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ABSTRACT: Chemical and Isotopic Constraints on the Sources and Flow of Fluids in the Peruvian Continental Margin

J. B. Martin, M. Kastner, H. Elderfield

Two types of interstitial waters dominate the Peruvian continental margin: a high-chloride brine (maximum 1043 mM) in the shelf, and a low-chloride fluid (minimum 454 mM) in the middle to lower slope. The brine has positive ^dgr18O values with maxima up to +0.8 ^pmil (SMOW) observed at shallow burial depths. The isotope ratios systematically deviate from the meteoric water line towards heavier ^dgr18O values and lighter ^dgrD values. The low-chloride fluid has negative ^dgr18O values, decreasing approximately linearly with depth. Most hydrogen and oxygen isotope values of this fluid fall on the meteoric water line. Ion/chloride ratios are enriched over seawater values in the following order: Li > Na > NH4 > K and Br > I.

One upper slope drill site, Ocean Drilling Program Site 679, exhibits chemical and isotopic characteristics of both fluid types. During the Pleistocene, 18O-enriched water was probably trapped in the shelf and upper slope, and has been preserved unmixed since. The low-chloride fluid may be derived from an influx of meteoric water (isotopic evidence) or from clay membrane ion filtration (chemical evidence), although these data do not exclude clathrate dissociation and mineral dehydration reactions. The light ^dgr18O values indicate that enriched Pleistocene fluids were flushed from the slope sediments. A source of meteoric water requires flow in the shelf beneath the brine, below a Miocene unconformity penetrated at Site 679 and possibly even through the continent l crust. The head driving this flow may contribute to the pressures necessary for clay membrane ion filtration.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990