NONSTEADY STATE DIFFUSION AND ADVECTION MODEL OF TRANSIENT CONCENTRATION-DEPTH PROFILES FROM THE BARBADOS ACCRETIONARY COMPLEX

We present a non-steady-state diffusion and advection model that descr ibes the distribution of methane, manganese and silica concentrations versus depth at sites 671, 676, and 672 of ODP Leg 110 through an ocea nic sedimentary pile undergoing horizontal shortening by means of thru st-faults and folds. The model suggests that the distribution of these chemical components above and below fluid flow conduits is controlled by molecular diffusion. Upward fluid advection from the pathways to t he surrounding sediments appears to be without significance for this e nvironment (about 1.10(-11) m.s-1, i.e. almost-equal-to 0.3 mm.yr-1). Estimates of the time necessary to produce the diffusion profiles indi cate that average ages of the chemical anomalies increase from Site 67 2 to Site 671, i. e. from the oceanic domain towards the internal part of the accretionary prism. Using a two-dimensional framework for the model, fluid-flow velocities along the decollement and protodecollemen t zones are estimated about 7.10(-9) m.s-1 (i.e. almost-equal-to 0.20 m.yr-1) and 1.10(-7) m.s-1 (i.e. almost-equal-to 3 m.yr-1), respective ly. Comparisons between ion-activity products of rhodochrosite and amo rphous silica, and their equilibrium constants suggest the dissolution of biogenic silica and precipitation of rhodochrosite within the deco llement zone, which is the major fluid conduit of the Barbados Accreti onary Complex.