Numerical modeling of multiphase flow and phase separation phenomena in the system H2O−NaCl with applications to magmatic hydrothermal systems at fast-spreading mid-ocean ridges

At mid-ocean ridges, the inner workings of hydrothermal systems are difficult to access directly and have to be investigated indirectly through geophysical measurements, petrological studies of disclosed oceanic crust brought to the surface, or geochemical analysis of venting hydrothermal fluids. Numerical models have become a beneficial tool to study hydrothermal fluid dynamics and allow researchers to better synthesize and understand observations and interpretations obtained by other scientific disciplines. At fast-spreading ridges, hydrothermal fluids are heated by shallow magmatism separating the fluids into a high-salinity liquid and a low-salinity vapor phase. In the first application of a self-developed hydrothermal simulator, I investigate the brine formation and mobilization in hydrothermal systems driven by a transient basal temperature boundary condition, which represents the axial magma lens. It was found that basal heating results in rapid phase segregation and the formation of a stable brine layer that thermally insulates the driving heat source of flow circulation. While this brine layer is stable under steady-state conditions, a reduction of the heat input mobilizes the brines. The second application studies dike intrusions for conditions found at the East Pacific Rise (EPR). At EPR 9°50.3’N, vent fluid salinity and temperature at individual vents in the axial summit trough have been repeatedly measured over a 25+ year-long period. After the 1991/92 diking event it was observed that low salinities are followed by higher salinities after a few years. The simulation analysis includes brine accumulation close to the dike as well as two-phase flow and the delayed brine upflow when the dike has cooled. In a comprehensive suite of model runs, I have identified key parameters, which control the vent salinity evolution, These are rock permeability and porosity plus the background fluid temperature and salinity