Stress-driven melt segregation in partially molten feldspathic rocks

High-strain torsion experiments were performed on a series of samples composed of anorthite plus <1 to 12% melt to investigate the formation of melt-rich bands produced by stress-driven melt segregation. Fine-grained (3-4 μm) samples were deformed in the diffusion creep regime at a temperature of 1450 K and a confining pressure of 300 MPa at shear strain rates of 1 × 10. ^-4 to 16 × 10. ^-4 s. ^-1 and shear stresses of 15-150 MPa to shear strains between γ= 1·9 and 6·6. The dependence of viscosity, η, on melt fraction, φ, for these partially molten aggregates can be expressed as η = 2·6 × 10. ¹² exp (-24 φ) Pa s. In each sample, melt-rich bands develop by a shear strain of γ = 1, forming a population of bands at an angle of 5-15° to the shear plane and 40-30° to the applied maximum principal stress. The spacing between and width of the melt-rich bands increases as melt fraction increases from <0·01 to 0·06, then roughly levels off as melt fraction increases to 0·12. This band spacing, δ. _s, increases linearly with increasing compaction length, δ. _c, according to the relation δ. _s = 0·07 δ. _c when the bulk viscosity is assumed to be twice the shear viscosity. In the Earth, spontaneous stress-driven segregation of fluids is an important mechanism for localizing deformation into shear zones.