HR: 1340h
AN: B13A-0202    [Abstracts]
TI: Physical Constraints On The Formation And Storage Of Brines In Mid-Ocean Ridge Hydrothermal Systems
AU: * Fontaine, F J
EM: fontaine@ocean.washington.edu
AF: University of Washington - School of Oceanography, Box 357940, Seattle, WA 98195 United States
AU: Wilcock, W S
EM: wilcock@ocean.washington.edu
AF: University of Washington - School of Oceanography, Box 357940, Seattle, WA 98195 United States
AU: Rabinowicz, M
EM: michel.rabinowicz@cnes.fr
AF: UMR CNRS 5562 "dynamique terrestre et planetaire", Observatoire Midi-Pyrenees 14 Avenue Edouard Belin, Toulouse, 31400 France
AB: Mid-ocean ridge hydrothermal systems are known to vent fluids with salinities substantially different from seawater. This is attributed to phase separation and the segregation of the resulting vapor and brine phases. Time series of vent temperature and salinity (chlorinity) show that some black-smoker vent fields have vented fluids with salinities well below seawater for over a decade. This raises important questions concerning chloride mass conservation and the fate of brines in these systems. One widely accepted model is that high-density brines formed during super-critical phase separation sink efficiently to the base of hydrothermal systems, leading to the development of a two-layer system in which a re-circulating brine layer underlies a single-pass seawater cell. However, there is no conclusive evidence for such a two-layer configuration or for the assumption that a brine layer will convect. In this study we first present theoretical arguments to constrain the dynamics of such a deep brine layer. From an analysis of brine properties in the two-phase area, we conclude that, if brines are stored in a layer at the base of high-temperature mid-ocean ridge hydrothermal systems they are unlikely to convect because phase separation will lead to a stable stratification. One consequence of this result is that the brine layer beneath black systems has to be thin ($<$ 10m) 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 and below vents near $9\deg$50'N on the East Pacific Rise suggest that the brine layer is likely at least 100 meter thick. To resolve this apparent paradox we propose an alternative model which we support with both conceptual arguments and inferences from single-phase numerical models. It is generally believed that the pressure gradients in mid-ocean ridge hydrothermal systems are close to cold hydrostatic. At the high temperatures and pressures characteristic of the deeper parts of these systems brines with salinities as high as 20-30 wt% NaCl have densities around 800-900 kg/m$^{3}$ and will be buoyant in a cold-hydrostatic system. 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 that will rise only slowly because of their higher density and viscosity and the low permeability of brine filled fissures. Our numerical models suggest that brines that rise will reach a level of neutral buoyancy as they cool and enter high permeability regions in which the pressure gradients decrease.
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting