Secondary clay minerals observed in the two uppermost basalt lava flows at ODP Site 1001, in the Caribbean Sea, drilled from the large igneous province of Cretaceous age, result from low-temperature alteration processes. Alteration mainly proceeds by circulation and diffusion of sea water. Six different types of clay mineral assemblage were recognized. Initial alteration with oxygenated sea water involves Fe and K fixation, creating visible oxidation halos parallel to the sides of cracks and fissures. A saponite/beidellite mixture, interstratified smectite-glauconite, interstratified glauconite-nontronite and Fe oxyhydroxides are obtained depending on the distance from fluid conduits. The presence of beidellite may be due to enhanced Al mobilization resulting from high fluid flux. These early minerals are cross-cut by thin veins of pure celadonite or glauconite with further vesicle infill. Late-stage alteration is typified by the formation of saponite and takes place under closed reducing conditions resulting from deposition of the sedimentary overburden.