Direct observation of deformation processes in crystal mushes

Isothermal microstructural development in deforming, melt-bearing crystal mushes is studied using synkinematic microscopy and a polyphase, thiocyanate system. The aim of the work is to contribute to the basis for understanding the origin of microstructures in rocks which have undergone melt-present deformation. During high strain rate, pure shearing experiments, microstructures indicating crystal plasticity and dynamic recrystallization are observed. During low strain rate, pure shearing experiments, a pressure solution-like process called contact melting is active, resulting in optically strain-free crystals. Crystal boundary sliding is active in both regimes. Framework scale processes such as filter pressing, grain flow, and development of a micro shear zone are illustrated. Some of these processes are cryptic microstructurally, and may be difficult or impossible to recognize in rocks. The validity of the experimental system as an analog for magmatic systems is discussed. We conclude that the experiments can serve as a source of ideas to test against the evidence from rocks, but that no direct analogy to the behavior of natural crystal mushes has been, or will readily be, established.