Fluid–sediment interactions at Eastern Mediterranean mud volcanoes: a stable isotope study from ODP Leg 160

Pore fluids from two ODP sites at Eastern Mediterranean mud volcanoes have been analyzed for their Cl concentration and their δ18O and δD isotopic composition. The Cl data span a wide range of concentrations, from extremely depleted with respect to seawater (as low as 60 mM) at the crest of Milano dome (site 970) to strongly enriched (up to 5.4 M) at Napoli dome (site 971). Chloride enrichment is known to be due to dissolving Messinian evaporites, whereas the source of the low-Cl fluid is deduced from stable isotope data presented here. The isotopic composition of the endmember fluid is found to be +10‰ for δ18O and −32‰ for δD for low- as well as for high-Cl waters. From this signature it can be concluded that neither gas hydrates nor meteoric water play a significant role in the freshening of the pore water. Several other processes altering the δ18O/δD composition of pore waters are discussed and considered to be of only negligible influence. The process characterizing the isotopic composition of the fluid is found to be clay mineral dehydration (mainly smectite–illite transformation), corresponding to a depth range of 3.5–7 km and an elevated temperature of about 120–165°C. A quantitative estimate shows that this reaction is capable of producing the observed extreme Cl depletion.