Periodic formation of magma fractures and generation of layered gabbros in the lower crust beneath oceanic spreading ridges

There must be a transition from continuous porous flow to transient formation of melt-filled fractures in the region of magma transport below oceanic spreading centers. This transition may occur at permeability barriers that impede porous flow of melt, at the base of the oceanic crust and within the lower crust. We summarize evidence for formation of melt-filled lenses at the base of the crust in the Oman ophiolite, and evidence indicating that melt is very efficiently extracted from these lenses, probably in fractures. We also discuss the possible formation of melt lenses elsewhere in the oceanic lower crust. We then present a simple physical model for the periodic formation of melt-filled fractures originating in a melt lens beneath a permeability barrier. Finally, quantitative models show how modal layering in lower crustal gabbros can form as a result of the periodic pressure changes associated with fracture formation. An ancillary result of chemical modeling is the quantification of a thermal gradient in the lower crust of the Oman ophiolite during igneous accretion beneath a spreading center, from 1165 to 1195°C near the dike/gabbro transition to ~1240°C near the crust/mantle transition. This and other data for the Oman gabbros support models in which much of the lower crust forms by crystallization in sills at a variety of depths, from the dike/gabbro transition to the base of the crust.