The depth to the axial magma chamber at three oceanic spreading centers, as indicated by seismic reflection surveys, is greater than that predicted by conductive cooling thermal models of ridge crests. The additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust. A theoretical model for the temperature distribution at spreading centers, which includes distributed heat sources and sinks, is presented. By representing the hydrothermal heat loss as a series of heat sinks, the depths to the axial magma chamber at the East Pacific Rise at latitude 9øN., the southern Juan de Fuca Ridge, and the Lau Basin spreading center are modeled. Heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys. The thermal modeling indicates that the axial hydrothermal heat flux is between 7.1 x 10 6 and 13.8 x 10 6 MJ/m 2, or 10 to 20% of the total missing heat at the ridge axis, requiring extensive low-temperature circulation off axis. High-temperature vents that are spaced 1 km apart and are active between 4 and 10% of the time would account for the axial hydrothermal heat loss.

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