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

Ravizza, G., Blusztajn, J. and Prichard, Hazel Margaret 2001. Re-Os systematics and platinum-group element distribution in metalliferous sediments from the Troodos ophiolite. Earth and Planetary Science Letters 188 (3-4) , pp. 369-381. 10.1016/S0012-821X(01)00337-5

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Abstract

Umbers from 8 of 11 localities studied yield 187Os/188Os ratios that range from 0.51 to 0.57. We interpret this range as reflecting the Os isotopic composition of mid-Cretaceous seawater. 187Os/188Os 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 187Os/188Os 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 186Os–187Os variations in Hawaiian basalts.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: Re/Os; platinum group; Troodos Ophiolite; ocher; ferromanganese composition
Publisher: Elsevier
ISSN: 0012-821X
Last Modified: 10 Oct 2017 13:15
URI: http://orca.cf.ac.uk/id/eprint/10445

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