Journal of Petrology Advance Access published online on July 22, 2009
Journal of Petrology, doi:10.1093/petrology/egp043
Stress-driven Melt Segregation in Partially Molten Feldspathic Rocks
1Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr. Se, Minneapolis, MN 55455, USA
2Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, USA
Received February 20, 2009; Revised typescript accepted June 11, 2009
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Abstract |
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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 x 10–4 to 16 x 10–4 s–1 and shear stresses of 15–150 MPa to shear strains between 
= 1·9 and 6·6. The dependence of viscosity, 
, on melt fraction, 
, for these partially molten aggregates can be expressed as = 2·6 x 1012 exp (–24 ) Pa s. In each sample, melt-rich bands develop by a shear strain of = 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, 
s, increases linearly with increasing compaction length, c, according to the relation s = 0·07 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.
KEY WORDS: melt segregation; rock deformation; magma transport; self-organization; shear zones
*Corresponding author. Fax: 612-25-19. E-mail: dlkohl{at}umn.edu