Flow-to-Sliding Transition in Crystal-Bearing Magma

摘要

The presence of crystals in magmatic flows introduces spatial heterogeneity, which can build up to the degree that the flow behavior of the crystal-bearing magma becomes substantially different from that of a pure melt. One example is the transition from flow to sliding, in which the deformation in the crystalline magma is concentrated almost entirely in a thin interfacial layer as opposed to being distributed in a typical flow profile throughout the domain. In this study, we use numerical reproductions of existing laboratory experiments of Fuji basalt in a rotational viscometer to identify when the flow-to-sliding transition is likely to occur in crystal-bearing magma. We utilize a direct numerical method to resolve the interactions between the crystals and the magmatic melt at the scale of individual interfaces in 2-D. All phase interactions and their aggregate effects on the flow emerge self-consistently from the simulation itself. We find that the long-range hydrodynamic interactions between crystals create nonlinear flow behavior even at low crystal fractions. By performing simulations in different geometries, we show that the formation and stability of force chains in crystal-bearing magma depend sensitively not only on the crystal shape and size but also on the geometry of the experimental apparatuses. Our simulations are consistent with the observed discrepancies in previous laboratory measurements of shear thinning behavior, highlighting the delicacies of using point measurements in experiments to constrain the bulk rheology of crystal-bearing magma.