MAJOR-ELEMENT AND TRACE-ELEMENT AND SR ISOTOPE CONSTRAINTS ON FLUID CIRCULATION IN THE BARBADOS ACCRETIONARY COMPLEX .2. CIRCULATION RATES AND FLUXES

Sr-87/Sr-86 ratios of pore fluids recovered from diapiric structures e ast of tile deformation front df the Barbados accretionary prism remai n quasi-constant from the surface to depths of 8 m. The Sr isotopic ra tios are non-radiogenic compared with seawater, which implies that non -local fluid was expelled at such a high rate that its Sr isotopic com position was dominated by the source fluid and not by pore fluid or se awater. Chloride depletion recorded in the fluids corresponds to a fiv e-fold dilution in the deepest sample from the Atalante diatreme: this also strongly suggests upward flow. The comparison of measured chlori de contents and Sr isotope depth profiles with theoretical values obta ined using a steady-state advection-diffusive model gives some constra ints on the fluid circulation rate in the two diatremes (Atalante and Cyclope) and in one mud diapir (C). The velocities are variable, but t he diatremes, which have the largest thermal and chemical anomalies, y ield the highest estimated advection rates. The fluid circulation rate in the centre part of the largest diatreme (Atalante: 0.47 km(2) tota l surface) is estimated at about 1 m/yr. The fluid flux from the deepe st to shallowest levels of the Atalante diatreme was estimated on the basis of the fluid circulation rate calculations. Fluid release is cle arly favoured if active conduits are present, as they are in the diatr emes but not in the mud diapirs. However, even though this steady-stat e advection-diffusion model provides a rough estimation of flow rates, modification of the fluid pressure (and therefore fluid volume) durin g earthquakes may superimpose large but transient fluid expulsions.