The viscosity of shoshonitic melts (Vulcanello Peninsula, Aeolian Islands, Italy): Insight on the magma ascent in dikes

The viscosity of shoshonitic melts and glasses from Vulcanello Peninsula (Vulcano Island, Italy) was experimentally determined at temperatures between 733 K and 1673 K. The water content of the melts varies from 0.03 to 4.75 wt.% H2O. The micropenetration technique was employed at ambient pressure in the high viscosity range (109–1012 Pa·s). Falling sphere(s) experiments were performed at 500 and 2000 MPa in the low viscosity range (100.5–103 Pa·s). Results show a decrease of about 2 orders of magnitude in viscosity if ∼ 3 wt.% of water is added to the dry melt at 1300 K. At high temperature the viscosity of Vulcanello melts is intermediate between that of andesitic and basaltic melts. In contrast, at low temperatures (≤ 1050 K), the shoshonitic melt is characterized by a lower viscosity with respect to the two previous melts. The general viscosity models of Giordano et al. [Giordano, D., Mangiacapra, A., Potuzak, M., Russell, J.K., Romano, C., Dingwell, D.B., Di Muro, A., 2006. An expanded non-Arrhenian model for silicate melt viscosity: a treatment for metaluminous, peraluminous and peralkaline liquids. Chemical Geology 229, 42–56.] for dry silicate melts is in good agreement with the experimental data in the high temperature range (less than 0.3 log units) but shows significant differences in the low temperature range (up to 1.1 log units). The calculation model for hydrous silicate melts from Huy and Zhang [Huy, H., Zhang, Y., 2007. Toward a general viscosity equation for natural anhydrous and hydrous silicate melts. Geochimica et Cosmochimica Acta 71, 403–416.] reproduces the new viscosity data in the high temperature range within ± 0.6 log units but shows as well large discrepancies in the low-T range for hydrous melts (deviations up to 2 log units). Thus we propose a specific calculation model to predict the viscosity of the shoshonitic melts as a function of temperature and water content. This model reproduces the experimental data (24 measurements) with a 1σ standard deviation of 0.17 log units.The viscosity data are used to constrain the ascent velocity of shoshonitic magmas from Vulcanello within dikes. Using petrological data (temperature and crystal content of the magma) and volcanological information (geometrical parameters of the eruptive fissure and depth of magma storage), we estimate the time scale for the ascent of magma from the main reservoir to the surface. Results show time scales in the order of hours to few days. We conclude that the rapid ascent of poorly evolved melts from Moho depths should be taken into account for the hazard assessment of Vulcano Island.