STABLE-ISOTOPE GEOCHEMISTRY OF BENTONITES FROM THE ISLAND OF MILOS (GREECE)

Smectites from bentonite deposits from the island of Miles were analyz ed for their oxygen and hydrogen isotopic ratios. Based on published i sotopic fractionation data the results argue for a main meteoric water origin of the fluids responsible fur the transformation of the pyrocl astic rocks to bentonites. Alteration temperatures were estimated to b e in the range from 30 degrees to 90 degrees C. In addition, delta(18) O and delta(13)C ratios were also measured on calcite separated from t he bentonites. Dissolution of carbonates in the overlying marl and rep recipitation in the bentonite/pyroclastic rocks is considered as the o rigin of most of the calcites. In consequence of this polyphase histor y, delta(13)C-values indicate a marine carbon source while delta(18)O ratios suggest equilibrium of calcite with a fluid phase at elevated t emperatures. Isotopic data of smectites and calcite point to similar c onditions for the formation of both minerals. It is proposed that an a quifer in the NE of Miles was heated during volcanic activity in the e arly Quaternary and that heated groundwater circulated through pyrocla stic rocks with high hydraulic permeability to form extensive bentonit e deposits.