Complementary insights from orogenic and xenolithic eclogites as recorders of mass transfer in subduction zones: A unique case study from the Farallon plate

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Oceanic subduction zones arguably constitute the single most important interface on Earth where significant crust-mantle interaction, with transfer of volatiles and other elements, occurs - processes that are fundamental to planetary evolution. Some mass transfer is mediated by fluids and melts as a function of composition, pressure, temperature and oxygen fugacity. The effects of these parameters on element mobility in this complex environment remain fuzzy, also owing to the incomplete geological record available for any single subduction zone. Due to a unique geological happenstance, the oceanic crust of the Cretaceous Farallon plate has been sampled over an exceptionally large pressure interval (~2.0-4.7 GPa), by tectonically exhumed eclogites in the Franciscan Complex (western United States), and by eclogite xenoliths occurring in the Navajo Volcanic Field, 700 km to the east. Using a representative sample suite, this study aims to fingerprint the sources of the fluids that interacted with the eclogites during deep mass transfer processes in the Farallon plate with bulk-rock Li-B-Mg-Fe isotopes, and detect short-lived fluid flow events with in situ Li-Mg-Fe isotopes. A pressure-temperature-time-oxygen fugacity path will be delineated by combining geothermo-oxybarometry with a novel garnet U-Pb dating tool. Coupled bulk carbon and sulphur concentrations, speciations and isotopic compositions, enhanced by sulphide δ34S, garnet Fe3+/ΣFe and garnet δ56Fe, will be used to assess the interplay between the redox-sensitive elements Fe, C and S, and identify the reactions by which they are mobilised or retained in the slab. Combined, these data will allow to trace the physicochemical controls on mass transfer processes and volatile cycling from forearc to subarc depth, in a single slab, with unprecedented detail.