Stress-driven Melt Segregation in Partially Molten Feldspathic Rocks

Abstract

High-strain torsion experiments were performed on a series of samples composed of anorthite plus51 to 12% melt to investigate the formation of melt-rich bands produced by stress-driven melt segregation. Fine-grained (3^4 � m) 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 � ¼ 1� 9a nd 6� 6. The dependence of viscosity, � , on melt fraction, � , for these partially molten aggregates can be expressed as � ¼ 2� 6 � 10 12 exp (^24 � ) Pa s. In each sample, melt-rich bands develop by a shear strain of � ¼ 1, forming a population of bands at an angle of 5^158 to the shear plane and 40^308 to the applied maximum principal stress. The spacing between and width of the meltrich bands increases as melt fraction increases from50� 01 to 0� 06, then roughly levels off as melt fraction increases to 0� 12. This band spacing, � s, increases linearly with increasing compaction length,