HR: 1340h
AN: B13A-0207    [Abstracts]
TI: A Model for Hydrothermal Convection in the Near-Axis Region
AU: * Lowell, R P
EM: bob.lowell@eas.gatech.edu
AF: Georgia Institute of Technology, School of Earth and Atmospheric Sciences, Atlanta, GA 30332 United States
AB: Although high-temperature ridge axis convection and low-temperature circulation have received extensive study, relatively little is known concerning the transition zone between these environments. Recent seismic studies of the East Pacific Rise at 9$\deg$ 30'N indicates the presence of a large partial melt region, extending approximately 3 to 4 km on either side of the ridge axis. The partially molten body is capped by a thin melt lens, approximately 2 km wide at a depth of approximately 1.5 km, and the sides of the melt body are nearly vertical to a depth of 6 km, but gradually deepen over the next 6-7 km to a depth of roughly 8 km. This large region of partial melt is essentially defined by the 1150$\deg$C isotherm. Except very near the near-vertical sides of the partial melt zone, where isotherms are high compressed, the temperatures appear to increase nearly linear with depth. We consider two styles of hydrothermal convection in the oceanic crust in an attempt to explain these observations. In the first model, hydrothermal circulation is constrained the rate of heat conduction from below; and we assume the 1150$\deg$C isotherm simply deepens conductively as hydrothermal circulation extracts heat from the crust. We further assume that convection is constrained to lie above the 500$\deg$C isotherm, the crust being effectively impermeable at higher temperatures. Conductive cooling would place the 1150$\deg$C isotherm at a depth of 6 km in approximately 3.6 x 10$^{5}$ yr, but at a 6 cm/yr half-spreading rate, this depth would occur $\sim$ 20 km from the ridge axis. This is much farther than the seismic data indicate. In the second model, we assume that the steep- sided magma chamber drives a narrow vertical convective boundary layer. The application of boundary layer theory suggests that a boundary layer $\sim$ 100 wide would form if the permeability in the frozen gabbros were $\sim$ 10$^{-14}$ m$^{2}$. The heat transfer in the boundary layer appears to be roughly the same as the rate of heat input to the ridge axis to maintain seafloor spreading. Further from the axis, the linear isotherm distribution suggests weak cellular convection, perhaps restricted to the upper 2 km (pillows and dikes) of the crust.
DE: 8424 Hydrothermal systems (8135)
DE: 8135 Hydrothermal systems (8424)
DE: 4832 Hydrothermal systems
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting