Petrological and textural investigations of ultramafic clast in serpentine mud volcanoes of the Mariana forearc (IODP Expedition 366): Evolution of water-rock interactions between the décollement and the seafloor with special emphasis on the generation of

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Serpentine mud volcanoes (or serpentine seamounts) are large edifices that are abundant 50-100 km behind the trench axis in the outer Mariana forearc. These features form on top of faults reaching down to the serpentinized mantle wedge. Tectonic displacement comminutes the serpentinite and the fine-grained fault gouge material mixes with the rising slab-derived water to become a liquid-rich mud, which rises buoyantly to the seafloor, producing a mud volcano. Embedded within the fine-grained matrix are variably serpentinized clasts of harzburgite and dunite, ranging to meter-size, along with small clasts of metamorphosed mafic rocks. These clasts represent a rich archive of fluidrock interactions within the subducting slab and overlying mantle wedge. IODP Expedition 366 will drill three mud volcanoes that cover a large spectrum of slab depth (13 to 25 km) and presumed temperatures at the slab interface (from <80 to >200 °C). Only Conical seamount (drilled during ODP Leg 125) represents a greater depth and higher temperatures (30 km, >300 °C). We propose to conduct petrological, geochemical, and textural studies of serpentinite clasts from those four serpentine seamounts to determine the nature, extent and consequences of fluid-rock interactions rocks between the décollement and the seafloor. Our study will address the role of (1) pressure and temperature on serpentinization within the mantle wedge and (2) fracturing and continued water-rock interaction within the conduit of the mud volcano. A specific emphasis will be on identifying fluid characteristics, in particular in terms of trace element signatures and abundance of dissolved dihydrogen (H2, aq). The latter is important as a link between petrology and biology, as the microbial communities inhabiting the fluid seepage areas of the mud volcanoes thrive on dihydrogen as the principal metabolic energy source. In our work plan, we adopt approaches and methods developed in recent projects aimed at understanding serpentinization reactions and related hydrogen generation. Specifically, we propose whole-rock geochemical and thin-section microanalytical work (EMPA, LA-ICP-MS) along with phase petrological observations and geochemical reaction path modelling to deconvolute the sequences of water-rock reactions and their consequences for fluid compositional evolution and changes in mineralogical and physical (e.g., density, magnetic susceptibility) properties of serpentinite clasts and muds. To this range of methods, we add textural measurements using X-ray µ-CT to depict the 3D distribution of different alteration domains and establish relations to fracturing and veining events that repeated affect the clasts in the root zone of the serpentine mud volcanoes. The results will help understanding fluid-rock interactions in the mantle wedge and the rising column of serpentine mud and assessing the extent of hydrogen production, which seeps from the mud volcanoes and support life at and below the seabed.