Iron Isotopes Constrain Sub-Seafloor Hydrothermal Processes at the TAG Active Sulfide Mound, 26°N Mid-Atlantic Ridge

Abstract Sub-seafloor hydrothermal processes along volcanically active plate boundaries are integral to the formation of seafloor massive sulfide (SMS) deposits and to oceanic Fe cycling, yet their nature is poorly understood. Here we apply Fe isotope analysis to sulfides from the TAG mound and underlying stockwork, 26°N Mid-Atlantic Ridge, to trace hydrothermal processes inside an actively-forming SMS deposit in a sediment-free mid-ocean ridge setting. We show that data for recently formed chalcopyrite imply fluid–mound interactions cause small negative shifts (< -0.1‰) to the δ56Fe signature of dissolved Fe released from TAG into the North Atlantic Ocean. Texturally resolved pyrite, in turn, preserves a δ56Fe range from − 1.27 to + 0.56‰ that reflects contrasting modes of formation (fluid–seawater mixing vs. fluid conductive cooling) and variable degrees of progressive hydrothermal maturation during the > 20 kyr evolution of the TAG complex. The identified processes may explain Fe isotope variations found in fossil onshore sulfide deposits.