Viscosity changes during crystallization of a shoshonitic magma: new insights on a lava flow emplacement

Viscosity changes during the crystallization of a shoshonitic lava from the Vulcanello peninsula (Aeolian Islands, southern Tyrrhenian Sea, Italy) were measured by use of a electromagnetic rotational viscometer under Ni-NiO buffered conditions at temperatures ranging from 1539 K to 1381 K. Results showed an increase in effective viscosity from 131 Pa s to 15320 Pa s as the crystal content increased from 0 vol% to similar to 14 vol%. The crystallization processes in the nominally dry shoshonite began at 1420 K with the formation of clinopyroxene, which was followed by the formation of plagioclase and olivine at 1401 K. Experimental observations differed from predictions derived from the use of the Einstein-Roscoe equation with the Marsh (E-R-M) parameter at crystal contents higher than 8 vol%. The relative viscosity of samples was larger than that calculated by E-R-M by a factor of up to 9.5 as the temperature decreased to 1381 K. The large departure of the experimental results from the E-R-M equation predictions was likely caused by the interaction of elongated pyroxene crystals. The measured viscosity data were used to estimate the potential for emplacement of shoshonitic lavas.