MAJOR AND TRACE-ELEMENT AND SR ISOTOPE CONSTRAINTS ON FLUID CIRCULATIONS IN THE BARBADOS ACCRETIONARY COMPLEX .1. FLUID ORIGIN

We measured the chemical and Sr isotope compositions of vent and pore fluids and associated sediments from a diapiric field located near 13.5 degrees N, east of the deformation front of the Barbados accretionary complex. The two diapiric structures with the largest thermal anomalies and characterized by active fluid conduits exhibit the strongest chemical and Sr-87/Sr-86 variations. These two diatremes (Atalante and Cyclope) also have low chloride contents and non-radiogenic Sr-87/Sr-86, which may be explained by interaction between seawater and volcanic ash or with underlying oceanic crust. In the central zone of the two structures, Sr-87/Sr-86 varies little with depth, which implies that fluid expulsion is at such a high rate that it is not affected by seawater Sr-87/Sr-86. The expelled fluids probably circulate along the decollement zone and ascend along faults which act as channels, A deep component is probably located in the decollement zone whereas another component may be recirculated seawater. The ascending fluid interacted with numerous materials, causing dehydration of clay minerals and alteration of volcanic ash and/or underlying oceanic crust. To explain the low chloride contents of the Atalante fluids, gas hydrate decomposition, yielding dilution of the fluid without modifying its Sr isotope ratio, needs to be superimposed on the mixing between seawater and the deep component. The occurrence of gypsum in these two diatremes implies higher fluid temperatures and more intense activity, at least temporarily, to allow the deposition of such a mineral.