Katja Schmidt

• Hydrothermal circulation at spreading axes plays a fundamental role for global heat and chemical fluxes between the lithosphere, the hydrosphere, and the atmosphere. Slow-spreading ridges, which comprise about 50% of the cumulative global ridge axis length, likely contribute significantly to the overall exchange fluxes. During this study, the fluid geochemistry of five hydrothermal fields, located in different geological environments at the Mid-Atlantic Ridge (MAR), has been investigated: Nibelungen, Logatchev I, and Turtle Pits, Red Lion and Comfortless Cove. It has been demonstrated that fluid-rock interaction in heterogeneous lithosphere at high temperatures displays a unique geochemical fingerprint with respect to key parameters, such as low concentrations of dissolved H2S, Si, Li relative to basaltic-hosted hydrothermal systems, depletion of B relative to seawater and an enriched Δ2HH2O signature. At Nibelungen and Logatchev I, both the alteration of mafic rocks and pervasive serpentinization is evident. Extremely hot vapor-phase fluids at Turtle Pits and Comfortless Cove are venting at pressure-temperature conditions above the critical point of seawater (407°C, 298 bars). Such fluids have been sampled for the first time. They are characterized by different heat and chemical fluxes compared to systems in similar geological settings at fast-spreading ridges, likely as function of water depth and depth of heat source. The fluids are strongly enriched in metals and display a unique rare earth element and yttrium (REY) distribution with partly absent Eu anomaly. The variability in REE concentrations and distributions is controlled by precipitation/dissolution of hydrothermal anhydrite in the subseafloor. This study substantiates the relevance of hydrothermal activity at slow-spreading ridges for geochemical exchange budgets. Unique features of slow-spreading MAR hydrothermal systems disclose highly variable fluid compositions depending on the geological setting, physical conditions and tectonic activity, and striking temperature and compositional stability also in actively phase-separating hydrothermal systems, which have profound implications for global chemical fluxes.