Abstract: The first occurrence of thaumasite resulting from seawater-basalt interaction has been discovered in a drill hole on Mururoa atoll (French Polynesia, South Pacific). This sulphate-carbonate-silicate is associated with zeolites (phillipsite, chabazite, gmelinite), apophyllite, analcime, and gyrolite. In this environment thaumasite can be considered as the last mineral of an alteration sequence. It crystallized from a Ca-SO3-CO2-rich and Al-Na-K-poor fluid. The temperature of this solution is unknown, but was probably not greater than 100°C.