OSMIUM ISOTOPIC VARIATIONS IN METALLIFEROUS SEDIMENTS FROM THE EAST PACIFIC RISE AND THE BAUER BASIN

The osmium concentration and isotopic composition of metalliferous sediments from the East Pacific Rise (EPR) and Bauer Basin have been determined. Os-187/Os-186 ratios range from 7.9 to 8.6, with EPR samples between 8.4 and 8.6, and Bauer Basin samples between 7.9 and 8.2. The osmium isotopic composition of EPR samples is indistinguishable from the inferred osmium isotopic composition of seawater, requiring that nearly all the osmium in these sediments is hydrogenous in origin. This result is used to place broad constraints on the behavior of osmium in ridge crest hydrothermal systems. While Bauer Basin samples also contain predominantly hydrogenous osmium, an additional source of osmium with low Os-187/Os-186 is required to account for the lower measured ratios. Calculation of total osmium burial fluxes in the Bauer Basin show that the background flux of cosmic osmium is adequate to produce the observed depression in Bauer Basin Os-187/Os-186 ratios. Burial flux calculations also suggest that metalliferous sediment deposition is a significant sink in the marine osmium cycle.