Geochemistry of Deep-Sea Hydrothermal Vent Fluids from Broken Spur, Rainbow and Lost City Hydrothermal Vent Fields, Mid-Atlantic Ridge

Mid- Atlantic Ridge (MAR) is a very special tectonic environment which contains compositioanlly distinct hydrothermal vent fields as a result of its slow spreading character. Considering the increasing efforts on understanding the role of hydrothermal vents in both local and global ocean system, MAR enables to develop a wholistic approach to study formation, evolution and near field transportation of hydrothermal vent products in different vent fields at the same time. Here we present our results obtained from one of each representative vent settings that exists on the MAR (Type 1: Broken Spur, Type 2: Rainbow and Type 3: Lost City) during a research expedition with N/O L’Atalante and ROV Victor 6000 in 2018. ROV-assisted hydrothermal fluid sampling was conducted and samples were taken from the orifice as well as different heights of the hydrothermal plumes of corresponding vents. Upon the sample collection, both on-board and on-shore measurements for major ions, hydrogen sulfide and transition metals were performed by using ion chromatography, colorimetric methods, and ICP-MS. Our results, in support of previous reports for endmember fluid compositions, show that interplay of temperature, pH, Cl and H2S concentrations control the formation and transportation of dissolved species in Rainbow, Broken Spur and Lost City hydrothermal fluids. On the other hand, Rainbow and Lost City fields show high temporal stability over ~20 years’ time span however substantial changes were observed in Broken Spur fluids in ~25 years, which contrasts with previous findings describing high temporal stability of MAR hydrothermal vents. Considering intense cooling (~100 °C) upon the fluid rise to seafloor in Broken Spur hydrothermal field, we are proposing Broken Spur experiences extensive Si (up to 50%) and Fe precipitation in the subseafloor as Fe-silicate and Fe-sulfide minerals. Furthermore, plume geochemistry results indicate metal oxide and silicate phases are important fraction of Rainbow (nano)particle pool while low Fe:H2S dynamics in Broken Spur plume result in formation and domination of metal sulfide phases. Considering high removal trends and high correlations observed in metals, silica and cations, we are proposing reverse weathering reactions might be responsible in formation of metal silicate phases especially in Rainbow and possibly in Broken Spur plumes. Finally, our findings in conjunction with previous studies show that hydrothermal vents are factories for biologically important (nano)particles regardless of the setting. Hence, considering the possible global scale effects of such hydrothermal vent products, we are suggesting hydrothermal (nano)particles might have played much more important roles in major geological events in the Earth’s history than previously thought