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

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.

Active Basalt Alteration at Supercritical Conditions in a Seawater‐Recharged Hydrothermal System: IDDP‐2 Drill Hole, Reykjanes, Iceland

“…Phase separation by brine condensation leads to phase segregation of a halite‐saturated brine and vapor‐like fluid that further separate due to higher buoyancy forces of the vapor phase. Accumulation of brine and precipitation of halite in the deep reaction zone drastically changes the permeably and heat transfer in the deep reaction zone (Scott et al., 2017; Vehling et al., 2020). Rates of vapor mass flux and convective heat transfer are maximized in the brine condensation zones, in contrast to shallower systems that phase separate at lower pressures by boiling.…”

“…Examples of high temperature brines condensed from seawater by phase separation at high temperature are less common (Vehling et al., 2020). The highest salinity hydrothermal fluids (excluding those which have interacted with evaporites) are from the Cleft Segment of the Juan de Fuca ridge where vent fluid salinity reaches approximately twice seawater values.…”

Integrating Multidisciplinary Observations in Vent Environments (IMOVE): Decadal Progress in Deep-Sea Observatories at Hydrothermal Vents

“…While these data sets pertaining to seismic and fluid flow studies provide valuable constraints for coupled geodynamichydrothermal models (e.g., Theissen-Krah et al, 2016), these snapshots of the "current state" at a specific location are difficult to interpret in the context of a dynamically evolving hydrothermal and magmatic system. At the same time, numerical models have evolved in recent years and can now handle more realistic thermodynamic fluid properties and twophase flow phenomena (Coumou et al, 2009;Lewis and Lowell, 2009;Afanasyev et al, 2018;Vehling et al, 2021). An outstanding strength of numerical forward models is their ability to establish process-based causal links between different observations and data sets, including spatial and temporal dependencies.…”

Thermolab: A Thermodynamics Laboratory for Nonlinear Transport Processes in Open Systems

“…Predicting causes and effects of climate change, the reduction of global warming by storing CO 2 or the safety of storing nuclear waste, strongly rely on the understanding of underlying coupled physical processes (Orr, 2018; X. Zhang et al., 2022). Furthermore, the exploration of fossil fuels and raw materials in economic ore deposits, and most importantly the transition to renewable energy rely on a good understanding of the physics of geological processes (Feng et al., 2021; Vehling et al., 2020; Weis et al., 2012). In many cases, numerical models that solve the mathematical equations governing the physics are the only way to predict such natural processes.…”