Chemical heterogeneity in the upper mantle recorded by peridotites and chromitites from the Shetland Ophiolite Complex, Scotland

The timing, causes and extent of mantle heterogeneity preserved in the similar to 492 Ma Shetland Ophiolite Complex (Scotland) are evaluated using Re-Os isotope and whole rock highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance measurements of a suite of eight chromitites and 21 serpentinised harzburgites and dunites. Shetland dunites have more variable initial Os-187/Os-188, as well as absolute and relative abundances of the HSE, compared to spatially associated harzburgites. As is common for ophiolitic peridotites, the harzburgites (gamma(Os492Ma) of -5.3 to +2.6) preserve evidence for a Mesoproterozoic depletion event, but are dominated by contemporary chondritic, ambient upper mantle compositions. The dunites have gamma(Os492Ma) values ranging between -3.3 and +12.4, reflecting dunite formation by higher degrees of melt interaction with mantle rock than for the spatially associated harzburgites. Chromitite seams from three locations separated by <500 m have a large range in HSE concentrations (e.g., 0.09 to similar to 2.9 mu g g(-1) Os) with initial gamma(Os492Ma) values ranging only from +0.48 to +3.95. Sulphides, arsenides and platinum-group minerals are the primary hosts for the HSE in the chromitites. Their isotopic variations reflect initial isotopic heterogeneity in their primary magmatic signatures. Coupled with field observations that support chromitite formation in concentrated zones of enhanced melt flow, the isotopic dichotomy between the harzburgites and the chromitites suggests that chromitite Os-187/Os-188 compositions may better approximate the upper limit, rather than an average value, of the bulk convecting upper mantle. The Shetland peridotite compositions reflect protracted melt depletion (low-Al2O3) and melt percolation events in a supra-subduction zone (SSZ) setting at similar to 492 Ma, following an earlier (Mesoproterozoic) melt-depletion event. These results provide further evidence that ancient chemical complexities can be preserved in the upper mantle during ocean plate formation. Chromitites and peridotites from the Shetland Ophiolite Complex also attest to lithological and geochemical heterogeneities generated at scales of less than tens of metres during the formation of ancient oceanic lithosphere by high-degree SSZ melt extraction, percolation and during chromitite formation in the oceanic lithosphere. (C) 2012 Elsevier B.V. All rights reserved.