A REVIEW OF MELT MIGRATION PROCESSES IN THE ADIABATICALLY UPWELLING MANTLE BENEATH OCEANIC SPREADING RIDGES

We review physical and chemical constraints on the mechanisms of melt extraction from the mantle beneath mid-ocean ridges. Compositional con straints from MORB and abyssal peridotite are summarized, followed by observations of melt extraction features in the mantle, and constraint s front the physical properties of partially molten peridotite. We add ress 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 flo w, important in MORB genesis? (2) Where chemically isolated melt extra ction is required, does this occur mainly in melt-filled fractures or in conduits of focused porous flow? Reactive flow plays an important r ole, but somewhere in the upwelling mantle melting must be 'near fract ional', with intergranular porosities less than 1%, and most melt extr action must be in isolated conduits. Two porosity models provide the b est paradigm for this type of process. Field relationships and geochem ical data show that replacive dunites mark conduits for focused, chemi cally isolated, porous flow of mid-ocean ridge basalt (MORB) in the up welling mantle. By contrast, pyroxenite and gabbro dikes are lithosphe ric 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 d oes not rule out hydrofracture. Predicted porous flow velocities satis fy Th-230 excess constraints (ca. 1 m yr(-1)), provided melt extractio n occurs in porous conduits rather than by diffuse flow, and melt-free , solid viscosity is less than ca. 10(20) Pa s. Melt velocities of ca. 50 m yr(-1) are inferred from patterns of post-glacial volcanism in I celand and from Ra-226 excess. If these inferences are correct, minimu m conditions for hydrofracture may be reached in the shallowest part o f melting region beneath ridges. However, necessary high porosities ca n only be attained within pre-existing conduits for focused porous flo w. Alternatively, the requirement for high melt velocity could be sati sfied in melt-filled tubes formed by dissolution or mechanical instabi lities. Melt-filled fractures or tubes, if they form, are probably clo sed at the top and bottom, limited in size by the supply of melt. Ther efore, to satisfy the requirements for geochemical isolation from surr ounding 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 dun ite zones must be present before melt extraction in fractures or tubes .