Evidence from mantle xenoliths for an enriched lithospheric keel under the Outer Hebrides

Abstract

Several independent lines of evidence suggest the presence of old regions in the Earth's upper mantle, particularly beneath stable cratons, which are enriched in incompatible elements such as barium and the light rare earth elements (LREE). Such 'lithospheric keels' are well documented beneath the Archaean of South Africa^1–4 and North America^5,6. Recent studies⁷ have shown the Lewisian of Scotland to be underlain by heterogeneous mantle, fragments of which are entrained by magmas that later intrude the crustal rocks of north-west Scotland. Here we present chemical and isotopic data from megacryst suites in these magmas, which point to niineralbgical complexity in the lower crust and upper mantle. Moreover, variable trace-element ratios in the xenoliths are now recorded as extreme isotopic variations. The Sr and Fb isotopic compositions are similar to those of lamprbites⁶ and micaceous kimberlites⁸ respectively. In contrast, the Nd isotopic ratios are highly variable (ε_Nd = −3.4 to −32.6) and overlap with those observed in garnet inclusions⁹ in diamonds (South Africa) and Iamproitic⁶–potassic⁵ volcanic rocks (USA). We suggest that the Hebridean lithospheric mantle has evolved in a manner totally unlike that of the mantle source regions tapped by Tertiary volcanic rocks erupted in the North Atlantic volcanic province.