A model based on the fluid dynamics of intermixed fluids indicated that favorable conditions for segregation of melt from a mostly crystalline mush upwelling from the asthenosphere included a high concentration of melt, a narrow conduit containing the mush, a large grain size, and a large ratio of the viscosity of grains to the viscosity of melt. The greater depth, inferred geochemically, of the source regions of seamount lavas, compared with mid-oceanic ridge lavas, is attributable to the dependence of segregation on conduit width. For thermal reasons, conduit width is smaller at greater depths for small sources of material such as seamounts. Observed systematic enrichment of seamount and island-arc lavas in radiogenic isotopics (which, for chemical reasons, would be most abundant in domains containing high fractions of water) relative to mid-oceanic ridge lavas may be due to disproportionate representation of ubiquitous, small-scale, water-rich domains in the source region of seamount lavas. The fraction of melt is strongly dependent on water content in the deep source regions of seamount lavas but only weakly dependent in the shallow source regions of mid-oceanic ridge lavas. This also complicates identification of subducted sediments in island-arc lavas. The viscosity of partial melt in the asthenosphere, inferred from seismic attenuation studies, is too high for that melt to segregate efficiently.