Constraints on mantle plumes from the helium isotopic composition of basalts from the British Tertiary Igneous Province

Helium isotopes have been measured in olivines from igneous rocks spanning the history of the Palaeocene volcanic centre at the Isle of Skye, northwest of Scotland. 3He/4He in olivine phenocrysts from basalts from the Skye Lava Field are up to 22 times the atmospheric ratio which is consistent with their origin from the proto-Iceland mantle plume despite being erupted 1300 km from the plume axis. The 3He-anomaly of the Palaeocene starting plume in the North Atlantic appears to have been up to three times greater, for the same distance from the plume axis, than the modern Iceland plume. The Pb isotope and trace element (ΔNb) composition of the least crustally contaminated Skye magmas resemble North Atlantic MORB rather than Iceland basalts. The proto-Iceland plume head appears to have been dominated by upper mantle material with a small deep mantle He contribution rather than undiluted deep mantle material as predicted in the classic plume model.