OCEAN island basalts (OIBs) are thought to result from the melting of plumes rising from a boundary layer in the mantle, such as the core-mantle boundary or that between the upper and lower mantle at 660 km depth(1,2). OIBs display considerable compositional heterogeneity, generally attributed to mixing between components such as depleted mantle, subducted oceanic crust, continental lithospheric mantle and primitive mantle(3-8), Although the existence of multiple endmembers in the OIB array is now well established, the relationships between crustal recycling, plume volcanism and deep mantle structure and composition are controversial, We present isotope data for picritic lavas from seven volcanic centres, sampling 2 million years of Hawaiian plume activity(9), and show that the osmium and lead isotopic compositions form remarkably linear, negative arrays. These correlations are indicative of binary mixing, but are difficult to explain by combining recycled oceanic crust with depleted mantle. We argue that mixing between two distinct mantle components, within the Hawaiian plume provides a better explanation of the isotopic variations of the picrites.