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 from 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?
Reactive flow plays an important role, but somewhere in the upwelling mantle melting must be ‘near fractional’, with intergranular porosities less than 1%, and most melt extraction must be in isolated conduits. Two porosity models provide the best paradigm for this type of process. Field relationships and geochemical data show that replacive dunites mark conduits for focused, chemically isolated, porous flow of mid–ocean ridge basalt (MORB) in the upwelling mantle. By contrast, pyroxenite and gabbro dikes are lithospheric features; they do not represent conduits for melt extraction from the upwelling mantle. Thus, preserved melt extraction features do not require hydrofracture in the melting region. However, field evidence does not rule out hydrofracture.
Predicted porous flow velocities satisfy 230Th excess constraints (ca. 1 m yr-1, provided melt extraction occurs in porous conduits rather than by diffuse flow, and melt-free, solid viscosity is less than ca. 1020 Pa s. Melt velocities of ca. 50 m yr-1 are inferred from patterns of post–glacial volcanism in Iceland and from 226Ra excess. If these inferences are correct, minimum conditions for hydrofracture may be reached in the shallowest part of melting region beneath ridges. However, necessary high porosities can only be attained within pre–existing conduits for focused porous flow. Alternatively, the requirement for high melt velocity could be satisfied in melt–filled tubes formed by dissolution or mechanical instabilities.
Melt–filled fractures or tubes, if they form, are probably closed at the top and bottom, limited in size by the supply of melt. Therefore, to satisfy the requirements for geochemical isolation from surrounding peridotite, melt–filled conduits may be surrounded by a dunite zone. Furthermore, individual melt–filled voids probably contain too little melt to form sufficient dunite by reaction, suggesting that dunite zones must be present before melt extraction in fractures or tubes.