We review physical and chemical constraints on the mechanisms of melt extraction from the mantle beneath mid-ocean ridges. Compositional constraints from MORB and abyssal peridotite are summarized, followed by observations of melt extraction features in the mantle, and constraints front the physical properties of partially molten peridotite. We address two main issues. (1) To what extent is melting 'near-fractional', with low porosities in the source and chemical isolation of ascending melt? To what extent are other processes, loosely termed reactive flow, important in MORB genesis? (2) Where chemically isolated melt extraction is required, does this occur mainly in melt-filled fractures or in conduits of focused porous flow?
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