Lavas from the Easter Seamount Chain (ESC) between Salas y Gomez Island and the Ahu volcanic field are tholeiitic and alkalic basalts showing regular and systematic chemical variations with longitude. With progressive distance eastward from the east rift of the Easter microplate, the lavas become progressively richer in K2O, Na2O, FeO, TiO2, and P2O5, and have higher K2O/TiO2 and lower MgO and CaO. These changes reflect differences in the total extent of shallow fractionation and differences in the conditions under which it occurred. Below the Salas y Gomez ridge, where large isostatically compensated volcanoes lead to locally thicker crust, fractionation took place under higher pressure and/or conditions of higher H2O, compared with lavas of the Easter ridge and the east rift. Differences in K2O, P2O5, and K2O/TiO2 reflect differences in mantle source composition and binary mixing between an enriched plume component and a depleted mid-ocean ridge basalt (MORB)-like component, as indicated by Pb isotopic data. Mixing along the ESC apparently occurred in the solid state prior to melting, whereas mixing below the east rift involved fractionated liquids. We also see evidence for differences in the conditions of melting, using oxide abundances corrected for shallow fractionation and mantle heterogeneity. Melting below the Salas y Gomez region seems to be initially deeper and more extensive, with progressively shallower and less extensive melting toward the east rift. If this model is correct, it implies that some alkali basalts may form by larger extents of melting than previously thought on the basis of trace element modeling. Since independent evidence suggests the Easter plume has a modest to large excess temperature compared with ambient MORB mantle, we conclude the plume is under the Salas y Gomez region. ¿ 1998 American Geophysical Union