A model for the viscosity of melts within the Phlegraen Fields, Italy Matysek, Nikolas
The viscosity of silicate melts is perhaps the most important factor in volcanic processes. Presented here is a model that predicts the non-Arrhenian Newtonian viscosity of 25 anhydrous and hydrous naturally-occurring silicate melts as a function of temperature. This model was created from 437 measurements of viscosity on 16 real-world samples, of which 9 were enriched with water content ranging from 0.30 to 6.32 Wt%. The VFT equation [log η = A + B/(T(K)-C)] was used to describes the non-Arrhenian temperature dependence of viscosity. A hightemperature viscosity limit (A) was assumed and the optimal value was calculated to be -4.74 (e.g. η = 10ˉ⁴·⁷ Pa s). The parameters B and C account for the compositional dependence but a method to calculate them from chemical data was not determined. The model predicts viscosity continuously across 14 orders of magnitude (10³ – 10¹⁶ Pa s) and can calculate the glass transition temperature (Tg) and melt fragility (m). Model parameters B and C are compared against each other and show a strong negative correlation. Anhydrous parameters show noticeable trends when compared against all measures but is clearest against SiO₂. The affect of water is complex and seems to have little systemic influence expect for when B is compared against NBO/T. These cases provide the best avenues of investigation for determining the compositional dependence.
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