Egress of hot water from midocean ridge hydrothermal systems: Some thermal constraints

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Abstract
The flow rates necessary for hot water to vent directly into the ocean at midocean ridge hydrothermal systems are obtained by balancing heat lost by conduction through the walls of the hydrothermal passages with the heat brought up in the water. Widespread porous flow is unlikely to vent hot water even at the ridge axis. More spatially restricted flow was modeled by planar and tubular crack zones. The planar zones can vent water only through young (age <1 m.y. B.P.) crust and then must be spaced on the order of kilometers. Tubular zones vent hot water on moderately old crust (1 m.y. B.P. <age <11 m.y. B.P.). A factor of 2 or 3 change in flow rate can change the temperature in a crack zone from slow cooling as the fluid rises to venting of hot water. Clogging will eliminate vents where extensive cooling or mixing occurs in the uppermost regions. The flow volumes predicted for cracks are large enough that minor and trace constituents may clog crack zones.Appendix is available with entire article on microfiche. Order fromAmerican Geophysical Union, 1909 K Street, N. W., Washington, D. C. 20006. Document J78‐008; $1.00. Payment must accompanyorder.