Apart from being popular holiday destinations, oceanic-island volcanoes such as Hawaii, Tahiti, or the Canaries provide magmas that yield valuable information about the interior of our planet. Until recently, studies have concentrated on the easily accessible, subaerial parts of the volcanoes, largely ignoring their earlier-formed, submarine parts. These submarine parts, however, provide critical information about how the mantle begins to melt and about the lowest-melting-point mantle components—information not available from the subaerial volcanoes but highly relevant for the chemical evolution of the whole mantle. We present here compositional information from small (<500 m) volcanoes on the seafloor near Tahiti and Pitcairn Islands and show that these small volcanoes erupt only highly differentiated magmas. These early melts are derived exclusively from the most trace element–enriched, isotopically extreme mantle component, evidence that this component has the lowest melting temperature and is the first product of melting of a new batch of mantle. The geochemical mantle components (enriched mantle EM-I, EM-II) proposed in the 1980s to explain the compositional variations among oceanic volcanoes worldwide appear in reality to represent distinct rock masses in the mantle.