Viscosity of pantelleritic and alkali-silicate melts: Effect of Fe redox state and Na/(Na+K) ratio

The viscosity of two series of synthetic alkali silicates, corresponding to Al-bearing pantelleritic and Al-free tri-silicate compositions, has been investigated as a function of temperature, iron redox and Na/(Na + K) ratio. Low temperature (708–1000 K) viscosities were determined by the micropenetration technique in the 109.6 to 1013.6 Pa s range. The effect of Fe2 +/Fetot, from 0.15 to 0.86, was explored for [Na/(Na + K)] ratios from 0 to 1. The results demonstrate a strong decrease of viscosity with the replacement of Na for K in both the Al-bearing and Al-free compositions. In the Al-bearing pantelleritic glasses (Ebu) the viscosity as well as the activation energy for viscous flow decrease with an increase of Fe2 +/Fetot from 0.15 to 0.76. In contrast, no measurable changes in viscosity occur in the Al-free tri-silicate glasses (NFS and KFS) as Fe2 +/Fetot ratio varies from 0.15 to 0.86. The comparison of the pantelleritic glasses with peralkaline compositions from the literature stresses the strong influence of iron redox on the overall viscosity of such melts. This suggests that the contribution of iron species should be accounted for in the calculation of agpaitic index (AI) if magma Fe3 + contents are known. One peculiarity of peralkaline magmas is the excess of alkali to alumina, which generally results in lower viscosities of pantelleritic liquids compared with more common metaluminous rhyolites and exerts a primary control of the rheological behaviour of these melts. Our new viscosity data, combined with existing literature, allow more accurate constraints on the nature and eruption of the pantelleritic magmas. Moreover, the present experimental study can be used to improve the viscosity models that mostly do not take in consideration parameters as iron redox in the viscosity prediction.