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Stable isotopes (¹⁸O/¹⁶O) and ⁸⁷Sr/⁸⁶Sr ratios in pore fluids of the Chile Triple Junction accretionary prism : implications for diagenesis and fluid migration

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  • Oxygen and strontium isotopic composition of sedimentary pore fluids obtained by drilling at the Chile Triple Junction have been used to obtain information on the diagenesis and transport mechanisms of fluids. Samples were collected from three sites located on an east-west transect of the pre-collision zone (Sites 859, 860, and 861) and from Site 863 located at the base of the trench slope at the point where the Chile Ridge is being subducted. Closer to the toe of the wedge (Sites 859 and 860), non-steady state depth profiles were obtained, whereas linear chemical and isotopic profiles at the upper trench slope (Site 861) are indicative of diffusion of solutes at this site. The nature of fluids from the sites located in the transect prior to ridge subduction is quite different from that observed after ridge subduction (Site 863). Even though in all cases oxygen and strontium isotopic fingerprint indicates alteration of volcanogenic sequences to form authigenic clay mineral, the reactions that modified the chemical composition at Sites 859, 860, and 861 occurred at low temperatures, while high temperature reactions are responsible for the isotopic composition of the fluids at Site 863. At this later site, the ¹⁸O of pore fluids show values as high as 0.7‰ SMOW and a ⁸⁷Sr/⁸⁶Sr ratio as low as 0.706 in the interval from 400 to 500 mbsf, a sediment section characterized by the presence of a cementation front and steep chemical gradients in the pore fluids. These data suggest that the basal fluids (>400 mbsf) at this site are likely to have been influenced by the hydrothermal reactions occurring at high temperatures.
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  • Zheng, Y., Froelich, P., Torres, M., & Dia, A. (1995). Stable isotopes (¹⁸O/¹⁶O) and ⁸⁷Sr/⁸⁶Sr ratios in pore fluids of the Chile Triple Junction accretionary prism: implications for diagenesis and fluid migration. Proceedings of the Ocean Drilling Program, Scientific Results, 141, 313-320.
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  • 141
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