Os isotopes as tracers in seafloor hydrothermal systems: metalliferous deposits from the TAG hydrothermal area, 26°N Mid-Atlantic Ridge

Os isotopic analyses of a suite of samples from the TAG hydrothermal area, 26°N on the Mid-Atlantic Ridge, reveal evidence of release of ocean crust-derived Os to seawater and removal of seawater-derived Os to the seafloor. 187Os186Os ratios of sulfide samples from the active TAG mound vary between 8.7 and 1.3. The highest ratios are indistinguishable from the inferred 187Os186Os ratio of seawater. The incorporation of seawater-derived Os in these samples is interpreted as evidence of both entrainment of seawater into hydrothermal fluids and the addition of seawater Os to samples during sulfide oxidation. Several sulfide samples from the active TAG mound have 187Os186Os ratios lower than the inferred seawater value, with the lowest ratio approaching values expected for pristine oceanic crust. These low 187Os186Os ratios indicate that Os derived from alteration of the oceanic crust is released to seawater in association with venting of high temperature hydrothermal fluids. Precipitates from low-temperature hydrothermal fluids, Fe- and Mn-oxide deposits from the east wall of the axial rift valley, also have low 187Os186Os ratios (4.2–6.8) relative to seawater. These data suggest Os isotopes may be sensitive tracers of low-temperature hydrothermal circulation on the seafloor, and that supply of ocean crust-derived Os to seawater is not strictly limited to high temperature hydrothermal vents. concentration and isotopic composition of metalliferous sediment samples from a push core collected 125 m from the base of the active TAG mound are also reported. In this core metalliferous sediment horizons derived from mass wasting of altered sulfides exhibit variable 187Os186Os ratios (4.8–8.7), reflecting mixing of seawater-derived and ocean crust-derived Os. In contrast, sediments in which the metalliferous component is derived from fall-out of hydrothermal material from the neutrally buoyant hydrothermal plume have 187Os186Os ratios which vary over a narrow range (8.6–8.7), and appear to accurately record the Os isotopic composition of seawater.