Stable isotope geochemistry of bentonites from the island of Milos (Greece)

Smectites from bentonite deposits from the island of Milos were analyzed for their oxygen and hydrogen isotopic ratios. Based on published isotopic fractionation data the results argue for a main meteoric water origin of the fluids responsible for the transformation of the pyroclastic rocks to bentonites. Alteration temperatures were estimated to be in the range from 30° to 90°C. In addition, δ18O and δ13C ratios were also measured on calcite separated from the bentonites. Dissolution of carbonates in the overlying marl and reprecipitation in the bentonite/pyroclastic rocks is considered as the origin of most of the calcites. In consequence of this polyphase history, δ13C-values indicate a marine carbon source while δ18O ratios suggest equilibrium of calcite with a fluid phase at elevated temperatures. Isotopic data of Smectites and calcite point to similar conditions for the formation of both minerals. proposed that an aquifer in the NE of Milos was heated during volcanic activity in the early Quaternary and that heated groundwater circulated through pyroclastic rocks with high hydraulic permeability to form extensive bentonite deposits.