Phase separation is one of the most fundamental phenomena that create spatially inhomogeneous patterns in materials and nature. It has so far been classified into three types: (i) solid, (ii) fluid, and (iii) viscoelastic phase separation. Here we report another phase-separation behaviour accompanying fracture, which is observed under a sufficiently deep quench in polymer solutions. Surprisingly, fracture becomes a dominant coarsening process of the phase separation. Under a deep quench, a transient gel is formed by strong attractive interactions between polymers. The connectivity of the polymer network acts against phase separation and produces the internal stress field. When this stress field exceeds the mechanical stability limit of the transient gel, mechanical fracture takes place: fracture phase separation. The behaviour of viscoelastic and fracture phase separation originates from a strong coupling between composition and deformation field. We demonstrate that the same type of coupling between density and deformation field leads to cavitation of fluid under shear and mechanical fracture of glassy liquid and solid under deformation. The key common concept is "dynamic asymmetry". We discuss a common physics underlying these apparently unrelated phenomena and a selection principle of the kinetic pathway of pattern evolution. For example, the only difference between phase separation and fracture may stem from whether deformation is produced internally by phase separation itself or externally by loading.