The Hydrothermal “Clogged Shell” Model Revisited Using Coupled Reactive Fluid Flow (OpenFOAM + Reaktoro) – Feedback Between Vent Fluid Salinity, Temperature, and Anhydrite Precipitation

Black-smoker-type hydrothermal vent systems are a feature of all mid ocean ridges. They often sit atop developing massive sulfide deposits, such as the TAG mound in the central Atlantic. The measured apparent upper limit for vent fluid temperatures at these sites around 400°C can be explained with the thermodynamic properties of water [1]. However, continuum-scale numerical models of seawater and hydrothermal fluid circulation commonly fail to reproduce these high vent temperatures under realistic assumptions of host rock permeability. While most discharge of circulating seawater does occur diffusively and at low temperatures, an explanation for the extreme focusing of flow at hot vent sites is needed.

One common approach to resolve this is the so-called “clogged shell” model, where the precipitation of mainly anhydrite at the interface of rising hot fluids and entrained seawater locally lowers permeability around the hydrothermal plume, preventing mixing and increasing vent temperatures [2]. This concept has been validated in a number of studies [e.g., 3], but no fully coupled model of hydrothermal fluid flow and fluid-rock interaction in such systems exists.

Using a newly developed coupling of open-source C++ libraries to solve fluid flow in 2D and 3D (OpenFOAM) and local equilibrium thermodynamics (Reaktoro [4]), we investigate feedback between reactive fluid flow, anhydrite precipitation and vent temperatures.

Anhydrite solubility decreases with higher temperatures, leading to precipitation from heated seawater at the interface with rising hot hydrothermal fluids. Solubility also depends on salinity, increasing in saltier fluids [5]. Thus, we vary hydrothermal fluid salinity between 0 and 5 wt%, based on vent fluid measurements.

Our results clearly show that anhydrite precipitation occurs around the plume and inhibits mixing, focusing the hot upflow and increasing vent temperatures over time. These effects are strongly dependent on fluid salinity: Initial vent temperatures are highest with high salinity, linked to thermodynamic properties of water. Over time, lower salinity hydrothermal fluids produce a narrower anhydrite shell, leading to stronger focusing and a steeper vent temperature increase.

Figure 1. Model results: (a) 2D anhydrite shell (b) cut 3D Anhydrite shell (c) vent temperature over time with variable hydrothermal fluid salinity.

 

References

[1] Jupp, T. and A. Schultz, A thermodynamic explanation for black smoker temperatures. Nature, 2000. 403(6772): p. 880-3.

[2] Cann, J.R. and M.R. Strens, Modeling periodic megaplume emission by black smoker systems. Journal of Geophysical Research: Solid Earth, 1989. 94(B9): p. 12227-12237.

[3] Guo, Z., et al., Anhydrite‐Assisted Hydrothermal Metal Transport to the Ocean Floor—Insights From Thermo‐Hydro‐Chemical Modeling. Journal of Geophysical Research: Solid Earth, 2020. 125(7).

[4] Leal, A.M.M. Reaktoro: An open-source unified framework for modeling chemically reactive systems. 2015; Available from: https://reaktoro.org.

[5] Creaser, E.C., M. Steele-MacInnis, and B.M. Tutolo, A model for the solubility of anhydrite in H2O-NaCl fluids from 25 to 800 °C, 0.1 to 1400 MPa, and 0 to 60 wt% NaCl: Applications to hydrothermal ore-forming systems. Chemical Geology, 2022. 587.