Alteration mineralogy of Cretaceous basalt from ODP Site 1001, Leg 165 (Caribbean Sea)

Secondary clay minerals observed ill the two uppermost basalt lava flows at ODP Site 1001, in the. Caribbean Sea, drilled from the large igneous provi nce of Cretaceous age, result From low-temperature alteration processes. Al teration mainly proceeds by circulation and diffusion of sea water. Six dif ferent types of clay mineral assemblage were recognized. Initial alteration with oxygenated sea water involves Fe and It fixation, creating visible ox idation halos parallel to the sides of cracks and fissures. A saponite/beid ellite mixture, interstratified smectite-glauconite, interstratified glauco nite-nontronite and Fe oxyhydroxides are obtained depending on the distance from fluid conduits. The presence of beidellite may bu 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 in fill. Late-stage alteration is typified by the formation of saponite and ta l;es place under closed reducing conditions resulting from deposition of th e sedimentary overburden.