In the upper crust, where hydraulic gradients are typically 10 MPa km−1, the mean permeabilities required to accommodate the estimated metamorphic fluid fluxes decrease from ∼10−16 m² to ∼10−18 m² between 5- and 12-km depth Below ∼12 km, which broadly corresponds to the brittle-plastic transition, mean k is effectively independent of depth at ∼10−185±1 m² Consideration of the permeability values inferred from thermal modeling and metamorphic fluxes suggests a quasi-exponential decay of permeability with depth of log k ≈ −32 log z - 14, where k is in meters squared and z is in kilometers At mid to lower crustal depths this curve lies just below the threshold value for significant advection of heat Such conditions may represent an optimum for metamorphism, allowing the maximum transport of fluid and solute mass that is possible without advective cooling

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Permeability of the continental crust: Implications of geothermal data and metamorphic systems

We investigate groundwater flow near cooling plutons with a computer program that can model multiphase flow, temperatures up to 1200°C, thermal pressurization, and temperature-dependent rock properties. A series of experiments examines the effects of host-rock permeability, size and depth of pluton emplacement, single versus multiple intrusions, the influence of a caprock, and the impact of topographically driven groundwater flow. We also reproduce and evaluate some of the pioneering numerical experiments on flow around plutons. Host-rock permeability is the principal factor influencing fluid circulation and heat transfer in hydrothermal systems. The hottest and most steam-rich systems develop where permeability is of the order of 10−15 m2. Temperatures and life spans of systems decrease with increasing permeability. Conduction-dominated systems, in which permeabilities are ≤10−16 m2, persist longer but exhibit relatively modest increases in near-surface temperatures relative to ambient conditions. Pluton size, emplacement depth, and initial thermal conditions have less influence on hydrothermal circulation patterns but affect the extent of boiling and duration of hydrothermal systems. Topographically driven groundwater flow can significantly alter hydrothermal circulation; however, a low-permeability caprock effectively decouples the topographically and density-driven systems and stabilizes the mixing interface between them thereby defining a likely ore-forming environment.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JB00552

Multiphase groundwater flow near cooling plutons

[1] The dynamic behavior of magmatic hydrothermal systems entails coupled and nonlinear multiphase flow, heat and solute transport, and deformation in highly heterogeneous media. Thus, quantitative analysis of these systems depends mainly on numerical solution of coupled partial differential equations and complementary equations of state (EOS). The past 2 decades have seen steady growth of computational power and the development of numerical models that have eliminated or minimized the need for various simplifying assumptions. Considerable heuristic insight has been gained from process-oriented numerical modeling. Recent modeling efforts employing relatively complete EOS and accurate transport calculations have revealed dynamic behavior that was damped by linearized, less accurate models, including fluid property control of hydrothermal plume temperatures and three-dimensional geometries. Other recent modeling results have further elucidated the controlling role of permeability structure and revealed the potential for significant hydrothermally driven deformation. Key areas for future research include incorporation of accurate EOS for the complete H2O-NaCl-CO2 system, more realistic treatment of material heterogeneity in space and time, realistic description of large-scale relative permeability behavior, and intercode benchmarking comparisons.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009RG000287

Numerical simulation of magmatic hydrothermal systems

Sub-seafloor hydrothermal convection at mid-ocean ridges transfers 25% of the Earth's heat flux and can form massive sulfide ore deposits. Their three-dimensional (3D) structure and transient dynamics are uncertain. Using 3D numerical simulations, we demonstrated that convection cells self-organize into pipelike upflow zones surrounded by narrow zones of focused and relatively warm downflow. This configuration ensures optimal heat transfer and efficient metal leaching for ore-deposit formation. Simulated fluid-residence times are as short as 3 years. The concentric flow geometry results from nonlinearities in fluid properties, and this may influence the behavior of other fluid-flow systems in Earth's crust.

The structure and dynamics of mid-ocean ridge hydrothermal systems.

There is a remarkable difference between the maximum temperature of black smoker effluent (350 degrees C-400 degrees C) and the temperature of the solidifying magma which heats it (approximately 1,200 degrees C). It has been suspected for some time that the nonlinear thermodynamic properties of water might be responsible for this discrepancy. Here, we translate this hypothesis into a physical model, by examining the internal temperature structure of convection cells in a porous medium. We demonstrate that, at pressures appropriate to seafloor crust, plumes of pure water form naturally at approximately 400 degrees C for any heat source with temperature greater than approximately 500 degrees C. Higher temperatures are confined to a boundary layer at the base of the convection cell, where the flow is horizontal. The phenomenon is explained analytically using the thermodynamic properties of water, and is illustrated by numerical simulations. Our model predicts the existence of the high-temperature 'reaction zone' found in ophiolites and suggests that vent temperatures will remain steady as magma chambers solidify and cool.

A thermodynamic explanation for black smoker temperatures

Near-critical extrema in the properties of water may influence flow patterns in hydrothermal systems, but singularities in equations of state for H2O at its critical point have inhibited quantitative modeling Posing governing equations in terms of pressure (P) and enthalpy (H) avoids these singularities and facilitates computation Numerical simulations with a P-H based model show little near-critical enhancement in heat transfer for systems in which flow is driven by fixed pressure drops However, in density-driven systems, near-critical variations in fluid properties can enhance convective heat transfer by a factor of 10² or more (“superconvection”) if permeability is sufficiently high Near-critical two-phase processes (“heat pipes”) are at least equally effective at dissipating thermal energy The restriction to high-permeability environments within a fairly narrow P-H window suggests that superconvection may be quite rare in natural systems

S. E. Ingebritsen

Papers

Fluid flow and heat transport near the critical point of H2O