Postglacial basalts from Theistareykir (younger than 10,000 years), northern Iceland, define the depleted end of the spectrum of chemical and isotopic compositions observed in Icelandic volcanics but extend to some of the most enriched chemical and isotopic compositions found in Icelandic tholeiites. A spectacular feature of these basalts is the impressive correlations observed between radiogenic isotope ratios (Sr, Nd, Hf, and Pb) and almost the entire spectrum of major and trace element concentrations and ratios. The radiogenic isotope and major and trace element compositions are little affected by crystal fractionation and are essentially unaffected by interaction with the preexisting crust. The Theistareykir basalts must therefore be relatively close in composition to primary melts from the mantle. Consequently, their chemical and isotopic compositions provide a unique opportunity to investigate the nature of melting beneath Iceland and the geochemical character and origin of the mantle source. Large variations in incompatible element abundances require source heterogeneity, as well as variable extents of melting, to be important factors in determining the final chemical composition of the melts. Melting integrates over a large pressure range and is dominated by melting a depleted peridotite similar to the ambient depleted North Atlantic mantle. The isotopically enriched component is of relatively minor abundance and probably has a lower solidus temperature compared to the depleted component. More than one isotopically enriched component must be involved, but it is difficult to identify the end-member compositions using those of the lavas because of preeruptive averaging and damping of the enriched isotopic signals by mixing with the ambient depleted mantle or melts thereof, suggesting that the isotopic signals in Icelandic melts represent a somewhat muted isotopic signal of the enriched component(s) in the Icelandic source mantle. Comparison of the isotopic arrays of Icelandic basalts with those of global OIB suggests that the dominant enriched component may have a HIMU (high μ) affinity rather than representing a component similar to the enriched end of the Iceland isotopic arrays, and small amounts of an enriched component similar to enriched mantle (EMI)- type OIB sources are probably also involved.