MELT SEGREGATION DURING DEFORMATION OF PARTIALLY MOLTEN ROCKS

Not only does a small amount of melt weaken a partially molten rock, but also simultaneously deformation reorients and redistributes melt. To investigate the effect of deformation on melt distribution in partially molten rocks, we have sheared a series of synthetic rock specimens to high strain. Samples were deformed in a gas-medium apparatus to shear strains of g > 2 at a confining pressure of 300 MPa and a temperature of 1523 K. In samples of olivine + MORB, shear deformation locally redistributes melt from disc-shaped inclusions on grain boundaries and tubules along triple junctions into grain-scale sheets oriented ~15^o to the shear plane and antithetic to the shear direction. In samples of plagioclase + MORB, and olivine + albitic melt, olivine + chromite + MORB and olivine + MORB + molten iron sulfide, melt segregates into melt-rich bands again 10-20^o to the shear plane and antithetic to the shear direction during shear deformation. For these four solid-melt systems, the spacing between the melt-rich bands is several grain diameters. We propose that differences in the wavelengths of the melt-rich bands from system to system are due to differences in the compaction length. If the compaction length is larger than the scale of the sample, as it is for the olivine + MORB samples, melt remains localized at the grain scale. By decreasing the permeability, increasing the melt viscosity or decreasing the matrix viscosity, the compaction length can be reduced to less than the scale of the sample, as it is for the latter four types of samples. As a consequence, melt instabilities nucleate at perturbations in the stress field caused by the peaks and grooves in the pistons or at inhomogeneities in the interior of the sample. The spacing between the resulting melt-rich bands, which extend across the entire width of the sample, is directly related to the compaction length and not to the spacing between serrations in the pistons. This deformation-driven migration of melt into aligned bands significantly increases permeability in the direction parallel to the bands, localizes deformation, and produces shear wave splitting. Evidence for a strong coupling between deformation and melt localization may be evident, for example, in exposed mantle sections of ophiolites.