Basalt lavas with a high Nb/Y ratio for a given Nb/Zr ratio occur in the Polynesian “superswell” region of the South Pacific, which probably formed by upwelling of a deep-mantle superplume. The distinctive geochemical characteristics of the Polynesian basalts may be attributed to melting of a mantle source that is more enriched in a basaltic (ancient mid-oceanic-ridge basalt) component. Basalts displaying such chemical signatures have been found on Shatsky Rise, the Ontong Java Plateau, and greenstones from subduction-zone complexes of Sakhalin Island. The occurrence of Polynesian-type basalts, together with an estimate of their ages, suggests that the South Pacific superplume was active as long ago as 90–150 Ma. The superplume activity preceded the onset of the superchron, supporting an idea that the superplume acted as a trigger for such a global event.