Analog modelling of melt segregation and migration during deformation

Analog experiments of melt segregation and migration in lower crustal rocks have been conducted using paraffin wax. The wax has a mechanical planar an isotropy which reproduces the pervasive foliation of high-grade metamorphic rocks. The shortening of a layer of partially molten wax (melt fraction be tween 15 and 20%) results in the movement of a part of the liquid from the microscopic porosity of the wax to the outside of the layer in large accumu lation sites. Four stages can be identified: (1) from the beginning of the shortening, melt segregates into dilatant foliation-parallel veins; (2) the development of a fold occurs with an increasing accumulation of liquid in the limbs; (3) strain localization and vein connection allows the nucleatio n of shear bands; (4) melt migration is channelled by the shear band toward external pockets. The first two stages involve melt percolation from kinem atically controlled high-stress areas around growing veins. The third stage is associated with local attainment of a segregated melt critical concentr ation estimated at 14-15%. The last point involves both horizontal and upwa rd migration of the melt. Melt segregation and migration are highly scale- and strain-dependent mechanisms. (C) 2001 Elsevier Science Ltd. All rights reserved.