Re-Os systematics and platinum-group element distribution in metalliferous sediments from the Troodos ophiolite

Umbers from 8 of 11 localities studied yield Os-187/Os-188 ratios that range from 0.51 to 0.57. We interpret this range as reflecting the Os isotopic composition of mid-Cretaceous seawater. Os-187/Os-188 measured in samples from the remaining three localities fall above and below this range and are believed to result from post-depositional redistribution of Os. Extremely large Os-187/Os-188 ratios occur in hydrothermal sulfides (> 200) and in their oxidation products, ochres (3.6-13.2). Measured Re/Os ratios in these samples require open system behavior. Model age calculations indicate that ochre formation occurred over a 1-5 million year interval after sulfide precipitation. In most samples > 99% Re loss occurred during sulfide oxidation. Comparison of element ratios in Cyprus umbers to those of modern seawater suggest the following sequence of scavenging efficiency by hydrothermal Fe-oxides: Ir > Pt > Os > Re. Cyprus sulfides exhibit the opposite pattern of element enrichment. These patterns of aqueous platinum-group element (PGE) fractionation are potentially relevant to study of recycled Os in mantle-derived rocks because they give rise to the following phenomena: (1) ephemeral Re enrichment within the basal sediments of the oceanic crust, (2) PGE concentrations in metalliferous sediments (Os up to 1 ppb and Pt up to 22 ppb) that are comparable to those in the mantle, and (3) Pt/Re ratios in Cyprus umbers that exceed those required to explain the coupled Os-186-Os-187 variations in Hawaiian basalts. (C) 2001 Published by Elsevier Science B.V.