Numerical simulation of fluid flow in mid-ocean ridge hydrothermal systems

Located in the deep oceans, the 60,000km chain of mid-ocean ridge spreading centers hosts the world’s largest magmatic-hydrothermal systems. These so-called black smoker systems transport a major part of the total Earth’s heat flux, affect the chemistry of crust and overlying ocean significantly and provide nutrients for chemosynthetic life on and beneath the seafloor. In addition, they are active ore-forming systems and the modern day analogue of massive sulfide deposits which have successfully been developed into world-class mines onshore. To understand the physics of these sub-seafloor convection systems numerical modeling techniques are essential. However, the physics of these systems are complex especially because seawater can phase-separate, either by boiling or condensation, into a low-salinity vapor and high-salinity brine. To capture this process accurately in numerical models is challenging but key to understanding the true nature and dynamics of black smoker systems. Previous numerical studies avoided these complexities by taking severe assumptions on the thermodynamic properties of the fluid. In addition, high heat-flux thermal convection has only been studied in two dimensions so far. The object of this thesis is to contribute to our understanding of black smoker hydrothermal systems in two fundamental ways: The first goal is to study thermal convection in three dimensions using high-resolution models. The second goal is to accurately simulate multiphase thermohaline convection including the phase-separation of a seawater-like fluid into a low-salinity vapor and high-salinity brine. To do so, two novel computational tools were developed during this thesis work. The first is the addition of parallel computing techniques to CSMP++, a simulation platform developed by research groups at ETH Zurich and Imperial College London, allowing the use of supercomputers to calculate simulations. The second consists of a new pressure-enthalpysalinity simulation scheme, using finite element and finite volume numerical techniques, to solve mass and energy transport of multi-phase NaCl-H2O fluids in geologic media. In this thesis results from more than 50 simulations, including high-resolution 3D and multi-phase models, are presented, providing new insights into the nature and dynamics of sub-seafloor convection. The most important general conclusion from these simulations is that the non-linear dependence of the fluid properties on temperature, pressure and salinity can explain many of the key-observations at black smoker systems. Though all models are generic in nature, omitting geologic complexity of real systems, they accurately reproduce salient features as temperature and salinity variations measured in vent-fluids. Moreover, convection in three-dimensions naturally forms individual black smoker fields at regular spacing without any restrictions on the sub-seafloor permeability distribution. High-resolution 3D simulations demonstrate that recharge of black smoker systems dominantly happens in narrow concentric zones surrounding pipe-like upflow zones. Recharge fluids are relatively warm, which enhances mass fluxes and therefore the energy output of