Linear arrays in lead isotope space for mid-ocean ridge basalts (MORBs) converge on a single end-member component that has intermediate lead, strontium, and neodymium isotope ratios compared with the total database for oceanic island basalts (OIBs) and MORBs. The MORE data are consistent With the presence of a common mantle source region for OIBs that is sampled by mantle plumes. He-3/He-4 ratios for MORBs show both positive and negative correlation with the Pb-206/Pb-204 ratios, depending on the MORE suite. These data suggest that the common mantle source is located in the transition zone region. This region contains recycled, oceanic crustal protoliths that incorporated some continental lead before their subduction during the past 300 to 2000 million years.