TEXTURAL CONSTRAINTS ON MELT MIGRATION PROCESSES IN THE OCEANIC CRUST

Differences in the textures of gabbros from the Southwest Indian Ridge (SWIR) and the Oman Ophiolite can be used to constrain first order differences in compositional trends within these same rocks, and therefore, melt migration processes in the oceanic crust. The grain size of the SWIR gabbros is approximately a factor of ten greater than that of the Oman gabbros. Due to the strong grain size effect on the viscosity of rocks in the diffusion creep regime, this difference in grain size can lead to as much as 3 orders of magnitude difference in viscosity. Similarly, the permeability of rocks depends on the square of the grain size. Based on the concept of a compaction length for porous flow in a viscous matrix (e.g., McKenzie, 1984), reactions between melts and crystallizing cumulates could occur at length scales as much as 300 times larger for the SWIR rocks than the Oman gabbros. This hypothesis is consistent with compositional and modal variations observed within these gabbro units. In Oman, correlated variations in the composition of primary mineral assemblages are observed at length scales less than 100 m (Korenaga and Kelemen, 1998). In the "moho-transition-zone" such variations are observed at length scales as small as ~0.1 m (Korenaga and Kelemen, 1997). These trends would be destroyed if the melt flux through the lower crust were more than a few percent of the total melt flux into the base of the crust (Korenaga and Kelemen, 1998). By contrast, compositions of the SWIR gabbros suggest interactions of cumulate assemblages with more fractionated basaltic melt at length scales on order of 1 km (Dick et al., 2000, Dick et al., 2001). These observations suggest that the larger size of the SWIR gabbros inhibited viscous compaction and promoted a relatively large permeability during the crystallization process beneath the oceanic spreading center.