Numerical Models of Layered Convection in a Heterogeneous Medium and the Relationship Between Focused and Diffuse Flow in Ridge Crest Hydrothermal Systems

One of the outstanding issues in submarine hydrothermal research concerns the connection between focused high-temperature venting and nearby, low temperature diffuse flow. Some systems such as the Galapagos do not exhibit high-temperature venting at all despite geochemical evidence of a high-temperature end member fluid. We explore this issue be constructing a number of 2-D simulations using the two-phase finite difference code GTH, set to operate in the single-phase mode. We considered a rectangular system one kilometer deep and two kilometers wide with a heterogeneous permeability distribution to simulate a deep single-pass circulation system. To drive the single-pass system we constructed a 750 m-wide recharge zone with permeability k = 10^-13 m² and 50 m-wide cross-flow and discharge zones with k = 10^-12 m². In the interior of the cell, we assumed k = 10^-15 m². The side boundaries were impermeable and insulated; the top boundary was open and held at fixed temperature. The lower boundary was also impermeable and either constant temperature or constant heat flux, with values appropriate for black smoker flows, conditions were imposed. We then explored the effect of a surface layer of high permeability pillow basalts with thicknesses ranging between 100 m and 400 m and 10^-14 m² < k_p < 10^-10 m². At low values of k_p, convection in the pillows was negligible and the pillow had little affect on the high-temperature system. At high values of k_p, convection in the pillows significantly damped the high-temperature output. At intermediate values cold diffuse flow occurred at varying distances from the main high temperature vent. The results of these simulations thus show that fluid convection in the pillows can significantly impact deep-seated circulation. Depending upon the permeability and thickness of the pillow either low-temperature Galapagos-type venting or combined focused-diffuse flow venting may evolve.