THE ROLE OF DENSITY IN THE ACCUMULATION OF BASALTIC MELTS AT MIDOCEAN RIDGES

It is commonly assumed that magma ponds at a level of neutral buoyancy in the shallow crust where melt densities are equal to the bulk density of the surrounding crust. At the East Pacific Rise this neutral buoyancy level lies only 100-400 m below the sea floor, significantly shallower than the depths (> 1-2 km) of the magma bodies imaged in multichannel reflection data, suggesting that other factors must control the collection of melt in these reservoirs. The apparent inverse relationship between magma chamber depth and spreading rate at intermediate and fast spreading ridges suggests that the thermal structure of the rise axis, not the buoyancy of melt, is the primary factor that controls the depth at which melt ponds in crustal magma chambers beneath mid-ocean ridges.