  Recent insights to melt migration beneath ridges suggest that channelized
flow is a consequence of melting of a heterogeneous mantle, and that spreading
rate modulates the dynamics of the localized flow. A corollary of this finding
is that both mantle het- erogeneity and spreading rate have implications for
the speed and time scale of melt migration. Here, we investigate these
implications using numerical models of magma flow in heterogeneous mantle
beneath spreading plates. The models predict that a broad distribution of magma
flow speeds is characteristic of the sub-ridge mantle. Within the melting
region, magmatic flow is fastest in regions of average fusibility;
surprisingly, magmas from sources of above-average fusibility travel to the
ridge in a longer time. Spreading rate has comparatively simple consequences,
mainly resulting in faster segregation speeds at higher spreading rates. The
computed time scales are short enough to preserve deep origin 230Th
disequilibria and, under favourable parameter regimes, also 226Ra excesses. An
important prediction from the models is that mantle heterogeneity induces
significant natural variability into U-series disequilibria, complicating the
identification of relationships between disequilibria and ridge properties or
chemical signatures of heterogeneity.
