MELT MIGRATION THROUGH THE OCEANIC LOWER CRUST - A CONSTRAINT FROM MELT PERCOLATION MODELING WITH FINITE SOLID DIFFUSION

We present a melt percolation model incorporating finite solid diffusi on to provide a quantitative constraint on how melt migrates through t he oceanic lower crust at fast-spreading ridge axes. The lower crustal , layered gabbro in the Oman ophiolite, which was formed at a fast-spr eading ridge, shows correlated variations in primary mineral compositi ons with a vertical wavelength less than 100-200 m. Possible effects o f porous melt how on these compositional variations are considered. Th e results of our numerical modeling indicate that melt transport by po rous flow must be less than a few percent of the total incoming melt f lux from the sub-ridge mantle, in order to preserve the observed corre lations between different mineral compositions. It is also shown that rapid damping of compositional variations due to finite solid diffusio n is likely further reducing the potential role of porous flow melt tr ansport. The dominant mode of melt migration through the oceanic lower crust must be more focused than pervasive porous flow, such as ascent in melt-filled fractures. (C) 1998 Elsevier Science B.V. All rights r eserved.