Viscosity behaviour of silicate melts during cooling under variable shear rates

The viscosity of a crystal-bearing shoshonite from Vulcanello (Aeolian Archipelago, Italy) was measured using the concentric cylinder method. Experiments were performed in the temperature interval from superliquidus (1573 K) to 1373 K with the aim to investigate the rheological response of the magma during variable cooling rate (i.e. 1000, 100 and 10 K/h) and shear rate (0.01, 0.1 and 1 s(-1)). In one experiment, with a cooling rate of 10 K/h and shear rate of 1 s(-1), clinopyroxenes (ca. 11 vol.%) with hopper texture crystallized. One further experiment was performed for a longer time with a cooling rate of 100 K/h and shear rate of 1 s(-1) to the final temperature of 1323 K. In this case, the final product showed skeletal crystals of pyroxenes (ca. 17 vol.%) and an apparent viscosity of 1.10 x 10(4) Pa s, which was very close to the experiment performed at 10 K/h and shear rate of 1 s(-1) to 1373 K (1.18 x 10(4) Pa s). The comparison between experiments performed at cooling rate of 100 K/h, and finally quenched at to 1373 and 1323 K, respectively, showed that a temperature decrease of 50 K produces an increase in the apparent viscosity of ca. 1 log unit due to the transition from crystal-free to crystal-bearing melts. This study provides new data on apparent viscosity variation as cooling and shear rates vary in a crystallizing magmatic system. Results show that, on average, the viscosity of a shoshonitic system can increase by about two orders of magnitude due to temperature decrease from 1573 to 1373 K and the incipient crystallization occurring at the lowest temperatures. Finally, the results confirm that crystal nucleation and growth kinetics, as well as magma viscosity, are controlled by the dynamic state of the system.