The alkalic composition of the lavas resembles that of rejuvenated stage volcanics on Kauai, that of preshield volcanics at Loihi and that of the North Arch volcanic field; they share with the latter a high ratio of helium-3/helium-4 and a high carbon dioxide content. Mixing between mantle plume melts of the Hawaiian hotspot and components typical of mid-ocean ridge basalts may have given rise to the volcanics of the South Arch volcanic field, with metasomatism involved as well. The magmas of the South Arch volcanic field appear to have been unusually water-rich, perhaps because of an early melting of water-rich mantle.