Large outpourings of basaltic lava have punctuated geological time, but the mechanisms responsible for the generation of such extraordinary volumes of melt are not well known(1). Recent geochemical evidence suggests that an early-formed reservoir may have survived in the Earth's mantle for about 4.5 billion years (ref. 2), and melts of this reservoir contributed to the flood basalt emplaced on Baffin Island about 60 million years ago(3-5). However, the volume of this ancient mantle domain and whether it has contributed to other flood basalts is not known. Here we show that basalts from the largest volcanic event in geologic history-the Ontong Java plateau(1,6,7)-also exhibit the isotopic and trace element signatures proposed for the early-Earth reservoir2. Together with the Ontong Java plateau, we suggest that six of the largest volcanic events that erupted in the past 250 million years derive from the oldest terrestrial mantle reservoir. The association of these large volcanic events with an ancient primitive mantle source suggests that its unique geochemical characteristics-it is both hotter (it has greater abundances of the radioactive heat-producing elements) and more fertile than depleted mantle reservoirs-may strongly affect the generation of flood basalts.