Twenty-two young seamounts near the East Pacific Rise (EPR) consist predominantly of Ol-Hy normative baslat chemically identical to depleted mid-ocean ridge basalts (MORB). The basalt is systematically more primitive than the EPR lavas and is supplied directly to the seamounts from the mantle under the ridge, ridge-transform intersections, and fracture zones. The seamounts also contain small volumes of differentiated basalt with low Mg ♯ (Mg ♯=Mg/Mg+Fe2+) which fractionate from basalt parents of high Mg ♯ mainly by processes of fractional crystallization. Seamounts also contain Ne normative alkali basalts, which are enriched in light rare earth, other incompatible trace elements, and volatiles, as well as a complete-spectrum of transitional basalts of intermediate composition. We sugget that this spectrum of primitive basalt types forms owing to magma mixing during melt segregation in the region of melting of a chemically, isotopically, and perhaps mineralogically heterogeneous mantle. Melting probably takes place at about 100 MPa in the stability field of plagioclase or plagioclase-spinel, and the extent of partial melting is highly variable: relatively low to produce alkali basalts and relatively high to produce the tholeiitic MORB. All basalt types, however, are probably hybrids produces by mixing in the region of melt segregation. The thermal and mechanical regime which results in the magma diversity observed on young seamounts is evidently also present at very slow spreading ridges but not below fast-spreading ridges. Normal ridge crest segments and fracture zones (Seamount volcanism) thus exhibit a range of thermochemical characteristics which differ markedly from plume/hotspot volcanism.