Depletion of a brine layer at the base of ridge-crest hydrothermal systems

The variable salinity of fluid venting from mid-ocean ridges is indicative of mixing between hydrothermal seawater and fluids that have undergone supercritical phase separation. In order to study the stability of a brine-saturated layer that may form in the lowermost part of the hydrothermal system, we have performed numerical simulations of a system that has returned into the subcritical regime. For typical geological parameters, it is shown that the interface between the brine layer and the overlying fluids is not very stable, but vanishes by one of two dynamical mechanisms: convective breakdown or vertical migration. This contradicts the conventional picture of a steady, layered convective system in which the brine is depleted only by dispersion and diffusion across the interface. The depletion mechanism depends on the fluid-dynamical stability of the brine layer. Convection within the brine layer results either in the convective breakdown (for low excess salinity of the brine, as compared to seawater) or the upward migration of the interface (for higher excess salinities). Consequently, the depletion times are much shorter than for models with pure dispersion/ diffusion across the interface. If the brine layer is static, high-chlorinity liquid is entrained slowly by the convecting overlying fluids, leading to downward migration of the interface. This gradual depletion of the brine layer results in almost constant vent salinities, in agreement with measured salinities of chronic high-chlorinity vents. ß 2000 Elsevier Science B.V. All rights reserved

The formation and evolution of layered structures in porous media: effects of porosity and mechanical dispersion

Horizontally layered structures can develop in porous or partially molten environments, such as hydrothermal systems, magmatic intrusions and the early Earth's mantle. The porosity f of these natural environments is typically small. Since dissolved chemical elements unlike heat cannot diffuse through the solid rocks, heat and solute influence the interstitial fluid density in a different manner: heat advects slower than solute through the liquid by the factor f, while diffusion of heat through the bulk porous medium is larger by the factor f y 1 times the ratio between the thermal and chemical diffusivities. By performing numerical experiments in which a rigid low-porosity medium is heated from below, we have studied the formation and evolution of layers in an initially stably stratified liquid. Growth of a convective layer through convective entrainment, the formation of a stable density interface on top of the layer and destabilization of the next layer are intimately . . linked. By monitoring the heat solute fluxes, it is observed that the transport of heat solute across the interface changes . from convective entrainment towards a regime in which transfer is purely diffusive dispersive . Because this transition occurs before the stage at which the lower layer arrives at the thermal equilibrium, we conclude that the layer growth stops when the density interface on top has grown sufficiently strong to keep the ascending plumes in the lower layer from convectively entraining more fluid from above. A simple balance between the most important forces, exerted on a fluid parcel in the lower layer, is proposed to determine this transition. This force balance also indicates whether a density interface keeps intact, migrates upwards or breaks down during the further evolution of the layered sequence. Finally, mechanical dispersion tends to increase transport of chemically dissolved elements across the density interface. Since this reduces the density difference between the two adjacent layers, the thickness of the lower layer increases. q2000 Elsevier Science B.V. All rights reserved

Chaotic thermohaline convection in low-porosity hydrothermal systems

Fluids circulate through the Earth's crust perhaps down to depths as great as 5^15 km, based on oxygen isotope systematics of exhumed metamorphic terrains, geothermal fields, mesozonal batholithic rocks and analysis of obducted ophiolites. Hydrothermal flows are driven by both thermal and chemical buoyancy; the former in response to the geothermal gradient and the latter due to differences in salinity that appear to be ubiquitous. Topographically driven flows generally become less important with increasing depth. Unlike heat, solute cannot diffuse through solid matrix. As a result, temperature perturbations advect more slowly than salinity fluctuations by the factor P, but diffuse more rapidly by the factor U/D and are so smoothed out more efficiently. Here, P is porosity, while U and D denote the thermal and chemical molecular diffusivity, respectively. Double-advective instabilities may play a significant role in solute and heat transport in the deep crust where porosities are low. We have studied the stability and dynamics of the flow as a function of P and thermal and chemical buoyancy, for situations where mechanical dispersion of solute dominates over molecular diffusion in the fluid. In the numerical experiments, a porous medium is heated from below while solute provides a stabilizing influence. For typical geological parameters, the thermohaline flow appears intrinsically chaotic. We attribute the chaotic dynamical behavior of the flow to a dominance of advective and dispersive chemical transfer over the more moderate convective heat transfer, the latter actually driving the flow. Fast upward advective transport and lateral mixing of solute leads to formation of horizontal chemical barriers at depth. These gravitationally stable interfaces divide the domain in several layers of distinct composition and lead to significantly reduced heat flow for thousands of years. The unsteady behavior of thermochemical flow in low-porosity regions has implications for heat transport at mid-ocean ridges, for ore genesis, for metasomatism and metamorphic petrology, and the diagenetic history of sediments in subsiding basins. ß 1999 Elsevier Science B.V. All rights reserved