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The Rum Eastern Layered Intrusion (ELI) is the product of part of an ~60 Ma open-system magma chamber. The 16 coupled peridotite-troctolite ± gabbro macro-rhythmic units it contains represent crystallization of multiple batches of basaltic and picritic magma. Within the ELI, Unit 10 has been considered the type example of batch fractionation of magma on Rum for more than 50 years, successively producing peridotite, troctolite and olivine gabbro. Detailed field observations and logs of the Unit 10 peridotite cumulate are presented here, together with mineralogical and textural analyses of Cr-spinel seams and their peridotite host-rocks. Numerous harrisite layers are commonly associated with diffuse, laterally discontinuous, platinum-group element (PGE) enriched Cr-spinel seams. Multiple millimetre- to centimetre-thick Cr-spinel seams occur at the bases and tops and within harrisite layers. These relationships are inconsistent with simple batch fractionation of magma. Critically, the harrisite layers also exhibit centimetre- to metre-scale, upward oriented apophyses that point to injection of magma into the overlying cumulate, indicating an intrusive origin for the harrisite. Quantitative textural and chemical analysis suggests that the Cr-spinel seams formed via in situ crystallization within the crystal mush together with intrusive peridotites from an assimilation reaction between the replenishing magma and peridotitic crystal mush. Intrusive magma replenishment in Unit 10 caused significant compositional disequilibrium between the crystallizing phases in response to the postcumulus migration of reactive liquid, resulting in chemical zoning of intercumulus plagioclase crystals. We propose that the Unit 10 peridotite is intrusive and that repeated small-volume magma replenishments are responsible for incremental construction of a large proportion of the peridotite body, similar to recent interpretations of parts of Unit 12 and Unit 14. Moreover, it is suggested that some or all of the injections of magma occurred into the crystal mush, rather than at the magma chamber floor. This new model of intra-mush Cr-spinel formation may have significant economic implications for PGE enrichment in other layered intrusions, such as the peridotite-hosted chromitites of the Stillwater Complex Ultramafic Series (Montana, USA). It is also worth noting that thin platiniferous chromitite seams considered to have formed in situ occur below the Merensky Reef of the Bushveld Complex (South Africa). [ABSTRACT FROM AUTHOR]

Laterally extensive (∼2 mm thick) Cr-spinel seams in the Rum Layered Suite, NW Scotland, occur at the bases of several of the coupled peridotite–troctolite macro-rhythmic units that form the bulk of the eastern part of the intrusion. Detailed petrography, mineral chemical analyses and quantitative textural measurements of the rocks above and below two of these seams suggest that existing models for seam petrogenesis involving early crystallization and gravitational settling of Cr-spinel from a newly emplaced body of magma need to be reassessed. We argue for assimilation of troctolitic cumulate by a new influx of picrite magma at the crystal mush–magma interface, and subsequent in situ crystallization of the Cr-spinel seams. The bases of seams are characterized by Mg- and Al-rich Cr-spinel, with slightly more Fe-rich crystals toward the tops. These seams crystallized from a superheated hybrid magma generated by the initial assimilation and dissolution of the plagioclase-rich cumulate floor by the picrite. Coeval syn-magmatic deformation of the crystal mush at the unit boundary between peridotite and troctolite caused localized expulsion of Cr-spinel seed crystals several centimetres upward into the peridotite mush, resulting in the formation of overlying supra-seams as well as possibly developing a distinctive texture comprising chains of Cr-spinel around cumulus olivine crystals immediately above unit boundaries. The mineral compositional and textural evidence collected from the Rum Cr-spinel seams also indicates that they exhibit varying degrees of postcumulus chemical and textural equilibration. [ABSTRACT FROM PUBLISHER]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Thin (~2 mm) Cr-spinel seams are present at the bases of several of the coupled peridotite–troctolite units that comprise the Rum Eastern Layered Intrusion. In some cases, ‘subsidiary’ Cr-spinel seams have also developed at between 6 and 12 cm below the unit boundaries. The subsidiary seams are thinner than the normal seams (approx. 1 mm), discontinuous, and occur within and at the base of a thin (~10 cm) layer of anorthosite, which is sandwiched between peridotite and troctolite. The troctolite contains cumulus olivine, but in the Cr-spinel seam and anorthosite, olivine is intercumulus only. Cr-spinel is scarce in the troctolite, but common in the anorthosite, where it has a different composition (more Fe-and Cr-rich) and crystal size distribution (CSD) profile to the Cr-spinel in the subsidiary seam, suggesting that it represents a different crystal population. A model involving downward infiltration of hot picrite and resulting troctolitic cumulate assimilation is developed here to explain the subsidiary seams. This is based on petrographic observation, quantitative textural measurement and mineral chemical analyses. It is suggested that as the picritic magma was emplaced, downward percolation of this melt occurred into a troctolite mush. The anorthosite represents a layer of almost completely melted troctolite, formed by growth of high-anorthite zones from the contaminated picrite onto residual cumulus plagioclase. Assimilation of large amounts of the troctolite cumulate mush forced the contaminated picrite onto the olivine-spinel cotectic, leading to Cr-spinel crystallisation. Subsequently, the Cr-spinel crystals in the anorthosite have reacted with intercumulus melt over a wider temperature interval and have gained a more Fe3+-rich composition than the subsidiary seam Mg- and Al-rich Cr-spinels. It is suggested that the separation of a small fraction of immiscible sulphide liquid in the Cr-spinel seams is the result of locally changing SiO2 and oxygen fugacity at the crystal–melt interface, during the peritectic reaction of some of the Cr-spinel to produce olivine and plagioclase. [Copyright &y& Elsevier]

Abstract: The timing, causes and extent of mantle heterogeneity preserved in the ∼492Ma 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 187Os/188Os, as well as absolute and relative abundances of the HSE, compared to spatially associated harzburgites. As is common for ophiolitic peridotites, the harzburgites (γOs492Ma 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 γOs492Ma 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 <500m have a large range in HSE concentrations (e.g., 0.09 to ∼2.9μgg−1 Os) with initial γOs492Ma 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 ∼492Ma, 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. [Copyright &y& Elsevier]