Origin of complex magma rheology at the molecular scale: Insights from time-resolved X-ray diffraction and scattering experiments

Abstract

Magma rheology is a key factor in understanding and modelling volcanic eruptions. Until now, macroscopic rheology experiments reveal the viscosity of the magma and conditions at which shear thinning and brittle failure occur. However, it remains unclear what mechanisms control complex magma rheology from the atomic and molecular-scale structure perspective. More specifically, no experimental data on molecular-scale structure have been obtained for deforming magma in the non-Newtonian regime. To resolve this situation, we have developed an experimental system for time-resolved X-ray diffraction and scattering at SPring-8, Japan. Based on the experiments on this system, we found that intermediate-range ordering (IRO), which is related to the size of the ring formed by SiO₄ tetrahedra, expands under tensional deformation. In particular, the IRO shows elastic and anisotropic deformation in the non-Newtonian regime. On the other hand, the short-range ordering such as T-O and T-T distances, where T and O represent Si and Al in the T-site and oxygen, respectively, shows no clear change during the deformation. These results imply that shear thinning and brittle failure may originate from the expansion of the ring size because the large ring is relatively weak and its formation results in cavitation. According to this model, the magma fails when the stress is large enough, rather than the strain rate, because the IRO deforms according to the stress applied to the structure. Recent experiments also observed that small and anisotropic rings form under compression. Previous rheology experiments did not confirm the difference between the conditions, at which shear thinning and brittle failure occur, under tension and compression, but the experimentally-determined molecular-scale structure clearly shows different behavior. To fully understand the mechanism of magma rheology from the view of the molecular-scale structure, we need to perform additional studies including the experiments and theoretical approaches.