HR: 16:20h
AN: V24A-02    [Abstracts]
TI: Getting on the Band Wagon: analysis of melt localization instabilities due to mechanical shear
AU: * Spiegelman, M
EM: mspieg@ldeo.columbia.edu
AF: Lamont Doherty Earth Obs., Rt 9W, Palisades, NY 10964 United States
AU: * Spiegelman, M
EM: mspieg@ldeo.columbia.edu
AF: Dept. of Applied Physics/Applied Math, Columbia University, New York, NY 10027 United States
AU: Katz, R F
EM: katz@ldeo.columbia.edu
AF: Lamont Doherty Earth Obs., Rt 9W, Palisades, NY 10964 United States
AU: Jung, M
EM: mj230@columbia.edu
AF: Dept. of Applied Physics/Applied Math, Columbia University, New York, NY 10027 United States
AB: Recent experiments by Holtzman et. al, (G-cubed, 2003) demonstrate that partially molten aggregates of mantle materials undergoing simple shear (from $\sim 100-500$% strain) can spontaneously develop localized melt-rich bands. These bands have been suggested as a source of seismic anisotropy in the shallow upper mantle, but the physics of their formation is not well understood. These experiments, can be modeled using the equations governing flow in deformable porous media, however, and thus provide an important opportunity to both validate this theory and to gain a better understanding of the rates and processes of melt-band formation in the Earth. Here, we present linear and numerical analysis of these equations and compare them to results of the experiments. The important feature of the experiments is that the melt-rich bands develop at small strains and persist at low angles to the plane of shear ($\sim15$--25$\deg$) even at large shear strains. They also appear to form localized weak regions that act as strain guides for the solid matrix flow. To model these, we consider the evolution of a deformable, permeable solid undergoing simple shear with a variable shear viscosity that weakens with increasing porosity. The linear analysis calculates the growth in porosity of a plane wave perturbation that starts at an initial angle $\theta_0$ to the plane of shear and grows with increasing strain. The perturbations grow exponentially with a rate that depends on the strain and the derivative of shear viscosity with respect to porosity $\alpha=\partial \eta/\partial\phi$. For a strain of 300%, the maximum growing melt band initiates at 16.8$\deg$ (but rotates to $\sim70\deg$ in the linear analysis). We also calculate the additional solid shear induced by the localized weak regions and show that it develops a sense of shear consistent with observations only for melt bands less that 45$\deg$. These results suggest that low angle bands are favored under shear, consistent with the observations. The linear analysis, however, does not allow the growth of the melt bands to interact with the enhanced shear and thus the initially low angle bands are rotated to higher angles by the background simple shear. To follow the further development of the bands requires solutions of the full non-linear equations and we present numerical solutions that also show the spontaneous development of melt rich bands. These bands tend to form near $45\deg$ and rapidly saturate until the inter-band regions are compacted dry (which currently halts the solution). Future work will explore the quantitative differences between the numerical solutions and experiments and explore additional physics such as surface energy and grain-boundary interactions that may also be important for a full description of melt-band formation.
DE: 8434 Magma migration
DE: 5114 Permeability and porosity
DE: 3035 Midocean ridge processes
DE: 3230 Numerical solutions
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting