SINGLE-PASS MODELS OF SEAFLOOR HYDROTHERMAL SYSTEMS

An integrated mathematical model of a seafloor hydrothermal system requires coupling of magmatic heat transfer, hydrothermal circulation, water-rock chemical reactions, ore deposition, and biological processes. As a first step towards such a model, we use a single-pass model together with simple observational constraints and parameter formulations to derive a number of first order results. We first derive the basic state relating conductive heat transfer from a magma body to a high temperature hydrothermal system and show that the heat transfer boundary layer must be thin (~ 10 meters) and that the bulk permeability of the upflow zone must be high (~ 10-12m2). With these constraints and a simple expression for iron concentration in vent fluids, we address the formation of sulfide chimneys and ore bodies such as the TAG mound. We then address the evolution of permeability as a result of thermoelastic stresses, and the precipitation of quartz and anhydrite. These results show that thermoelastic stresses can lead to different system steady states and that the slow kinetics of quartz precipitation is an important factor for keeping the permeability of the upflow zone open. Moreover, we show that anhydrite precipitation tends to rapidly clog downflow zones unless the recharge area is large or some other factor, such as biologically mediated sulfate reduction, inhibits anhydrite precipitation. We further show that event plumes can arise following dike injection, provided that dike emplacement generates a substantial permeability increase at the dike margins. Nevertheless a significant fraction of event plume heat is derived from heat stored within the crust.