The development and significance of geometric patterns in rock-melt mixtures: Insights from numerical modelling

The presence of melt in the Earth's lower and middle crust plays a crucial role in crustal evolution, rheology, and magmatic processes. Melt generated at depth can either remain in place or migrate towards shallower levels, forming magmatic bodies in the upper crust. The efficiency of this migration is strongly affected by the spatial distribution and connectivity of the melt. Zones of partial melting are complex environments and can be influenced by multiple interacting processes. These processes leave distinct signatures in the melt distribution, indicating that melt geometrical patterns can be used to interpret the mechanisms active during their formation. However, the exact link between the processes and their effect on the melt patterns has not been fully understood.
This study uses a hybrid discrete-continuum model to investigate the interplay between the rates of melt production, melt pressure diffusion, extensional deformation and pre-existing structures in shaping melt distribution patterns. Our numerical experiments show that when the melt production rate exceeds pressure diffusion, melt pressure accumulates, leading to the formation of well-connected hydrofracture networks. In contrast, systems dominated by pressure diffusion exhibit fewer and smaller fractures, favouring porous flow rather than fracture-driven migration. The introduction of extensional deformation promotes the development of shear fractures and organised, asymmetrical patterns. In systems that show compositional layering, hydrofractures develop inside fertile layers and parallel to their boundaries. The interactions between such structures and deformation create complex fracture geometries, enhancing connectivity and facilitating melt transport.
We compare the geometric patterns that emerge from the numerical experiments with those observed in natural melt veins or dykes from two study areas, one in the Lewisian Complex in NW Scotland and the other in the Rogaland region in Norway. These comparisons provide insights such as the timing of deformation, the role of melt pressure and the influence of compositional banding. This work highlights the importance of understanding the links between geometrical patterns in melt distribution and active processes, providing a framework for interpreting melt networks in natural systems.