Combined K–Ar and Ar–Ar dating of a xenolith-bearing alkaline dyke, previously 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 intrusive activity. High 3He/4He ratios of 11–16Ra, potentially up to 23Ra (Ra=atmospheric ratio of 1.39×10−6) have been measured from mantle xenoliths in the dyke and are consistent with previous determinations of the helium isotope composition of Tertiary lavas in Greenland and Scotland linking the magmatism with the proto-Iceland mantle plume.

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