Molecular-scale structural changes of silicate melts under tension revealed by time-resolved X-ray diffraction

Abstract

The non-Newtonian behavior of silicate melts influences magma flow dynamics in a volcanic conduit. Shear thinning causes a reduction in viscosity and brittle failure induces magma fragmentation, resulting in explosive eruptions that release magma fragments into the surface with volcanic gases. The conditions under which they occur have been investigated by macroscopic rheological experiments; however, the understanding of the molecular-scale origin of shear thinning and brittle failure of silicate melts remains poor, partially because of the experimental difficulty of in situ investigations of molecular-scale melt structures under deformation. In this study, we performed tension experiments for silicate melts at temperatures of 760–840 °C and investigated the melt structure forming intermediate-range ordering using time-resolved X-ray diffraction. Our experiments demonstrate that the contribution of anisotropic elastic deformation to molecular-scale strain increases with increasing stress. The mechanical data for stress relaxation indicate that the relaxation timescale decreases with an increase in stress, implying viscosity reduction with an increase in stress. Based on this, we infer that when the stress is large, the molecular-scale structure elastically dilates in the direction of the deformation under tension, causing a decrease in melt viscosity, and the melt may cause macroscopic failure through local anisotropic dilation and the resultant cavitation. This process seems to depend on experimental geometry, such as tension, compression, and torsion; hence, the relationship between experimental geometry, molecular-scale structure, and non-Newtonian behavior must be further investigated in future studies.