Non-Newtonian behavior of plagioclase-bearing basaltic magma: Subliquidus viscosity measurement of the 1707 basalt of Fuji volcano, Japan

Laboratory measurements of viscosity were done for basalt erupted in 1707 AD from Fuji volcano, Japan, using a concentric cylinder rotational viscometer at temperatures of 1297–1157 °C, 1 atm pressure, and fO2 near the Ni–NiO buffer. On cooling, elongated plagioclase crystals with a mean length/width ratio of ca. 8.5 appeared at 1237 °C, followed by olivine at 1157 °C. At progressively lower temperatures, the total crystal volume fraction increased monotonously to ca. 0.25; viscosity increased from 38.9 to 765 Pa s at a shear strain rate of 1 s− 1. This basalt magma behaves as a Newtonian fluid at temperatures greater than 1217 °C, but shear-thinning behavior occurs at temperatures less than 1197 °C because of the suspended plagioclase crystals. This behavior is well approximated as a power law fluid. At the onset of shear thinning, the crystal volume fraction was between 0.06 and 0.13, which is attributed to the pronounced lath-shape of plagioclase crystals. The relative viscosity increases monotonously with increase of crystal volume fraction at a constant shear strain rate, and with decrease of shear strain rate at a constant crystal volume fraction. A modified form of the Krieger–Dougherty equation is introduced herein. It enables us to describe the dependencies of relative viscosity on both the crystal volume fraction and shear strain rate, and consequently the onset of shear-thinning behavior.