Evidence of the proto-Iceland plume in northwestern Ireland at 42 Ma from helium isotopes

Combined K Ar and Ar-Ar dating of a xenolith-bearing alkaline dyke, previou sly considered to be of late Palaeozoic age, suggest a mid-Eocene age of c. 43 Ma, indicating that it is part of extensive rift-related Tertiary intru sive activity. High He-3/He-4 ratios of 11-16R(a), potentially up to 23R(a) (R-a=atmospheric ratio of 1.39 x 10(-6)) have been measured from mantle xe noliths in the dyke and are consistent with previous determinations of the helium isotope composition of Tertiary lavas in Greenland and Scotland link ing the magmatism with the proto-Iceland mantle plume.