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  1. International Ocean Discovery Program (IODP) Expedition 366 focused, in part, on the study of geochemical cycling, matrix alteration and transport, and deep biosphere processes in the Mariana subduction zone. This research was accomplished by sampling the summit and flank regions of three active serpentinite mud volcanoes in the Mariana forearc: Yinazao (Blue Moon), Fantangisña (Celestial), and Asùt Tesoro (Big Blue) Seamounts. These mud volcanoes represent a transect with increasing distance from the trench. Because these mud volcanoes discharge fluids and materials from the subduction channel, they provide a means to characterize thermal, geochemical, and pressure conditions within the seismogenic zone. Previous coring on Ocean Drilling Program (ODP) Legs 125 and 195 at two other serpentinite mud volcanoes (Conical and South Chamorro Seamounts, respectively) and piston, gravity, and push cores from several other Mariana serpentinite mud volcanoes add to this transect of deep-sourced material that is discharged at the seafloor. Pore waters were squeezed from cored serpentinite materials to determine the composition of deep-sourced fluid from the subduction channel and to assess the character, extent, and effect of diagenetic reactions and mixing with seawater on the flanks of three serpentinite seamounts (Yinazao, Fantangisña, and Asùt Tesoro). In addition, two water-sampling temperaturemore »probe (WSTP) fluid samples were collected in two of the cased boreholes, each with at least 30 m of screened casing that allowed formation fluids to discharge into the borehole. Here we report shore-based Li, Rb, Cs, Ba, V, Mo, and U measurements of pore waters and one of the WSTP samples. The alkali metals were analyzed to constrain the temperature of reaction in the subduction channel. The other elements were analyzed to assess potential biogenic and diagenetic reactions as the serpentinite material weathers and exchanges with bottom seawater via diffusion. Results were generally consistent with earlier coring and drilling operations, resulting in systematic changes in the composition of the deep-sourced fluid with distance from the trench.« less