Oxygen isotopic composition of low-temperature authigenic clinoptilolite

Oxygen isotope ratios were obtained from authigenic clinoptilolites from Barbados Accretionary Complex, Yamato Basin, and Exmouth Plateau sediments (ODP Sites 672, 797, and 762) in order to investigate the isotopic fractionation between clinoptilolite and pore water at early diagenetic stages and low temperatures. Dehydrated clinoptilolites display isotopic ratios for the zeolite framework (δ¹⁸O_f) that extend from +18.7‰ to +32.8‰ (vs. SMOW). In combination with associated pore water isotope data, the oxygen isotopic fractionation between clinoptilolite and pore fluids could be assessed in the temperature range from 25°C to 40°C. The resulting fractionation factors of 1.032 at 25°C and 1.027 at 40°C are in good agreement with the theoretically determined oxygen isotope fractionation between clinoptilolite and water. Calculations of isotopic temperatures illustrate that clinoptilolite formation occurred at relatively low temperatures of 17°C to 29°C in Barbados Ridge sediments and at 33°C to 62°C in the Yamato Basin. These data support a low-temperature origin of clinoptilolite and contradict the assumption that elevated temperatures are the main controlling factor for authigenic clinoptilolite formation. Increasing clinoptilolite δ¹⁸O_f values with depth indicate that clinoptilolites which are now in the deeper parts of the zeolite-bearing intervals had either formed at lower temperatures (17–20°C) or under closed system conditions.