Chromitite seams from three locations separated by <500 m have a large range in HSE concentrations (e.g., 0.09 to ?.9 ¦Ìg g? Os) with initial ¦ÃOs492Ma 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 187Os/188Os 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 ?92 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.