The development of accretionary prisms at convergent plate margins is one of the focuses of current marine geological research. Of particular interest is the compaction of sediments and the accompanying expulsion of water from the prism that occurs over geologic time. This project will examine the role of interlayer water of expandable clays (smectites) in the development of subduction zones. The investigators propose to determine the oxygen and hydrogen isotope fractionation between interlayer and bulk water in order to determine how much of the porewater extruded from an accretionary prism originates as interlayer water. As part of this research the oxygen and hydrogen isotope composition and clay mineralogy of clays from the Barbados accretionary prism will be determined.