Major- and trace-element and Sr isotope constraints on fluid circulation in the Barbados accretionary complex. Part II: circulation rates and fluxes

87Sr86Sr ratios of pore fluids recovered from diapiric structures east of the deformation front of the Barbados accretionary prism remain quasi-constant from the surface to depths of 8 m. The Sr isotopic ratios are non-radiogenic compared with seawater, which implies that non-local fluid was expelled at such a high rate that its Sr isotopic composition was dominated by the source fluid and not by pore fluid or seawater. de depletion recorded in the fluids corresponds to a five-fold dilution in the deepest sample from the Atalante diatreme: this also strongly suggests upward flow. The comparison of measured chloride contents and Sr isotope depth profiles with theoretical values obtained using a steady-state advection-diffusive model gives some constraints on the fluid circulation rate in the two diatremes (Atalante and Cyclope) and in one mud diapir (C). The velocities are variable, but the diatremes, which have the largest thermal and chemical anomalies, yield the highest estimated advection rates. The fluid circulation rate in the centre part of the largest diatreme (Atalante: 0.47 km2 total surface) is estimated at about 1 m/yr. The fluid flux from the deepest to shallowest levels of the Atalante diatreme was estimated on the basis of the fluid circulation rate calculations. release is clearly favoured if active conduits are present, as they are in the diatremes but not in the mud diapirs. However, even though this steady-state advection-diffusion model provides a rough estimation of flow rates, modification of the fluid pressure (and therefore fluid volume) during earthquakes may superimpose large but transient fluid expulsions.