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Much of Earth’s magma is stored as extensive crystal mush systems, yet the prevalence of physical processes operating within mushes and their importance in volcanically active regions remain enigmatic. In this Review, we explore the physical properties and key processes of crystal mush systems. The initiation, evolution and decline of volcanic systems, modulated by heat supply and loss, could generate differences in the prevalence of mush processes through space and time. Additionally, regional tectonics alter mush properties, with mushes in cool wet settings having persistent residual melt, permitting more effective melt segregation than in hot dry settings. Disaggregation of mushes results in crystal mush material being mobilized or entrained into lavas and erupted, presenting opportunities to define the timescales and chemistry of some mush processes in volcanically active regions. Mush systems can be observed on length scales ranging from kilometres (using geological mapping) to micrometres (using crystal textures). Therefore, it is difficult to integrate data and interpretations across different fields. Improved integration of thermodynamics, textural analysis, geochemistry, modelling and experiments, alongside inputs from adjacent fields such as porous media dynamics, engineering and metallurgy will help to advance understanding of mush systems and ultimately improve hazard evaluation at active and dormant volcanic systems. 

Abstract Antigorite is a hydrous sheet silicate with strongly anisotropic seismic and rheological properties. Hydrous minerals such as antigorite have been invoked to explain numerous geologic observations within subduction zones including intermediate‐depth earthquakes, arc volcanism, the persistent weakness of the subduction interface, trench‐parallelSwave splitting, and episodic tremor and slip. To understand how the presence of antigorite‐bearing rocks affects observations of seismic anisotropy, three mylonites from the Kohistan palaeo‐island arc in Pakistan were analysed using electron backscatter diffraction. A fourth sample, which displayed optical evidence for crystallographically controlled replacements of olivine, was also investigated using electron backscatter diffraction to identify potential topotactic relationships. The resulting data were used to model the bulk seismic properties of antigorite‐rich rocks. The mylonitic samples exhibit incredibly strong bulk anisotropy (10–20% for the antigorite + olivine). Within the nominally undeformed protomylonite, two topotactic relationships were observed: (1) (010)ant//(100)ol with [100]ant//[001]ol and (2) (010)ant//(100)ol with [100]ant//[010]ol. However, the strength of a texture formed by topotactic replacement is markedly weaker than the strength of the textures observed in mylonitic samples. Since antigorite is thought to be rheologically weak, we hypothesise that microstructures formed from topotactic reactions will be progressively overprinted as deformation is localised in regions with greater percentages of serpentine. Regions of highly sheared serpentine, therefore, have the potential to strongly influence seismic wave speeds in subduction settings. The presence of deformed antigorite in a dipping structure is one explanation for observations of both the magnitude and splitting pattern of seismic waves in subduction zones.