Rhenium-osmium isotopes and platinum-group elements in the Rum Layered Suite, Scotland: Implications for Cr-spinel seam formation and the composition of the Iceland mantle anomaly

The Rum Layered Suite is a layered mafic-ultramafic body that was emplaced during Palaeogene North Atlantic margin rifting. It is a classic open-system magma chamber, constructed of 16 repeated coupled peridotite-troctolite units, some of which have laterally extensive ~ 2 mm-thick platinum-group element (PGE) enriched (~ 2 μg g^{- 1}) Cr-spinel seams at their bases. In order to investigate Cr-spinel seam petrogenesis and enrichment of the PGE, abundances of these elements and Re-Os isotopes have been determined at three stratigraphic levels of the Rum Layered Suite that represent major magma replenishment events. Individual units preserve a range of initial ¹⁸⁷Os/¹⁸⁸Os ratios, demonstrating heterogeneity in the composition of replenishing magmas. Data for both the Cr-spinel seams and overlying silicates reveal that the processes that formed the Cr-spinel also concentrated the PGE, following magma replenishment. There is no evidence for structurally-bound PGE in Cr-spinel. Instead, the PGE budget of the Rum Layered Suite is linked to base metal sulphides, especially pentlandite, and to PGE alloys contained within the Cr-spinel seams, but which exist as separate phases at Cr-spinel grain boundaries. The range in initial Os isotope compositions (γ_Os = 3.4 to 36) in the Rum Layered Suite can be successfully modelled by 5-8% assimilation of Lewisian gneiss coupled with changing PGE contents in the replenishing magmas associated with sulphide removal. Initial ¹⁸⁷Os/¹⁸⁸Os ratios for Rum rocks range from 0.1305 to 0.1349 and are atypical of the convecting upper mantle, but are within the range for recently erupted picrites and basalts from Iceland and Palaeogene picrites and basalts from Baffin Island, Greenland and Scotland. Thus, the Os isotope data suggest that the North Atlantic Igneous Province magmas were collectively produced from a mantle source with components that remained relatively unchanged in Os isotopic composition over the past 60 Ma, and that likely contain a recycled lithospheric component.