It has been accepted for some time that mixing between MORB-source mantle and the Iceland plume (geochemically relatively ‘reriched’ mantle) occurs along the Reykjanes Ridge. The composition of the basalts becomes progressively more enriched in lithophile elements towards Iceland. Within Iceland itself, there is significant variation in the composition of the mafic volcanic rocks. Although variations in degree of melting of a single source could have produced the elemental compositions of the Icelandic picritic, tholeiitic and alkalic basalts, their isotope systematics are not consistent with this model and require differences in source chemistry. Therefore, either MORB-source mantle has been mixed into the plume or the plume itself is heterogeneous. The clearest indications come from Pb isotopic data, which suggest that MORB-source mantle was excluded from the generation of Icelandic volcanic rocks.