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, northwes t of Scotland. He-3/He-4 in olivine phenocrysts from basalts from the Skye Lava Field are up to 22 times the atmospheric ratio which is consistent wit h their origin from the proto-Iceland mantle plume despite being erupted 13 00 km from the plume axis. The He-3-anomaly of the Palaeocene starting plum e 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 (Delta Nb) composition of the least crustally contaminated Skye magmas resemble North Atlantic MORB rather than Iceland b asalts. The proto-Iceland plume head appears to have been dominated by uppe r mantle material with a small deep mantle He contribution rather than undi luted deep mantle material as predicted in the classic plume model. (C) 200 0 Elsevier Science B.V. All rights reserved.