Triple iron isotopes inform iron mineralization and sedimentation at the TAG hydrothermal mound

Abstract

Hydrothermal seafloor massive sulfide deposition at ocean spreading centers modifies the flux of iron from vents while producing an archive of these processes in both active and fossil hydrothermal complexes. Despite hopes of stable iron isotopes tracing mineral formation and iron cycling at these sites, the competing fractionations accompanying various mineralization processes have presented an obstacle to confidently interpreting iron isotopic datasets. We have developed a triple iron isotope proxy that can resolve some of these previously indistinguishable mineral formation histories, and applied it to a suite of samples from the Trans-Atlantic Geotraverse (TAG) active hydrothermal mound on the Mid-Atlantic Ridge. We show that massive pyrite-dominated sulfides formed near the TAG mound surface retain primary kinetic isotope signatures indicative of their rapid formation, likely from an iron monosulfide intermediate. Pyrite-anhydrite breccias retain mixed isotopic signatures of reworked primary massive sulfides together with secondary pyrite, grown in confined conditions, in some cases in equilibrium with subsurface hydrothermal fluid. We also suggest that iron oxyhydroxide-rich chert precipitation across parts of the mound surface trapped isotopically evolved fluid within the mound, and a later generation of sulfides precipitated from this evolved fluid. Metalliferous sediments from a core recovered near the base of the TAG mound mostly fall along a primary mass fractionation law defined by mound sulfides. This is consistent with a dominant contribution of collapsed mound debris and/or black smoker sulfide fallout to the seafloor near the base of the TAG mound. However, the triple iron isotopic composition of surface sediment from this core suggests it may also contain iron oxyhydroxide fallout from the dispersing non-buoyant plume that had already undergone extensive sulfide precipitation. Triple iron isotope studies of non-buoyant plume sediments may be used to quantify the mineral fate of iron vented to the oceans, estimate variations in iron to sulfide ratios of primary vent fluids, and resolve the relative importance of low and high temperature hydrothermal flow to basin-scale iron-rich plumes in the global oceans.