Ocean Drilling Program, United States

Low-temperature aqueous alteration (weathering) of serpentinitic basement rocks at Sites 897 and 899 has produced dramatic textural and compositional modifications. Fe-oxyhydroxides, clay minerals, and calcium carbonate fill multiple generations of brittle fractures. Replacement of serpentinite minerals by smectite, carbonate, quartz, and possibly other clay minerals was accomplished through a complex sequence of repeated dissolution and precipitation. The youngest generation of brittle fractures is filled with particulate debris, including surviving fragments of serpentinite and fragments of all of the authigenic minerals. Enrichments of relatively immobile elements in altered samples (on a carbonate-free basis) suggest volume losses, relative to the initial serpentinitic material, in excess of 50% at Site 897 and 20% at Site 899. The magnitude of Mg loss (on a carbonate-free basis) roughly confirms these apparent volume changes but is difficult to assess given possible additions of oxidized iron to these samples.