Magma plumbing at fast-spreading mid-ocean ridges

Recent years have seen a step change in understanding of components of mid-ocean ridge magma plumbing systems. At fast-spreading ridges, seismic studies have now imaged sills in the low-velocity zone beneath the axial melt lens. Combined with the observation that primitive cumulates are largely restricted to the deep parts of the lower crust, and that these rocks generally lack strong lattice preferred orientations, this suggests that a significant proportion of primary melts are emplaced and differentiated deep in the lower crust. Mineral chemical data require that these melts are then transported through the lower crustal crystal mush both by rapid focused flow and porous flow, the latter imparting a reactive geochemical signature on the melts. Both types of melts are collected in the axial melt lens, where they mix prior to eruption, accounting for the observed over-enrichment in MORB trace element relative to fractional crystallization models. Paradoxically, the rocks representing the fossilized axial melt lens are evolved, and generally not in equilibrium with MORB. This suggests that the residence time of MORB in the melt lens is limited. These combined seismic and petrological constraints lead us to propose an in-situ, dual-porosity model for the fast-spreading ridge magma plumbing system.