Abstract Olivine crystals from Reykjanes Ridge tholeiites are generally homogeneous and represent a distinct suite compared to the strongly zoned olivines in the tholeiites from the median Neovolcanic zone of Iceland which exhibit Mg-rich cores enclosed by more Fe-rich rims or overgrowths. Partitioning of MgO/FeO between olivines and liquid compositions approximated by the aphyric rocks are systematic, and the compositions of olivine are highly dependent upon lava compositions, indicating a close approach to equilibrium crystallization. Whole-rock MgO/FeO ratios, phenocrystic olivine Fo contents, and estimated temperatures of olivine crystallization along the Reykjanes Ridge—median Neovolcanic zone profile show similar V-shaped trends characterized by a minimum located at approximately 63°30′N. These parameters exhibit complete overlap for the two suites of rocks, discounting the possibility that the Icelandic tholeiites simply represent more fractionated rocks than the ridge tholeiites. The major element chemistry of ridge tholeiites can be accounted for by shallow depth extraction of approximately 30% of the assemblage olivine, clinopyroxene, and plagioclase in the ratio 1 : 2 : 3. This amount of fractionation cannot explain the high enrichment factors observed for the LIL elements, nor the LIL element ratios as well as the Sr and Pb isotopic ratios, which remain best explained by the two-mantle mixing model.