MELTING AND MANTLE FLOW BENEATH A MIDOCEAN SPREADING CENTER

A series of numerical experiments on mantle flow, melt generation, and melt migration beneath mid-ocean ridges is reported. In these experiments, flow in the mantle is driven by the motion of the surface plates and melting-induced mantle density variations. Mantle densities are controlled by the proportions of minerals in the residual mantle and their Fe/Mg ratio. Flow due to thermal variations in density are ignored. Melting results from the decompression of the upwelling mantle beneath the ridge, and melt production rates are determined from variations in mantle temperature above a solidus that depends explicitly upon pressure, mantle mineralogy, and mantle oxide composition. Melt migration is driven by mantle pressure gradients and melt buoyancy forces. The mantle temperature is influenced by the latent heat of melting and the advection of heat due to flow of the melt and mantle. Experiments with solely plate driven flows illuminate the results of including these latter thermal effects. The latent heat of melting lowers mantle temperatures to the solidus wherever melting occurs and also has an influence on the distribution of melting. Advection of heat due to melt migration increases melting rates up the melting column. For low mantle viscosities (10(18) Pa s) the latter effect is minor as melt rises essentially vertically. The thermal effect of the melt, however, will become more important as melting rates increase and if melt is focused into a narrow region. At slow spreading rates (1 cm yr-1), depletion-driven mantle flow markedly increases crustal thicknesses as melting rates increase to match the enhanced advection of mantle heat. This additional flow also causes the melting region to become narrower but is an insufficient mechanism for focusing melt to the ridge axis. The rapid extraction of the melt phase from the mantle results in an enhanced upwelling beneath the ridge which in turn leads to higher melt production rates and greater crustal thicknesses. Solidification of the melt phase at its liquidus results in melt freezing rates of order ten times greater than average melt production rates. High freezing rates are the result of high vertical temperature gradients in the cool oceanic lithosphere where the freezing occurs. The release of latent heat upon solidification has a negligible effect upon melting rates within the melting regime. Buoyancy forces resulting from the presence of an interstitial melt phase can result in a significant narrowing of the mantle upwelling beneath the ridge and also the width of the melt regime. The importance of melt buoyancy forces in shaping mantle flow depends on their magnitude relative to buoyancy forces resulting from density changes in the solid mantle.