Isotope analyses of mid-ocean-ridge basalts have led to the identification of large-scale geochemical provinces, with a clear distinction between the Pacific and the Atlantic or Indian Ocean basins(1,2), It is widely believed that Pacific ridges are formed from a single, fairly well mixed mantle reservoir(3), extending from the Australian-Antarctic discordance to the Juan de Fuca ridge and representing one of the largest chemically coherent mantle domains on the Earth(4,5). However, the evidence for this conception is mostly based on samples from the northern Pacific ridges. Here we report Sr, Nd and Pb isotope data from the Pacific Antarctic ridge that reveal different isotopic signatures north and south of the Easter microplate (25 degrees S). The evidence for two large-scale geochemical domains is further strengthened by the observation of different average depths of the ridge axes north and south of the 25 degrees S boundary. This boundary is located at the southeastern end of the Darwin rise/Pacific Superswell area(6), which is interpreted as a zone of upwelling(7) from the lower mantle that has persisted since Cretaceous times. We propose that this upwelling has led to the separation into true mantle domains with their own convective histories, producing slight differences in their average isotopic signatures and thermal regimes.