Viscosity of microlite-bearing rhyolitic obsidians: An experimental study

To investigate the influence of microlites on lava flow rheology, the viscosity of natural microlite-bearing rhyolitic obsidians of calc-alkaline and peralkaline compositions containing 0.1-0.4 wt.% water was measured at volcanologically relevant temperatures (650-950°C), stresses (10³-10⁵ Pa) and strain rates (10^-5 to 10^-7 s^-1). The glass transition temperatures (T_g) were determined from scanning calorimetric measurements on the melts for a range of cooling/heating rates. Based on the equivalence of enthalpic (calorimetric) and shear (viscosity) relaxation, we calculated the viscosity of the melt in crystal-bearing samples from the T_g data. The difference between the calculated viscosity of the melt phase and the measured viscosity for the crystal-bearing samples is interpreted to be the physical effect of microlites on the measured viscosity. The effect of <5vol.% rod-like microlites on the melt rheology is negligible. Microlite-rich and microlite-poor samples from the same lava flow and with identical bulk chemistry show a difference of 0.6 log₁₀ units viscosity (Pa s), interpreted to be due to differences in melt chemistry caused by the presence of microlites. The only major differences between measured and calculated viscosities were for two samples: a calc-alkaline rhyolite with 1 vol.% branching crystals, and a peralkaline rhyolite containing crystal-rich bands with >45 vol.% crystals. For both of these samples a connectivity factor is apparent, with, for the latter, a close packing framework of crystals which is interpreted to influence the apparent viscosity.