Unraveling the Mystery of Melt Migration in Migmatites: From Channeling Instability to Pattern Formation

Melt migration in partially molten rocks is commonly described by porous flow models controlled by the hydro‐mechanical compaction length, which effectively explains melt extraction at mid‐ocean ridges. However, this framework cannot account for the paradoxical accumulation of small melt fractions into rhythmic leucosome–melanosome bands in migmatites under high confining pressures. Here, we develop a thermodynamically consistent reaction–cross‐diffusion (RXD) framework that reconciles this paradox with Onsager's micro‐reversibility principle. We show that melt segregation in migmatites can be driven by Turing‐type instabilities, in which nonlinear rheology and cross‐diffusion feedbacks generate self‐organized channeling and banded structures. Analytical reduction to a cnoidal wave solution highlights the role of compaction length as a critical bifurcation parameter, while time‐dependent RXD simulations reveal the transition from homogeneous flow to localized melt channels. Two‐dimensional simulations predict a spectrum of patterns: straight bands, spots, zigzags, and labyrinthine structures, that align with field observations of leucosome spacing and morphology in migmatites and igneous mushes. Our results extend classical poromechanical theory, identifying a fundamental instability mechanism that couples stress, melt pressure, and reaction feedbacks. This work provides a unifying framework linking nonlinear thermodynamics, geomechanics, and field geology to explain enigmatic melt migration pathways in the Earth's crust.