Petrologic/geochemical studies of basalts drilled in Cretaceous oceanic crust (DSDP legs 51-53 in the western Atlantic Ocean) are synthesizd in quantitative magma fractionation models. From least squares analysis of whole-rocks glass, and phenocryst phase compositions it is concluded that fractionation by crystallization of olivine, plagioclase, and clinopyroxene involved gravitative separation of mafic phases from plagioclase. Flotation of plagiclase together with disequilibrium reaction morphologies suggest that crystallization began shortly after the release of primitive liquids from the mantle. Polybaric fractionation and mixing of derivative magma batches are indicated by evidence for solid-liquid reaction and low-pressure reequilibration. Distinct eruptive series bounded by magnetic and other stratigraphic discontinuities correspond to chemically defined magma fractionation series. The latter are probably differentiated from one another through small differences in degree of partial melting. Their lithophile element character shows a evidence for homogeneous, ''depleted'' source material persisting during crystal construction at the Atlantic ridge axis. The whole-rock and glass selvedge compositional averages for eruptive units reflects the importance of posteruptive phenocryst redistribution and syn-eruptive changes in liquid fraction compositions.