MASSIVE BARITE DEPOSITS IN THE CONVERGENT MARGIN OFF PERU - IMPLICATIONS FOR FLUID CIRCULATION WITHIN SUBDUCTION ZONES

The convergent margin of Peru, characterized by an extensional tectoni c regime and the lack of a well-developed accretionary prism, has been investigated by a deep-sea submersible during the Nautiperc cruise (M arch-April, 1991). This allowed the collection of fluid samples, soft sediments, and barite concretions in the vicinity of biological commun ities associated with fluid seeps. Major and trace element contents as well as strontium, oxygen, hydrogen, and sulfur isotopic compositions have been measured on fluid and/or solid samples to constrain the nat ure and origin of fluid circulating in this extensional tectonic conte xt. Chemical variations with respect to bottom seawater composition ha ve been recorded in the fluid samples and suggest the presence of a no nlocal component in the fluid expelled at the seafloor. The major vari ations correspond to elevations of the Cl, Na, and the Ba contents as well as the Sr-87/Sr-86 ratios. This is interpreted as the expulsion o f a radiogenic, continent-related (basinal brine and/or meteoric water ) fluid. Massive barite concretions have been collected at the seafloo r in two areas of major fluid venting. The radiogenic signature (stron tium isotopic composition) of the barite concretions implies that they are related to the nonlocal deep fluid component identified in the fl uid samples. Furthermore, it is shown that these barite deposits testi fy to a hot, short, and intensive fluid circulation event. Compared to subduction zones that exhibit venting fluid with a strong oceanic wat er signature, the nature and origin of venting fluid along the subduct ion zone of Peru are different. The extensional tectonic regime of the Peru continental margin, locally associated with a dense E-W trending fault network, is an agent which may help to drain continent-related fluid as deep as the subduction scarp at the trench-slope boundary. Co pyright (C) 1997 Elsevier Science Ltd.