Metasomatism and volatile mobilization in the oceanic mantle

During Earths evolution, volatile elements (C, O, H, S, N, and halogens) move between the planets surface and interior, facilitating melting and mass transport: they are released from active volcano- tectonic areas, and transported deep into the mantle during subduction processes at convergent plate boundaries. In the mantle, volatiles are heterogeneously distributed at all scales, from kilometers down to the micro-scale, reflecting processes such as extraction of melt, recycling of subducted material and interaction with circulating melts and fluids. Our abilities to unequivocally link the signature and abundance of volatile species measured in surface gaseous emissions to specific geodynamic processes is still rather limited, not least due to our incomplete understanding of how volatiles are introduced into, stored within and ultimately mobilized out of the mantle. The lithospheric mantle beneath oceanic islands is an effective trap for volatiles, as its residual nature and composition facilitate the infiltration of fluids and melts, which induce its chemical and modal modification the so-called mantle metasomatism. This takes place with the entrapment of small fluid and melt inclusions within minerals and in reaction zones at crystal boundaries, and with the incorporation of volatiles in the lattice of crystal phases. In this project, a unique collection of more than 400 fragments of lithospheric mantle rocks (xenoliths) brought to the surface by magmas in all the islands of the Madeira and Canary archipelagos will be studied to investigate how, where, when and why volatiles are stored and/or mobilized within the mantle. To this purpose, state-of-the-art high-resolution petrological and geochemical analyses on mineral phase constituents of mantle xenoliths will be combined to the determination of the OH concentration in crystal lattices, and of the concentration and isotopic signature of CO2 in mineral- hosted fluid inclusions. Combined with thermodynamic modeling and textural studies, such approach will bring to a comprehensive understanding of the volatile cycling in the lithospheric mantle beneath Madeira (largely unexplored) and Canary Islands, and its relationships with long-term geodynamics. Through a comparison with surface gaseous emissions in active volcano-tectonic areas, this project will enhance our comprehension of the outgassing of deep Earth volatiles and its relationship to the ocean-atmosphere-biosphere system, but also improve our knowledge of the short-term climate variations and seismic/volcanic unrest.