Non-linear feedback loops in the rheology of cooling-crystallizing felsic magma and heating-melting felsic rock

At least six major parameters control the rheology of partially molten systems: melt content, rate of melt production, reaction to strain of the solid component, reaction to strain of the molten component, temperature and chemical composition of the source rocks. We examine their interactions to understand the rheology of partly molten rocks and partly crystallized magmas. In particular, this paper focuses on the rheology in the transitional domains between two pairs of thresholds that bracket a transitional regime between solid state and fluid behaviour during melting and crystallization, respectively. We review related information and point out non-linear effects that develop during heating of melting rocks and cooling of crystallizing magmas. Owing to the non-linear interactions, positive or negative feedback loops accelerate or damp the system. Melt in migmatite experiences shear-softening which, along with strain partitioning, facilitates melt segregation. Conversely, the increasing number of rigid crystals during cooling increases the suspension viscosity (shear hardening), which soon inhibits magma movement. These effects reinforce the asymmetry between solid-to-melt and melt-to-solid transitions. They severely contradict the concept of one rheological critical melt percentage valid for both melting and crystallization transitions. Fabric acquisition competes with nucleation and crystal growth, thus leading to hysteresis of the stress-strain rate curves. Implications for field observations include horizontal magma segregation, magma extraction and successive magma intrusions.