Transient convective circulation in a sedimented ridge setting: Implications for applying geochemical data to estimate permeability

Abstract

For an open, sedimented ridge setting we investigate by means of transient numerical simulations hydrologic factors that can influence chemical signatures of mineral alteration, and how these features relate to the permeability of the upper oceanic crust. The transient state we consider involves three stages: (1) starting from a static flow field and a conductive thermal regime, flow in basement rocks is initiated as unstable convection; (2) with the inflow of seawater to the basement, the convection pattern evolves into a stable convection cell linking the recharge and discharge sites on the seafloor; and (3) convection weakens through time as the system offsets the initial thermal condition to attain a steady state. The duration of each stage, and the extent of the region in the basement undergoing fluid exchange with seawater varies with permeability. By the end of stage 2, for basement permeabilities between 1x10(-14) and 1x10(-13) m(2), most of the basement and the deeper sediments surrounding the discharge zone is hydrologically isolated, but with active recirculation inside this zone. Through-flow of seawater is restricted to a relatively narrow boundary layer. The unstable flow geometry during stages one and two is reflected in fluctuations of recharge and discharge rates into and out of the basement, and periodic variations in temperature of the discharging fluids. Fluid discharge at steady state is approximately one-half of the average occurring during stage one. Changing flow patterns through time lead to a strongly heterogeneous evolution of water/rock ratios in the basement. Average water/rock ratios for the basement approach a log-log linear correlation with basement permeability as steady state is reached.