Numerical modelling of magma transport in dykes

摘要

The rheology and dynamics of an ascending pure melt in a dyke have been extensively studied in the past. From field observations, it is apparent that most dykes actually contain a crystalline load. The presence of a crystalline load modifies the effective rheology of such a system and thus the flow behaviour. Indeed, the higher density and viscosity of each crystal, compared to the melt, cause a decrease of the ascent velocity and modify the shape of the velocity profile, from a typical Poiseuille flow, to a Bingham-type flow. A common feature observed in the field is the arrangement of crystals parallel or at a very low angle to the edge of the dyke. Such a structural arrangement is often interpreted as the result of magma flow, which caused the crystals to rotate and align within the flow direction, but this process remains unclear. Another issue related to the introduction of a crystalline load concerns the possibility for crystals to be segregated from a viscous granitic melt phase during magma ascent. The implications of such a process on magmatic differentiation have not previously been considered, nor has such a process been previously investigated via numerical models. In this study, we examine the flow dynamics of a crystal bearing granitic melt ascending in a dyke via numerical models. In our models, both the crystal and melt phases are represented as highly viscous fluids in a Stokes regime. Our results reveal that the presence of crystals in the melt modifies the magma velocity profile across the dyke. Furthermore, we observe that whilst crystals continually rotate in the shear flow, over one period of revolution, their major axis has a high probability to be aligned parallel to the flow direction. Moreover, some experiments showed that the melt phase can effectively be squeezed out from a crystal-rich magma when subjected to a given pressure gradient range. This demonstrates that crystal-melt segregation in dykes during granitic magma ascent constitutes a viable mechanism for magmatic differentiation.