Highly Siderophile Elements as Tracers of Mantle-crust Interactions in Subduction Zone Metamorphic Rocks

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Intellectual merit. Arc magmas are generated by complex processes involving release of aqueous fluids/melts from the subducting slab, which rise and interact with overlying mantle. These processes are still not well understood. Metamorphic rocks from subduction related mélange zones provide a critical window into these processes. Mélange zones, found in many subduction zone metamorphic complexes around the world, are thought to represent the interface between the subducting slab and the overlying mantle wedge. Their position above the dehydrating, subducting slab is the ideal location to record a variety of processes of aqueous infiltration, melting, and tectonic mixing of crust and mantle. Moreover, these processes act to hybridize crustal and mantle material creating a volatile-rich, mixed rock type with chemical and isotopic compositions reflecting the multiple sources of material in the mélange. Dehydration and melting of this hybridized rock type could potentially result in the production of fluids/melts that bear the signatures of multiple source materials. The tracers currently applied to subduction systems mainly provide information about the input of sedimentary and mafic components of the subducting slab to arc magma. Much less information is available regarding the contributions of ultramafic mantle peridotite to subduction zone processes. It is proposed here to determine the mechanisms of mass transfer at contacts between ultramafic and crustal lithologies and determine the relative proportion of ultramafic material within the hybridized rock types found in mélange zones. Highly siderophile element (HSE) concentrations and Os isotopic compositions will be used as the primary tracers of the ultramafic component in the fluids or rocks. Previous research has suggested that O isotopes and large ion lithophile elements (LILE) are modified in the Catalina Schist by infiltration of a fluid derived from sedimentary rocks. Therefore O isotopic compositions and the LILE will be used as tracers of the sedimentary component involved in material transport. We will additionally use the high field strength elements to determine whether mechanical mixing is a likely mechanism for mass transfer. Preliminary results from blocks and reaction rinds in the Franciscan Complex, Samana Metamorphic Complex and the Catalina Schist show that HSE are elevated in reaction rinds compared to mafic blocks, suggesting some relative mobility of HSE. The mélanges of the Catalina Schist and on Syros have well-exposed mafic rocks and reaction zones in ultramafic matrix, and will provide the ideal natural localities to investigate processes of mass transfer of HSE, LILE, and O.Broader impacts of the overall project include the training and education of students in the areas of petrology and geochemistry, as well as in scientific writing and communication of research results. Graduate and undergraduate students are critical to the implementation of the proposed research. Both PIs have involved women and underrepresented minorities in their research groups and expect to continue to do so in the future, particularly through the NSF-funded Louis Stokes Alliance for Minority Participation program at UMd. The Department of Geology at the University of Maryland requires all its undergraduate students to complete a senior thesis, and one or two seniors will likely be involved in this research. In addition, this grant will enable the lead PI (who started her career as a faculty member at the University of Maryland in 2007) to assist her in building a strong research group.