Intro

Transport of magma

Currently, my main research interest is the transport of magma, from the lower mantle (D'' layer) to the upper mantle or from lower/mid-crustal source rocks to (often) upper crustal emplacement levels. This is one of the most important mass and heat transfer processes in the earth and the whole process, from initial melt formation, segregation, accumulation, to emplacement, spans an enormous range of length scales and the volume concentration factor is a staggering 10²⁷ (Fig. 1). This large range in scales hampers the study of the complete process.

Fig. 1: Interaction of the different processes at different scales. Small scale(nm-cm) processes such as grain boundary migration due to (anisotropic) surface energy, change of the wetting angle due to pressure solution or changes of dislocation densities and the development of brittle cracks all change the rheology of the bulk rock (e.g. its permeability, strain tensor etc.) on the cm – m scale. These changes in rheology greatly affect the larger scale (m – km) transport mechanism and the flow of magma through the rock.

Research over the past decades has had a tendency to investigate the steps of segregation, accumulation, ascent and, in the case of crustally derived magma, emplacement, separately because their typical length scales necessitate different research methods. Nobody so far has linked these different models and evaluated their feasibility when combined. Can they still work even if one of them (e.g. melt segregation) is a rate controlling process that determines the overall speed with which the other processes can work?

Field evidence suggest that the transport of magma through the earths crust mainly occurs by melt draining into melt-filled veins (leucosomes) from the 1-10 cm scale upwards. Veins normally develop parallel to existing foliations (layering, cleavage) and/or in dilatant sites. Once melt can migrate through this network of veins or dykes, the flow rate becomes very important. If the flow rate is too slow, the dyke will solidify prohibiting any further movement of magma. If the flow rate is to fast, a high pressure gradient between the source region and the final site of emplacement will develop which essentially prohibits any further transport due to the closure of veins and dykes within the source region. Therefore, the flow rate appears to be one of the most influential properties of melt transport. But what controls the flow rate? Is it the density difference between the melt and the host rock? Is the flow rate controlled by the self organization of the network? And how does an externally applied stress field and the resulting pressure gradient change the flow? How does the “injection” of melt into upper crustal levels change the rocks at the site of emplacement? How is the space for these large amounts of melt/magma made?

Other mechanisms have been proposed for the movement of magma. Porous flow is a mechanism that may transport melt into dykes and veins, granular flow describes the movement of melt through a rock without focusing it into veins or dykes. How fast can melt move under such flow conditions? What are the implications for the host rock? What can we learn from migmatites?

Independent from the process by which the melt moves, melt considerably weakens the rock and therefore has great implications on its bulk rheological properties. But what is the relationship between e.g. melt fraction and viscous behavior of the partially molten rock? Is there a threshold, as has been suggested by some researchers, over which the bulk rock strength decreases dramatically, or is there a linear relationship between the melt fraction and the bulk rock strength? And how do other material properties of a rock change? How do, for example, the seismic properties or the electrical conductivity react to an increased melt fraction? How does the permeability change? What is the effect of preexisting textures on these processes?

To better understand the aforementioned processes, the use of numerical simulations in combination with experiments is essential. Experiments have the drawback that scaling them to natural dimensions is not easy and large scale processes such as dyking are hard to reproduce experimentally. Numerical simulations can overcome these problems and are a very useful tool if they are benchmarked against experimental results and, where possible, field observations. Migmatite terrains or pluton margins are probably the best areas to study these problems and to compare both, theoretical predictions from numerical simulations and experiments with results from detailed field studies.