Isla Marchena is the subaerial exposure of a shield volcano located between the Galapagos Platform and the Galapagos Spreading Center. The geologic evolution of the island can be divided into two lava series separated by formation of a cadera and a period of explosive activity. Both lava series contain evolved alkali olivine and tholeiitic basalts (average mg # 55.2) and are virtually aphyric with the exception of a plagioclase-rich horizon in the pre-caldera section. Although the major-element compositions of the two series are similar, the abundances of incompatible elements and ratios of highly/moderately incompatible elements increase systematically with decreasing stratigraphic age. These compositional differences, together with textural and mineral chemical evidence for disequilibrium, are not easily explained by any single petrologic mechanism. The combined effects of a periodically replenished magma chamber, assimilation-fractionation, and in situ crystallization of a solidification zone are possible mechanisms that may be responsible for the temporal changes in lava composition. MORB-like abundances of trace elements, in addition to unusually depleted 87Sr j86Sr and 143Nd;144Nd ratios for ocean-island basalts, suggest that magmas associated with the Galapagos plume interacted with those of the Galapagos Spreading Center.