Elemental evidence for an enriched small-fraction-melt input into Tertiary Mull basalts, Western Scotland

Abstract

Continental basalts enriched in the incompatible trace elements have recently been interpreted as mixtures of depleted asthenospheric melt and an enriched small-fraction-melt lithospheric component. Hebridean Tertiary basalts are relatively depleted in incompatible trace elements, and it has been suggested that this is because the enriched lithospheric component had already been extracted beneath that region, during the Permo-Carboniferous. It was therefore not widely available to contaminate Tertiary magmas. A flow-by-flow geochemical study of the Mull lava succession has nevertheless revealed the presence of lava flows relatively enriched in the incompatible trace elements, at the base of the succession, in some parts of the island. Fractional crystallization, contamination by Archean and Moinian crust, variation in the degree of mantle melting and an asthenospheric, ocean island basalt-like source, have all been ruled out as possible mechanisms of enrichment in favour of 5–10% contamination by an enriched, fusible, small-fraction-melt from the lithospheric mantle. The overall lack of basalts enriched in trace elements in the British Tertiary Igneous Province, argues strongly against the widespread presence of an enriched Tertiary lithospheric mantle below the Hebrides. The origin of the a sodic (rather than potassic) contaminant within the Hebridean lithosphere is also discussed.