Melt transport rates in heterogeneous mantle beneath mid-ocean ridges

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 localised flow. A corollary of this finding is that both mantle heterogeneity 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.