Abstract:
We present a melt percolation model incorporating finite solid diffusion to provide a quantitative constraint on how melt migrates through the oceanic lower crust at fast-spreading ridge axes. The lower crustal, layered gabbro in the Oman ophiolite, which was formed at a fast-spreading ridge, shows correlated variations in primary mineral compositions with a vertical wavelength less than 100-200 m. Possible effects of porous melt flow on these compositional variations are considered. The results of our numerical modeling indicate that melt transport by porous flow must be less than a few percent of the total incoming melt flux from the sub-ridge mantle, in order to preserve the observed correlations between different mineral compositions. It is also shown that rapid damping of compositional variations due to finite solid diffusion is likely, further reducing the potential role of porous flow melt transport. 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.