Heat-flow measurements have been made in five deep crustal holes drilled into the Mid-Atlantic Ridge by Glomar Challenger that provide new information on the hydrothermal circulation that probably controls the crustal temperature distribution near ocean ridge spreading centers. Nearly constant very low heat flow of 0.6 ±0.1 µcal cm" 2 sec" 1 (25 ±4 mW m~2) was found for three holes and three ocean probe measurements across one 3-km-wide sediment pond on 3.5 m.y. old sea floor and to a depth of 400 meters into the underlying basaltic basement. The heat flow predicted for theoretical models of plate accretion is much higher, about 6.4 (269). Heat must be carried to the surface by extensive hydrothermal circulation that may extend to the lower part of the crust. Temperature measurements in the basement part of one borehole into 13 m.y. old sea floor indicate that seawater was flowing rapidly down the hole into a permeable horizon near the bottom. The heat flow of 0.5 (21) measured in the overlying sediments suggests that before the hole was drilled, water may have been flowing horizontally in this horizon at about 10°C.

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