OS ISOTOPES AS TRACERS IN SEA-FLOOR HYDROTHERMAL SYSTEMS - METALLIFEROUS DEPOSITS FROM THE TAG HYDROTHERMAL AREA, 26-DEGREES-N MID-ATLANTICRIDGE

Os isotopic analyses of a suite of samples from the TAG hydrothermal a rea, 26 degrees N on the Mid-Atlantic Ridge. reveal evidence of releas e of ocean crust-derived Os to seawater and removal of seawater-derive d Os to the seafloor. Os-187/Os-186 ratios of sulfide samples from the active TAG mound vary between 8.7 and 1.3. The highest ratios are ind istinguishable from the inferred Os-187/Os-186 ratio of seawater. The incorporation of seawater-derived Os in these samples is interpreted a s evidence of both entrainment of seawater into hydrothermal fluids an d the addition of seawater Os to samples during sulfide oxidation. Sev eral sulfide samples from the active TAG mound have Os-187/Os-186 rati os lower than the inferred seawater value, with the lowest ratio appro aching values expected for pristine oceanic crust. These low Os-187/Os -186 ratios indicate that Os derived from alteration of the oceanic cr ust is released to seawater in association with venting of high temper ature hydrothermal fluids. Precipitates from low-temperature hydrother mal fluids, Fe- and Mn-oxide deposits from the east wall of the axial rift valley, also have low Os-187/Os-186 ratios (4.2-6.8) relative to seawater. These data suggest Os isotopes may be sensitive tracers of l ow-temperature hydrothermal circulation on the seafloor, and that supp ly of ocean crust-derived Os to seawater is not strictly limited to hi gh temperature hydrothermal vents. The Os concentration and isotopic c omposition of metalliferous sediment samples from a push core collecte d 125 m from the base of the active TAG mound are also reported. In th is core metalliferous sediment horizons derived from mass wasting of, altered sulfides exhibit variable Os-187/Os-186 ratios (4.8-8.7), refl ecting mixing of seawater-derived and ocean crust-derived Os. In contr ast, sediments in which the metalliferous component is derived from fa ll-out of hydrothermal material from the neutrally buoyant hydrotherma l plume have Os-187/Os-186 ratios which vary over a narrow range (8.6- 8.7), and appear to accurately record the Os isotopic composition of s eawater.