Beneath mid-ocean ridges, magma is thought to rise through a network of high porosity channels that form by reactive flow. Partial mantle melts travel rapidly through these channels to the surface, and retain the geochemical signature of their source rock. Global analyses of mid-ocean ridge lavas indicates that the mantle is chemically heterogeneous, but the consequences of this heterogeneity for reactive porous flow remain unclear. Using numerical models of coupled magma/mantle dynamics, we investigate the relationships between mantle heterogeneity, melting, and magmatic channelization. The models are based on conservation mass, momentum, energy, and composition in a system with two phases and two thermodynamic components in local thermodynamic equilibrium. One of these components is more fusible than the other. In this context, we find that heterogeneities enriched in the more fusible component can nucleate magmatic channels. To understand this result we consider an expression for the melting rate derived from the conservation principles. This expression quantifies the relationship of decompression, reactive flow, and thermal diffusion to the melting rate. With it, we assess their relative importance in the ambient mantle, channels, and enriched heterogeneities. In our models, heat diffuses into fertile channels and powers melting, in combination with reactive flow. These results suggest that thermal diffusion influences the dynamics of magmatic channelization.