Phase change and melt dynamics in partially molten rock

Fundamental mechanisms of chemical differentiation in magmas rely on grain-scale processes and corresponding physical properties of partially molten igneous microstructures. Two such processes are the kinetic control on the evolution of the igneous microstructure during magmatic phase change and both the ability and pattern of interstitial melt flow within the porous crystalline igneous microstructure. Numerical simulations of igneous microstructural development during progressive crystallization using a stochastic algorithm developed here reveal that a kinetic model involving exponential nucleation and constant growth rates best reproduces textural characteristics of natural rocks. Bounds on the percolation thresholds of the solid and melt phases are estimated from the simulated microstructures. Both the simulated microstructures and a natural microstructure obtained from X-ray Computed Tomography are used as inputs in a lattice-Boltzmann model to simulate low Reynolds number, interstitial melt flow. For both microstructure types, the pattern of porous melt flow is channelized at the grain-scale and permeabilities determined from the steady flow field are fit to two commonly employed correlation models. Field observations of silicic melt generation, segregation, and injection by dolerite partial melting of granitic wall rock are made in the McMurdo Dry Valleys, Antarctica. Numerous, long (100s m), thin (<30 cm), interconnected fine-grained granitic dikes cut Ferrar dolerite sills and the source of at least one dike emanates from a thin (5 cm) melt sheet separating chilled dolerite from partially melted granite country rock. Higher than expected calculated dolerite contact temperatures of 900--950°C suggest that the dolerite feeder acted as an conduit for sustained flux of magma generating a partial melting front to propagate into the granite country rock. Granite partial melting occurred as a closed-system beyond 50 cm but as an open-system closer to the dolerite chilled margin and is reconciled by a calculated compaction length scale. Melt generation, segregation into a melt rich reservoir parallel to the dolerite chilled margin, and finally dike emplacement likely occurred within a period of years. All of these processes are central to understanding the generation and separation of melt in partially molten areas within Earth, which are intimately associated with planetary magmatism and volcanism.