Influence of Compaction Length on Radial Melt Segregation in Torsionally Deformed Partially Molten Rocks

To investigate the influence of compaction length on radial melt segregation during torsional shear deformation of partially molten rocks, experiments were performed on samples composed of olivine plus ∼7 vol.% of either an albite, alkali basalt, or lithium silicate melt. These three melts cover a range of three orders of magnitude in viscosity, yielding samples that vary by approximately two orders of magnitude in compaction length. Samples were deformed in torsion at 1,473 K and 300 MPa in constant strain rate experiments to outer-radius shear strains of up to 14.3. Radial melt segregation occurred toward the axial center in all three types of samples that were sheared to γ(R) ≥ 4. At the same strain, samples with the largest compaction length exhibited the highest segregation rate, while samples with intermediate and smallest compaction lengths exhibited similar segregation rates. The experimental observations qualitatively agree with previously published results from two-phase flow theory for base-state melt segregation with anisotropic viscosity; specifically, the segregation rate for radial melt segregation increases with increasing compaction length. However, quantitatively, the segregation rate in experiments is smaller than the rate predicted by simulations for the same compaction length. This discrepancy may, for example, reflect the difference in rheological behavior between that observed in our experiments (non-Newtonian, dislocation-accommodated creep) and that incorporated into the numerical models (Newtonian, diffusion-accommodated creep). Our results thus provide a baseline for testing current and future models of two-phase flow, particularly as applied to understanding melt migration, segregation, and extraction from Earth's deeper interior.