THE EFFECT OF DEFORMATION ON THE PERMEABILTY OF PARTIALLY MOLTEN ROCKS: AN ANALOGUE EXPERIMENT

Changes in melt distribution and melt permeability during simple shearing of a partially molten analogue aggregate (norcamphor-benzamide) were investigated at a constant shear strain rate of 10^-5 s ^-1. Both the experimental strain rate and melting rate were faster by ~ 8 orders of magnitude with respect to rates expected in nature. The starting material was first deformed in the solid state at 38°C. After a strain increment of g=1, the temperature was raised above the solidus (42° C) of the system to 45° C, without interrupting deformation. The solid grains are inferred to undergo dislocation creep associated with grain boundary migration recrystallization during both subsolidus and suprasolidus deformation. Melt permeability was investigated with the aid of image analysis techniques, which allow one to calculate the parameters needed to solve a Kozeny-Carman-type of relationship. At low strain (g=0.05 after the onset of melting) melt films were preferentially distributed subparallel and subperpendicular to the foliation plane. At higher strain (g=0.75 after the onset of melting) melt films were mainly oriented subparallel to the incremental shortening direction. These melt films formed as intergranular, extensional fractures. At the highest strain (g=1.4 after the onset of melting) the maximum concentration of melt occurred at a low angle to the shear zone boundary, along melt-bearing shear bands. The direction of highest permeabilty was subparallel to the foliation plane at low strain and it became close to the incremental shortening direction at high strain. However, the ratio of permeability parallel to foliation to permeabilty normal to foliation was small (~1.5) at all stages of deformation. The absolute values of permeability showed a positive correlation to the melt fraction, but no correlation to increasing strain. Therefore, under the present experimental conditions, the effect of deformation on permeability changes was negligible, compared to the effect of varying melt fractions during melting.