DIFFERENT parts of the global mid-ocean ridge system create oceanic crust at rates that differ by more than a factor of ten. The rates of supply of heat to these different systems must also vary substantially, and it is this heat supply, in the form of hot mantle material rising beneath the ridge axis, that determines the nature of the magmatic, tectonic and hydrothermal processes that control the production of new oceanic crust. Here we present evidence suggesting that a systematic relationship exists between the depth to zones of elevated temperature and partial melt beneath mid-ocean ridges and their spreading rate. Recent seismic measurements provide robust determinations at six different locations of the depth to the top of the axial low-seismic-velocity zone which suggest that as full spreading rate decreases from 150 to 20 mm yr-1, the depth below the sea floor to the top of the LVZ increases from approximately 1 to approximately 4 km. Although this relationship might arise in a number of ways, our preferred explanation is that the major faults that extend to greater depths on slower-spreading ridges 1 allow a vigorous circulation of cooling sea water to reach greater depths, blocking the upward migration of large melt bodies.
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