge. This apparent paradox can be largely resolved in two possible ways: either mass transfer must be accomplished almost entirely at depths >100 km, or localized migration of small fluid volumes is of fundamental importance to subduction zone cycling. We request support for a series of linked field and analytical studies aimed at documenting how small-scale fluid-rock communication is accomplished in different parts of the subduction system, including high- and ultrahigh-pressure rocks and metasomatized mantle material. A secondary outcome will be better characterization of Cl-O-H isotopic reservoirs in different parts of the system. Fluid migration at different scales will be addressed by combining a strongly hydrophilic tracer, chlorine, with oxygen and hydrogen isotopic data and petrologic observations. Because there should be little to no isotopic fractionation of chlorine during metamorphic devolatilization or infiltration, d37Cl values should preserve a record of fluid sources throughout the subduction cycle. By coupling d37Cl values with d18O and dD data from sampling transects along and across lithologic units, specific episodes of fluid-rock interaction and reaction will be tracked. Whole rock and microprobe data, along with fluid inclusion analyses, will also be used to track small-scale mass transfer processes associated with specific fluid release or infiltration events. Specific field areas in the Alps have been selected as representative of deeply subducted oceanic crust (Zermatt-Saas/Piemonte zone), subducted continental flake (Sesia Zone), and depleted vs. metasomatized mantle wedge rocks (Balmuccia and Finero peridotite bodies in the Ivrea Zone). Although these sites were never part of a single subduction system during the Alpine orogeny, they do provide exceptionally well-characterized exposures of relevant rock types within a relatively restricted geographic area. Careful documentation of field relations coupled with detailed petrologic analysis on all samples will allow isotopic data to be interpreted in context. Small-scale processes are likely to dominate over large-scale ones over P-T intervals between major episodes of metamorphic devolatilization (such as serpentine breakdown), and may play a key role in modifying shallow levels of the mantle wedge. Small-scale fluid-rock interaction in subduction settings is potentially also important to studies of earthquake nucleation processes at a variety of depth. This study will complement the growing body of data on large-scale geochemical cycling by placing constraints on specific mechanisms of fluid-rock interaction in different parts of the subduction system.
Broader Impacts: This project will contribute to the training of two (female) graduate students in international fieldwork, stable isotope geochemistry, petrography, microstructural characterization, and whole-rock geochemical techniques. Both graduate students will also gain experience and close mentoring as teaching assistants in petrology at least once during their tenure at UNM. At least one undergraduate student per year will also be involved in the project, resulting in at least two senior theses. The stable isotope laboratory at UNM is widely recognized for its development of new procedures, such as chlorine isotope analysis, and this project will allow new personnel to be trained in these procedures, as well as contributing to further technique development. Samples will be made available to other researchers for future work involving other trace element and isotopic systems (sample characteristics will be posted online). Both PIs routinely incorporate aspects of their current research into undergraduate and graduate classes, and both have strong records of student mentoring. The project will strengthen ties between UNM and colleagues in Italy and at the University of Texas, and will lead to future visits and exchanges between these respective institutions, as well as laboratory cross-calibration.
