Numerical experiments are used to examine the effect of buoyant mantle flow on melt generation near spreading centers. Buoyancy results from (1) the depletion of residual mantle in Fe relative to Mg (mantle-depletion buoyancy), (2) the presence of low-density melt (melt-retention buoyancy), and (3) thermal expansion (thermal buoyancy). Thermal buoyancy drives off-axis convective rolls that develop closer to the spreading axis at slower spreading rates. Melt-retention buoyancy drives focused upwelling into isolated melting centers. With increasing spreading rate, the amplitude of on-axis crustal segmentation decreases, the melting region being essentially two-dimensional at full spreading rates greater than 100 km/m.y. These numerical experiments predict centers of mantle upwelling that are more widely spaced than the actual wavelength of inferred crustal thickness variations. This discrepancy indicates that short wavelength segmentation of slow spreading centers requires some process not included in our models of mantle flow. At fast spreading rates, when the lithosphere is thinner, melt-retention buoyancy gives rise to isolated, focused centers of off-axis upwelling and melt production. Off-axis melting centers begin to form at about 300 km from the axis, with a characteristic along-axis spacing of 200 km. Successive off-axis melting centers are shifted along axis, so that crustal thickness far from the axis is increased at a 100 km wavelength. ¿ 1997 American Geophysical Union