Oxygen isotopic and mixed-layer compositional data are used to infer modes of formation of smectite and illite-smectite from the Barbados accretionary complex. Nearly pure smectites have δ18O values between 26‰ and 30‰ and apparently formed authigenically by alteration of volcanic ash. Illite-smectites are thought to have formed in situ at temperatures less than 40 δC, although a detrital origin cannot be ruled out for several samples. The unusually low temperature of transition of smectite to illite indicates that fluid generation by mineral dehydration and consequent structural effects can occur at shallow depths in accretionary complexes.

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