The concentration and isotopic composition of osmium in the oceans

Abstract

Osmium is one of the rarer elements in seawater. Analytical difficulties have previously prevented the direct measurement of the osmium concentration and isotopic composition in seawater. We report a chemical separation procedure that yields quantitative extraction of osmium standard and of osmium tracer by iron hydroxide precipitation from seawater doped with osmium standard, osmium tracer, and FeCI_3. The iron hydroxide precipitate is processed to extract osmium, using techniques developed for iron meteorites. Utilizing this procedure, water samples from the Pacific and Atlantic oceans were analyzed for osmium concentration and isotopic composition. Direct determination of the osmium concentration of seawater gives between 15 and 19 fM kg^(−1). Detailed experiments on different aliquots of one seawater sample from the North Atlantic Ocean, keeping the amounts of reagents constant, yield concentrations from 16 to 19 fM kg^(−1). The variability in concentration is outside the uncertainty introduced because of blanks and indicates a lack of full equilibration between the osmium tracer and seawater osmium. The most reliable osmium concentration of the North Atlantic deep ocean water is 19 fM kg^(−1) with the ^(187)Os^(186)Os ratio being 8.7 ± 0.2 (2σ). Detailed experiments on one seawater sample from the Central Pacific Ocean indicate that the most reliable osmium concentration of the deep ocean water from the Central Pacific is 19 fM kg^(−1) with the ^(187)Os^(186)Os ratio being 8.7 ± 0.3 (2σ). The directly measured osmium isotopic composition of the oceans is in good agreement with that obtained from the analysis of some rapidly accumulating organic rich sediments (Ravizza and Turekian, 1992). A sample of ambient seawater around the Juan de Fuca Ridge gave ^(187)Os^(186)Os= 6.9 ± 0.4. This is distinctly lower than the deep-sea water value and may reflect local hydrothermal activity or some analytical difficulty with this sample. The osmium isotopic composition of the deep oceans indicates that ∼80% of the osmium is derived from the continents and the rest from extraterrestrial and hydrothermal sources. Using the iridium data of Anbar et al. (1996, 1997) we find that the Os/Ir ratio is ∼22 for seawater. The mean residence time of osmium (τ_(Os)) cannot be derived directly from these data. However, using the Os/Ir ratio of seawater and the Os/Ir ratio of continental rocks, a lower limit can be established of τ_(Os) ≥ 4.4 × 10^4 years. The Os/Ir in modern deep sea sediments should be relatively constant and is distinctly less radiogenic than seawater osmium due to limited evaporation (26%) of osmium from infalling cosmic dust. Relative to the cosmic dust infall the hydrothermal sources may, however, play a smaller but significant role in the bulk economy of dissolved osmium and other Platinum Group Elements in the oceans.

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