Length-scales of chemical and isotopic heterogeneity in the mantle section of the Shetland Ophiolite Complex, Scotland

TitleLength-scales of chemical and isotopic heterogeneity in the mantle section of the Shetland Ophiolite Complex, Scotland
Publication TypeJournal Article
Year of Publication2018
AuthorsO'Driscoll B., Walker R.J, Clay P.L, Day JMD, Ash R.D, Daly J.S
JournalEarth and Planetary Science Letters
Volume488
Pagination144-154
Date Published2018/04
Type of ArticleArticle
ISBN Number0012-821X
Accession NumberWOS:000430032900014
Keywordsabyssal peridotites; earths upper-mantle; elements; Geochemistry & Geophysics; Highly siderophile elements; mantle heterogeneity; melt percolation; oceanic mantle; oceanic upper-mantle; Os isotopes; platinum-group minerals; podiform chromitites; Re-Os isotopes; scottish caledonides; Shetland Ophiolite Complex; siderophile; situ; Troodos ophiolite
Abstract

Kilometre to sub-metre scale heterogeneities have been inferred in the oceanic mantle based on sampling of both ophiolites and abyssal peridotites. The similar to 492 Ma Shetland Ophiolite Complex (SOC) contains a well-preserved mantle section that is dominated by harzburgite (similar to 70 vol.%) previously reported to have variable major and trace element compositions, yet dominantly chondritic initial(187)Os/Os-188 compositions. To assess the preservation of compositional heterogeneities at sub-metre length-scales in the oceanic mantle, a similar to 45 m(2) area of the SOC mantle section was mapped and sampled in detail. Harzburgites, dunites and a pyroxenite from this area were analysed for lithophile and highly-siderophile element (HSE) abundances, as well as for Os-187/Os-188 ratios. Lithophile element data for most rocks are characteristic of supra-subduction zone (SSZ) metasomatic processes. Two dunites have moderately fractionated HSE patterns and suprachondritic gamma Os-(492 Ma) values (+5.1 and +7.5) that are also typical of ophiolitic dunites generated by SSZ melt-rock interactions. By contrast, six harzburgites and four dunites have approximately chondritic-relative abundances of Os, Ir and Ru, and gamma Os-(492 Ma) values ranging only from -0.6 to +2.7; characteristics that imply no significant influence during SSZ processes. Two harzburgites are also characterised by significantly less radiogenic gamma Os (492 ma) values (-3.5 and -4), and yield Mesoproterozoic time of Re depletion (T-RD) model ages. The range of Os isotope compositions in the studied area is comparable to the range reported for a suite of samples representative of the entire SOC mantle section, and approaches the total isotopic variation of the oceanic mantle, as observed in abyssal peridotites. Mechanisms by which this heterogeneity can be formed and preserved involve inefficient and temporally distinct melt extraction events and strong localised channelling of these melts. (C) 2018 Elsevier B.V. All rights reserved.

DOI10.1016/j.epsl.2018.02.020
Short TitleEarth Planet. Sci. Lett.
Student Publication: 
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