Sketch of MORB petrogenesis in the case of a heterogeneous mantle composed of pyroxenite veins (stippled, folded layers at the bottom) in a peridotite matrix. Processes acting at a given depth are listed in the boxes on the right: melting processes in red boxes, and peridotite-pyroxenite interactions in yellow boxes. The outcome of each process for the chemical and mineralogical evolution of mantle is summarized in the left column. The grey triangle is the melting zone: in its lower part, only pyroxenites are partially molten; in its upper part (hatched area), both pyroxenites and peridotites are partially molten. Black arrows above the peridotite solidus mark the onset of penetrative porous flow; white arrows represent melt focusing into high-permeability channels.

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