Abstract
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.
Highlights
Hydrothermal venting at the ocean floor is a key mechanism of biogeochemical exchange between the solid earth and the global ocean
We explore brine formation and mobilization in a simplified setup that mimics the situation at intermediate to fast spreading ridges, where hydrothermal circulation is mainly driven by heat released from a cooling axial melt lens (AML)
We find that a basal internally stable stratified brine layer progressively forms with a maximum vertical extent that is controlled by a balance between basal heat input and hydrothermal heat transport
Summary
Hydrothermal venting at the ocean floor is a key mechanism of biogeochemical exchange between the solid earth and the global ocean. When seawater is heated and intersects the two-phase boundary, it splits into a low salinity vapor and a higher salinity brine phase (Bischoff and Rosenbauer 1984; Driesner and Heinrich 2007) Segregation of these phases can result in the venting of fluids that have a higher or lower salinity than seawater. Observed vent fluid salinities vary between 5 and 200% of the seawater value (3.2 wt% NaCl) These salinity variations are interesting in the context of metal mobilization and transport. Most metals, including iron (Fe), are chlorocomplexing and tend to travel with the more saline fluid phase, implying that phase separation has a first-order effect on vent fluid chemistry (Butterfield et al 1997; Jr. Seyfried et al 1991; Von Damm 2004).
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