AUTHIGENIC BARITES AND FLUXES OF BARIUM ASSOCIATED WITH FLUID SEEPS IN THE PERU SUBDUCTION ZONE

Large deposits of barite were discovered in association with biologica l communities, indicative of active fluid seepage on the middle slope of Paita and in the Chiclayo Canyon, in the Peru mar in. We postulate that the barium source for the deposits is associated with the high co ncentration of non-detrital barite buried in sediments from this high productivity region. Barite is remobilized within the sediment column due to sulfate depletion. Subsequent flushing of the barium-rich fluid s from the sediment to the bottom water, leads to the formation of bar ite deposits at the cold vent sites. High barium concentrations measur ed in pore fluids of sediments are consistent with remobilization of b arium sulfate below the zone of sulfate depletion. Fluid samples -coll ected in a time sequence using a benthic chamber in the Paita middle s lope vent sites-document a contemporaneous release of barium to the bo ttom water at a rate of 23 mu mol cm(-2) yr(-1). Fluid seepage in the Peru margin is not restricted to the middle slope of Paita and the Chi clayo Canyon where barite deposits occur, but is also evident in the u pper and lower slopes of Paita and in the Chimbote upper slope. Deploy ment of a benthic chamber on the Chimbote upper slope site show no mea surable release of barium; even though the dissolved barium concentrat ion in the pore fluids is high. These observations indicate that the b arite deposits associated with fluid seepage in the Peru margin are re stricted to areas where slope failure has exposed sequences deep enoug h such that the barium-rich fluids do not encounter sulfate-bearing po re fluids before emanating at the seafloor.