Differential stress‐induced melt migration: An experimental approach

Abstract

A gradient in the dilatational component of a differential state of stress will cause migration of the melt phase in a texturally (quasi)equilibrated partial melt. An experimental approach to image such deformation‐induced melt migration is presented. Two‐phase, solid‐liquid aggregate beams (prepared by a glass‐ceramic technique) having a primary crystalline phase of MgSiO3 (orthoenstatite with a limited amount of clinoenstatite intergrowths) in chemical and textural equilibrium with a sodium aluminosilicate glass are subjected to four‐point flexure; a first‐order thermodynamic analysis, based on the energy balance between grain boundaries (solid‐solid interfaces) and solid‐liquid interfaces, indicates that the melt phase flows from that side of the specimen under a compressive principal stress to the specimen side under a tensile principal stress. When the solid‐liquid aggregate is characterized by a Newtonian rheology (i.e., the deformation occurs via a solution‐precipitation‐enhanced diffusional creep mechanism), the melt migration is easily observed as a large deformation transient accompanying the flexural flow of a specimen. The melt migration is thus characterized as a completely recoverable, anelastic strain in the two‐phase system; the rheology of the partially molten beams is well modelled by where εT is the total inelastic strain, ε0 is the total anelastic strain due to melt migration, is the steady‐state strain rate for the two‐phase aggregate, t is time and B is a function of either the viscosity of the liquid phase or of the rheology (viscosity) of the two‐phase aggregate. In the experiments reported here, the melt migration is shown to be rate limited by the kinetics of compaction and/or dilation of the crystalline residuum. The impact of the experimental approach on compaction‐based models of melt transport and segregation is discussed.