EXPERIMENTAL OBSERVATIONS OF COUPLED EVOLUTION OF MELT DISTRIBUTION AND RHEOLOGICAL PROPERTIES IN PARTIALLY MOLTEN PERIDOTITE

We present an experimental study on synthetic partially molten olivine-dominated rocks deformed at high temperatures and pressures. During progressive shear deformation of samples of olivine + MORB, olivine + FeS melt + MORB, and olivine + chromite + MORB, an initially homogeneous melt distribution evolves into well-defined networks of melt-rich bands or channels. These experiments demonstrate the effectiveness of deviatoric stress as a driving force for melt segregation and organization. Bulk rheological properties are strongly influenced by the segregation of melt and demonstrate complex relationships between strain partitioning and deformation mechanisms. Crystallographic preferred orientation data provide relatively direct information on the mechanisms of deformation in the samples. We compare olivine fabrics from deformed samples of olivine alone, olivine plus homogeneously distributed MORB, and olivine plus segregated MORB. The differences from one pattern to the next correspond directly to changes in the melt distribution, implying that the mechanism of melt redistribution is closely coupled to the mechanisms of deformation in the sample. We also present new results from experiments designed to understand the driving forces working for and against melt segregation. Olivine + chromite + MORB samples were statically annealed for 0, 10, or 100 h at elevated P-T conditions. Melt-rich bands in statically annealed samples are fewer in number and appear more diffuse when compared to the deformed but not annealed samples. These annealing experiments provide new quantitative constraints on the kinetics of melt migration driven by surface tension.