Viscous heating in silicate melts: An experimental and numerical comparison

The transition from Newtonian to non-Newtonian flow of silicate melts is commonly manifested as shear thinning at conditions of high stress and strain rate. Shear thinning may strongly influence the dynamics of magmatic flows, but the details of its microscopic origins are not fully understood. Here we consider viscous heating and thermomechanical coupling as a potential cause of shear thinning. We compare the results of laboratory, uniaxial compression experiments of a silicate melt with the results of thermomechanical numerical simulations corresponding to the experimental setup. Both the experimental and numerical results concord and indicate that the reduction of the temperature-dependent viscosity in flowing silicate melts is a result of viscous heating. Viscous heating was quantified for glasses with viscosities ranging from 10⁸ to 10¹¹ Pa s and strain rates from 10 ^-5 to 10⁰ s^-1. The results of 48 compression experiments indicate that the transition from Newtonian to non-Newtonian flow in the silicate melt occurs at a Brinkmann number (i.e., ratio of heat gained to heat lost) around 1 whereas brittle behavior dominates the melt deformation when the Deborah number (i.e., ratio of viscoelastic relaxation time to characteristic deformation time) is larger than around 0.01. The observed viscous heating significantly contributes to the viscosity decrease observed in high stress-strain rate experiments and questions our current understanding of the non-Newtonian deformation behavior of silicate melts.