OXYGEN ISOTOPIC COMPOSITION OF LOW-TEMPERATURE AUTHIGENIC CLINOPTILOLITE

Oxygen isotope ratios were obtained from authigenic clinoptilolites fr om Barbados Accretionary Complex, Yamato Basin, and Exmouth Plateau se diments (ODP Sites 672, 797, and 762) in order to investigate the isot opic fractionation between clinoptilolite and pore water at early diag enetic stages and low temperatures. Dehydrated clinoptilolites display isotopic ratios for the zeolite framework (delta(18)O(f)) that extend from +-18.7 parts per thousand to +32.8 parts per thousand (vs. SMOW) . In combination with associated pore water isotope data, the oxygen i sotopic fractionation between clinoptilolite and pore fluids could be assessed in the temperature range from 25 degrees C to 40 degrees C. T he resulting fractionation factors of 1.032 at 25 degrees C and 1.027 at 40 degrees C are in good agreement with the theoretically determine d oxygen isotope fractionation between clinoptilolite and water. Calcu lations of isotopic temperatures illustrate that clinoptilolite format ion occurred at relatively low temperatures of 17 degrees C to 29 degr ees C in Barbados Ridge sediments and at 33 degrees C to 62 degrees C in the Yamato Basin. These data support a low-temperature origin of cl inoptilolite and contradict the assumption that elevated temperatures are the main controlling factor for authigenic clinoptilolite formatio n. Increasing clinoptilolite delta(18)O(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 degre es C) or under closed system conditions. (C) 1998 Elsevier Science B.V . All rights reserved.