Abstract
Hydrothermal circulation at the ridge axis removes heat from the oceanic crust more rapidly than would conduction alone. The top of the axial magma chamber is thus deeper and possibly wider than the theoretical shape computed from conductive thermal models. At 9°N on the East Pacific Rise seismic reflection indicates that the roof of the magma chamber is relatively flat, 2 km deep, and extends 4 km from the axis. This is about a kilometer deeper than predicted by a purely conductive model.
We believe that the magma chamber is mostly filled with mush at ridges with both fast and slow spreading rates. At fast rates the mush is formed by crystallization at the top of a magma chamber that is wide and flat topped. At slow rates a narrow magma chamber is probably an anastomosing complex of partially molten dikes and associated cumulate layers. Thermal modeling indicates that the hydrothermal heat flux is between 0.7×108 and 1.5×108 cal/cm2, or less than 1/10 of the total missing heat flux (the difference between obsereved and theoretical heat flow) at the ridge axis. By using the observation that Mg is totally depleted from exiting axial fluids, we find that the minimum amount of crust which reacts with axial hydrothermal flow is equivalent to a 80 m thick section of crust. A minimum thickness of 200 m is obtained from K which is leached from the basalt into the hydrothermal fluid. These estimates indicate that there is no requirement that the bulk of the oceanic crust react strongly with the axial hydrothermal fluid.
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Sleep, N.H., Morton, J.L., Burns, L.E., Wolery, T.J. (1983). Geophysical Constraints on the Volume of Hydrothermal Flow at Ridge Axes. In: Rona, P.A., Boström, K., Laubier, L., Smith, K.L. (eds) Hydrothermal Processes at Seafloor Spreading Centers. NATO Conference Series, vol 12. Springer, Boston, MA. https://doi.org/10.1007/978-1-4899-0402-7_4
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