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. (C) 2011 Elsevier B.V. All rights reserved.