Constraining the high-temperature viscoelastic behavior of basaltic melts through oscillatory rheometry

The rheological behavior of magma impacts the eruption style and related hazards during a volcanic eruption. For example, the range of viscoelastic behavior of volcanic melts affects whether magma undergoes brittle fragmentation prior to eruption or not. Previous research has shown that solid-state (elastic) behavior is promoted by high strain rates, high viscosities, and high crystal fractions. However, few studies have been carried out to constrain the viscoelastic behavior of natural samples. This study is the first to investigate the Newtonian behavior of a natural sample at superliquidus temperature using oscillatory rheology. We selected a basaltic sample from the 2023 Litli-Hrútur eruption in Iceland. During experiments, the sample was first fully remelted and then cooled in a concentric cylinder rheometer from 1550 °C to superliquidus and subliquidus target temperatures between 1500 °C and 1151 °C. At each temperature step, oscillation measurements were performed to constrain the viscoelastic properties of the melt. The results show that the rheology of the melt is Newtonian at superliquidus temperatures, but becomes frequency-dependent at subliquidus temperatures, displaying shear thinning and an increase in the elastic component at high frequencies. This suggests that in silicate melts viscoelastic behavior is mainly promoted by processes other than viscosity increase alone (e.g. crystallization and particle interaction). Although current method limitations in sample retrieval prevent the explicit correlation of these results to crystal fraction and sample texture, this study shows that high-temperature oscillatory measurements yield valuable information about processes in volcanic systems.