ORIGIN OF SMECTITE AND ILLITE-SMECTITE IN THE BARBADOS ACCRETIONARY COMPLEX - OXYGEN ISOTOPIC EVIDENCE

Oxygen isotopic and mixed-layer compositional data are used to infer m odes of formation of smectite and illite-smectite from the Barbados ac cretionary complex. Nearly pure smectites have deltaO-18 values betwee n 26 parts per thousand and 30 parts per thousand and apparently forme d authigenically by alteration of volcanic ash. Illite-smectites are t hought to have formed in situ at temperatures less than 40-degrees-C, although a detrital origin cannot be ruled out for several samples. Th e unusually low temperature of transition of smectite to illite indica tes that fluid generation by mineral dehydration and consequent struct ural effects can occur al shallow depths in accretionary complexes.