Dynamics and storage of brine in mid‐ocean ridge hydrothermal systems

Abstract

[1] Mid-ocean ridge hydrothermal systems are known to vent fluids with salinities substantially different from seawater as a result of phase separation and segregation of the resulting vapor and brine phases. Time series of vent temperature and salinity (chlorinity) show that some black-smoker vent fields such as the Main Endeavour Field on the Juan de Fuca Ridge have vented fluids with salinities well below seawater for over a decade, which raises important questions concerning the fate of brines in these systems. One widely accepted model is that high-density brines formed by supercritical phase separation sink to the base of hydrothermal systems, leading to the development of a two-layer system in which a recirculating brine layer underlies a single-pass seawater cell. We first present theoretical arguments to constrain the dynamics of such a deep brine layer in a system still undergoing phase separation, and we conclude that if brines are stored in a basal layer, they are unlikely to convect because they will be stably stratified. One consequence of this result is that the brine layer beneath black smoker systems has to be thin (<10 m) to match the high heat fluxes. However, estimates of the rate at which brines are accumulating in the crust below the main field on the Endeavour segment of the Juan de Fuca Ridge suggest that the brine layer is likely at least 100 m thick. To resolve this apparent paradox, we propose an alternative model. We argue that interfacial tensions between fluid and solid phases will likely favor the segregation of vapor into the main fractures and brine into the smaller fissures and backwaters. This allows the vapor to flow efficiently through the system and transport large heat fluxes while most of the porosity in the lower part of the system fills with brines. It is generally believed that the pressure gradients in mid-ocean ridge hydrothermal systems are close to cold hydrostatic. At the high temperatures and pressures characteristic of the deeper parts of these systems, brines with salinities as high as 20 wt % NaCl have densities around 800-900 kg m À3 and will be buoyant in a cold-hydrostatic system. Rather than sinking to the base of the system, it is possible that brines produced by supercritical phase separation rise slowly until they reach a level of neutral buoyancy as they cool or enter high-permeability regions in which the pressure gradients decrease.

A preliminary 1‐D model investigation of tidal variations of temperature and chlorinity at the Grotto mound, Endeavour Segment, Juan de Fuca Ridge

“…Unlike the vertical temperature gradient caused by conductive and adiabatic heat loss, which can persist through out the discharge zone, the chlorinity gradient may only exist within layer 2B assuming a permeability contrast between layers 2A and 2B. According to Fontaine and Wilcock [2006], when layer 2A has much larger permeability than layer 2B, the vertical pressure gradient driving the upflow in layer 2A is much smaller than the pressure gradient in layer 2B. As a result, the rising brine becomes negatively buoyant after it crosses the interface and ultimately starts sinking.…”

“…The conceptual model of the storage of brine within the discharge zone of a hydrothermal circulation cell (whereby brine preferentially fills small fissures, dead ends, and covers the inner walls of the main conduit through which the vapor flows [Fontaine and Wilcock, 2006]) should allow another explanation for the tidal oscillations in venting chlorinity. As illustrated in Figure 10, if the inner walls of the main conduits through which the vapor rises are covered by brine, then chloride will be transferred from brine to vapor through Figure 9.…”

“…Also, notice the cutoff of brine density at q b 51000 kg=m 3 . Such a cutoff density is chosen based on the assumption that the pressure gradient within the lower hydrothermal discharge zone in layer 2B is close to cold hydrostatic [Jupp and Schultz, 2004a;Fontaine and Wilcock, 2006] and thus only brines with density lower than that of the cold pore fluid ($1000 kg=m 3 ) will rise from the basal reaction zone and reach 700 mbsf. Consequently, Figure 8 suggests the minimum vapor compressibility required to interpret the tidal oscillations of temperature and chlorinity is b f 51:9310 27 Pa 21 at P r 50:8.…”

“…The hydrothermal circulation system consists of a broad recharge zone (presumably primarily on axis), fluid heated at the base of the sheeted dikes just above the axial magma chamber (AMC), and a focused upflow zone [e.g., Fontaine and Wilcock, 2006;Coogan, 2008;Coumou et al, 2009]. Based on the seismic study of Van Ark et al [2007], the hydrothermal upflow zone beneath the MEF is represented in the model as a poroelastic medium comprising the typical seismic layers 2A and 2B of zero-age oceanic crust (Figure 3).…”

“…For the current study, brine and vapor refer broadly to fluids that are enriched and depleted in chloride, respectively, compared to seawater. According to Fontaine and Wilcock [2006], as a result of interfacial tensions, rising brine preferentially fills small fissures, dead ends, and backwater porosity thereby covering the inner walls of the main conduits through which vapor flows (Figures 3b and 3c). This is because brine is denser and thus forms a higher density of hydrogen bonds and likely contains a higher proportion of free ions that will enhance the adhesion of brine to rock compared to vapor.…”