Brine Formation and Mobilization in Submarine Hydrothermal Systems: Insights from a Novel Multiphase Hydrothermal Flow Model in the System H2O–NaCl

Numerical models have become indispensable tools for investigating submarine hydrothermal systems and for relating seafloor observations to physicochemical processes at depth. Particularly useful are multiphase models that account for phase separation phenomena, so that model predictions can be compared to observed variations in vent fluid salinity. Yet, the numerics of multiphase flow remain a challenge. Here we present a novel hydrothermal flow model for the system H2O–NaCl able to resolve multiphase flow over the full range of pressure, temperature, and salinity variations that are relevant to submarine hydrothermal systems. The method is based on a 2-D finite volume scheme that uses a Newton–Raphson algorithm to couple the governing conservation equations and to treat the non-linearity of the fluid properties. The method uses pressure, specific fluid enthalpy, and bulk fluid salt content as primary variables, is not bounded to the Courant time step size, and allows for a direct control of how accurately mass and energy conservation is ensured. In a first application of this new model, we investigate brine formation and mobilization in hydrothermal systems driven by a transient basal temperature boundary condition—analogue to seawater circulation systems found at mid-ocean ridges. We find that basal heating results in the rapid formation of a stable brine layer that thermally insulates the driving heat source. While this brine layer is stable under steady-state conditions, it can be mobilized as a consequence of variations in heat input leading to brine entrainment and the venting of highly saline fluids.

Progress in Deciphering the Controls on the Geochemistry of Fluids in Seafloor Hydrothermal Systems

Over the last four decades, more than 500 sites of seafloor hydrothermal venting have been identified in a range of tectonic environments. These vents represent the seafloor manifestation of hydrothermal convection of seawater through the permeable oceanic basement that is driven by a subsurface heat source. Hydrothermal circulation has fundamental effects on the transfer of heat and mass from the lithosphere to the hydrosphere, the composition of seawater, the physical and chemical properties of the oceanic basement, and vent ecosystems at and below the seafloor. In this review, we compare and contrast the vent fluid chemistry from hydrothermal fields in a range of tectonic settings to assess the relative roles of fluid-mineral equilibria, phase separation, magmatic input, seawater entrainment, and sediment cover in producing the observed range of fluid compositions. We focus particularly on hydrothermal activity in those tectonic environments (e.g., mid-ocean ridge detachment faults, back-arc basins, and island arc volcanoes) where significant progress has been made in the last decade in documenting the variations in vent fluid composition.

Vein networks in hydrothermal systems provide constraints for the monitoring of active volcanoes

Vein networks affect the hydrothermal systems of many volcanoes, and variations in their arrangement may precede hydrothermal and volcanic eruptions. However, the long-term evolution of vein networks is often unknown because data are lacking. We analyze two gypsum-filled vein networks affecting the hydrothermal field of the active Lipari volcanic Island (Italy) to reconstruct the dynamics of the hydrothermal processes. The older network (E1) consists of sub-vertical, N-S striking veins; the younger network (E2) consists of veins without a preferred strike and dip. E2 veins have larger aperture/length, fracture density, dilatancy, and finite extension than E1. The fluid overpressure of E2 is larger than that of E1 veins, whereas the hydraulic conductance is lower. The larger number of fracture intersections in E2 slows down the fluid movement, and favors fluid interference effects and pressurization. Depths of the E1 and E2 hydrothermal sources are 0.8 km and 4.6 km, respectively. The decrease in the fluid flux, depth of the hydrothermal source, and the pressurization increase in E2 are likely associated to a magma reservoir. The decrease of fluid discharge in hydrothermal fields may reflect pressurization at depth potentially preceding hydrothermal explosions. This has significant implications for the long-term monitoring strategy of volcanoes.