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.
Bibliographical noteFunding Information:
This research was supported by the NSF through grant OCE-0648020. Experiments at BGI were carried out while D.L.K. was the recipient of an Alexander von Humboldt Senior Award. The authors are grateful for the superb help of Julian Mecklenburgh and Iona Stretton in carrying out the torsion experiments at BGI. Constructive and insightful reviews by Georg Dresen, Julian Mecklenburgh, Erik Rybacki, and Jean-Louis Vigneresse resulted in significant improvements to the manuscript. This paper is LPI Contribution 1473.
- Magma transport
- Melt segregation
- Rock deformation
- Shear zones