Deposition of osmium and other platinum-group elements beneath the ultramafic-hosted rainbow hydrothermal plume

Cave, R.R., Ravizza, G.E., German, C.R., Thomson, J. and Nesbitt, R.W. (2003) Deposition of osmium and other platinum-group elements beneath the ultramafic-hosted rainbow hydrothermal plume. Earth and Planetary Science Letters, 210 (1-2), 65-79. (doi:10.1016/S0012-821X(03)00135-3).

Abstract

Osmium and other platinum-group elements (PGEs) have been measured in sediments accumulating directly under the Rainbow hydrothermal plume at 36[deg]N on the Mid-Atlantic Ridge. 187Os/188Os and PGE concentrations in the sediments of four cores taken 2-25 km from the active vent site reveal evidence of both radiogenic Os scavenged from seawater and, importantly, unradiogenic Os released by hydrothermal venting. The majority of analyses (n=16) yield 187Os/188Os ratios that fall in a narrow range (0.99-1.06) close to that of modern seawater. In these samples, Os concentrations correlate linearly with hydrothermal V enrichments, which are also derived from seawater scavenging. A smaller subset of samples, characterized by unusually high concentrations of Ni and Cr derived from ultramafic rock fragments, exhibit distinctly lower 187Os/188Os ratios. Their Ir content is also higher, and their PGE ratios are more similar to mantle material than the PGE patterns of the larger set of samples dominated by seawater-scavenged Os. Although the two most proximal cores lack evidence for any similar ultramafic 'detritus' in their surficial sediments, 187Os/188Os ratios in these samples are also lower than that of ambient seawater, suggesting they are likely to have been influenced by unradiogenic Os from vent fluid incorporated into the plume. Samples from the most distal core, by contrast, exhibit 187Os/188Os ratios indistinguishable from that of seawater. Compared with 'background' NE Atlantic sediments, Os is enriched four-fold in the seawater-dominated samples, while Pd, Pt and Ir are twice the background value. Burial fluxes of Os scavenged by hydrothermal Fe in these sediments are 200-400 pg cm-2 kyr-1, up to twice that found on the EPR, and several times the background (bg) of 50 pg cm-2 kyr-1. For Pt, burial fluxes are 1800-3000 pg cm-2 kyr-1 (bg ca. 1000 pg cm-2 kyr-1) and for Pd 2700-4600 pg.cm-2 kyr-1 (bg ca. 800 pg cm-2 kyr-1)

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