Modelling melting and melt segregation by two-phase flow: New insights into the dynamics of magmatic systems in the continental crust

The physics of magmatic systems within continental crust is poorly understood. We developed a thermomechanical compositional two-phase flow formulation based on the conservation equations of mass, momentum and energy for melt and solid, including compaction of the solid matrix, melting, melt segregation, melt ascent and freezing. We use a simplified melting law to track the enrichment or depletion in SiO₂ of the advected silicic melt and solid. The nonlinear viscoplastic rheology includes the effect of melt porosity. 2-D models with different heat input are carried out for cases without and with differential melt-matrix flow. The retention number, Rt, as a measure of melt mobility is varied between 1 and infinity. In the case of no melt segregation (large Rt) our models show transient oscillatory behaviour followed by stationary convection in the lower crust enforced by a solid-melt phase transition. In the case of two-phase flow (i.e. small Rt) melt separates from the solid matrix and accumulates in high melt porosity magma bodies within 10 s ka. We find a new melt ascent mechanism, termed CATMA, for Compaction/decompaction Assisted Two-phase flow Melt Ascent. This is a combination of compaction and decompaction of the contact zones between accumulated magma and solid rock that dislodges solid material from the roof that sinks through and partly dissolves in the magma. This process can be seen as an efficient microstoping mechanism and is associated with the formation of melt rich and chemically enriched channels within the magma body. The emplacement depths of magma change from >20 km for low heat flows to <10 km for high heat flows. In most models with high degrees of melting, two stacked SiO₂-enriched magmatic zones form interpreted as granitic layers. Models with stronger crustal rheology show porosity waves on a few km scale. Deviatoric stresses immediately above the evolving magma bodies are of the order of a few MPa, too small to overcome brittle or plastic yield stresses. The models predict significant chemical separation of depleted versus enriched composition, resulting in significant chemical stratification of the crust with spatial variations in solidus temperatures, and in a dual melt porosity distribution with crystal-poor magma bodies (>60 per cent melt) on top of low melt fraction mushes (<20 per cent). Comparison with the Altiplano-Puna magma body shows that the best agreement with observational data is obtained for a moderate (85-90 mW m^-2) heat flux and retention number of the order of 3 to 30.