The sub-lithospheric source of North Atlantic basalts: Evidence for, and significance of, a common end-member

Palaeogene basalts from the margins of the North Atlantic often show geoche mical variations that are consistent with their parental magmas having inte racted with the lithosphere en route to the Earth's surface. These geochemi cal trends vary depending on the nature of the local lithospheric contamina nts. Using examples from the British Tertiary Igneous Province and SE Green land, we construct coherent contamination trends, which converge on a restr icted Pb isotope composition, apparently indicating a common uncontaminated asthenospheric mantle component. Significantly, this composition is also s uitable as one end-member of the Pb isotope arrays recorded in Recent Icela ndic basalts. We conclude that this composition has been a persistent compo nent of the Iceland plume over 60 my, dominating the mantle contribution to the Palaeocene phase of flood basalt magmatism but constituting only one e nd-member on Iceland. The Pb isotope composition of this 'North Atlantic en d-member' is consistent with, but not necessarily demanding of, a primordia l source. Recent evidence suggesting a lower-mantle origin for mantle plume s encourages investigation of whether the geochemical evidence supports tha t hypothesis. Helium isotope data from Palaeogene North Atlantic basalts su pport a lower-mantle contribution. However, mixing models suggest that it i s untimely that the lower-mantle contribution is large enough to dominate t he Sr-Nd-Pb isotope compositions and lithophile trace element signatures of any plume-derived basalts.