The Control of End-Member Iron-to-Sulfide Ratios on Hydrothermal Plume Geochemistry at the Rainbow, Broken Spur, and Lost City Vent Fields in the Mid-Atlantic Ridge

The Mid-Atlantic Ridge (MAR) represents a unique tectonic environment hosting distinct hydrothermal vent fields. Considering the growing interest in understanding the role of hydrothermal vents in both local and global ocean systems, the MAR offers an opportunity to investigate hydrothermal geochemistry within a single basin. Here, we present findings from three representative vent settings along the MAR: Broken Spur, Rainbow, and Lost City, as explored during a research expedition aboard the R/V L’Atalante in 2018. We conducted high temperature hydrothermal fluid sampling with ROV Victor 6000 to assess the concentrations of major ions, ∑H₂S, Fe, and Mn in each vent field. Notably, our results highlight that Rainbow and Lost City vent fluids exhibit minimal spatial and temporal variations over a 20-year timeframe while Broken Spur fluids displayed significant variation since their last sampling 25 years ago, specifically losing ∼60% of their dissolved Fe concentration. We further analyzed hydrothermal plume composition evolution using geochemical models and suggested that the Fe:∑H₂S ratio plays a key role in controlling the minerals precipitated during hydrothermal fluid-seawater mixing. In the Broken Spur hydrothermal plume, a low Fe:∑H₂S ratio favors the early dominance of metal sulfides, whereas in the Rainbow hydrothermal plume, a high Fe:∑H₂S ratio might promote the formation of Fe-oxide and Fe-silicate phases alongside metal sulfides. These findings provide a framework for tracing hydrothermal fluid evolution along the subseafloor-plume interface. They also highlight the potential impact of hydrothermal vents on the near-field transport of micronutrient metals in the deep ocean.