Origin and evolution of fluids from mud volcanoes in the Barbados accretionary complex

A large collection of fluids (54 interstitial fluids and four expelled fluids) were sampled at the Manon site, at the outer edge of the Barbados accretionary complex. These warm fluids (up to 20degreesC) are expelled by sub-marine (5000 mbsl) mud volcanoes consisting of diapirs (unchanneled flow) and diatremes (channeled). Chlorine stable isotope ratios of these fluids were measured by IRMS with a reproducibility of +/- 0.05parts per thousand (1sigma) versus SMOC (Standard Mean Ocean Chloride). A large range of delta(37)CI between -5.3parts per thousand and +0.1parts per thousand is observed. Data from each volcanic structure describe a mixing between seawater and a low-delta(37)Cl fluid. The whole set of data is interpreted as the result of a mixing between two deep components and seawater. The two deep fluids are chemically distinct (e.g., in Ca, Mg, K, Li. Sr and Br contents and Br/Cl ratio). They display low and significantly different Sr-87/Sr-86 ratios (0.707790 and 0.707892, respectively) and delta(37)CI values (-4.51 and -5.24parts per thousand, respectively). Physicochemical processes such as mineralogical transformation, diffusion, compaction or ion filtration are known to fractionate chlorine stable isotopes and can produce fluids with negative delta(37)CI values. Ion filtration due to sediment compaction appears to be the more likely process to explain the negative delta(37)CI values observed at the Manon site. A model for the generation of these signatures is proposed where a residual negative delta(37)CI fluid reservoir is created at the bottom of the prism or the sediment pile. Further compaction/ fracturing and/or dewatering of the slab may flush out these fluids and focus them towards the decollement zone. Mixing between the fluids and ultimately with seawater and water released during gas hydratedestabilizations may explain the data set within the individual cores and between the different structures. Copyright (C) 2004 Elsevier Ltd.