Modeling the Viscosity of Anhydrous and Hydrous Volcanic Melts

Abstract

The viscosity of volcanic melts is a dominant factor in controlling the fluid dynamics of magmas and thereby eruption style. It can vary by several orders of magnitude, depending on temperature, chemical composition, and water content. The experimentally accessible temperature range is restricted by melt crystallization and gas exsolution. Therefore, modeling viscosity as a function of temperature and water content is central to physical volcanology. We present a model that describes these dependencies by combining a physically motivated equation for temperature dependence of viscosity and a glass transition temperature (Tg) model for the effects of water. The equation uses the viscosity at infinite temperature η∞, Tg, and the steepness factor m as fitting parameters. We investigate the effect of leaving η∞ free as a parameter and fixing its value, by fitting anhydrous viscosity data of 45 volcanic melts using the temperature dependent model. Both approaches describe experimental data well. Using a constant η∞ therefore provides a viable route for extrapolating viscosity from data restricted to small temperature intervals. Our model describes hydrous data over a wide compositional range of terrestrial magmas (26 data sets) with comparable or better quality than literature fits. With η∞ constrained, we finally apply our model to viscosities derived by differential scanning calorimetry and find—by comparing to viscometry based data and models—that this approach can be used to reliably describe the dependence of viscosity on temperature and water content. This introduces important implications for modeling the effects of nanostructure formation on viscosity.

Determination of cooling rates of glasses over four orders of magnitude

“…Both the viscosity-temperature space and melt fragility of our melts well match the spectrum of viscosity and fragility of anhydrous and hydrous volcanic melts (shaded areas in Fig. 2) recently compiled by Langhammer et al (2021).…”

“…Natural and synthetic melts (including those of volcanological interest) can undergo crystallization or exsolution of volatiles on the timescale of viscosity measurements near T g (Liebske et al 2003;Di Genova et al 2017, 2020a. Because crystallization and dehydration around T g lead to the absence of pure melt viscosity data near T g (Dingwell C-DSC + F-DSC Model log 10 q cm (K s −1 ) log 10 q cm (K s −1 ) log 10 q cm (K s −1 ) log 10 q cm (K s −1 ) log 10 q cm (K s −1 ) Chevrel et al 2013), the use of DSC offers a viable route to derive melt viscosity due to its lower impact on melt chemistry and texture (Dingwell et al 2004;Chevrel et al 2013;Di Genova et al 2020b;Langhammer et al 2021;Stabile et al 2021).…”

A chemical threshold controls nanocrystallization and degassing behaviour in basalt magmas

“…Empirical models of melt viscosity [28][29][30][31] are routinely used to approximate magma viscosity under eruptive conditions. These models are based on experimental data obtained over several decades from viscometry measurements (ref.…”

“…These models are based on experimental data obtained over several decades from viscometry measurements (ref. 31 and references therein). However, numerous studies have highlighted the challenges associated with accurately determining the viscosity of melts prone to partial crystallization [21][22][23][32][33][34][35] , which can lead to overestimating the viscosity of the liquid by up to two orders of magnitude 21,32 .…”

“…Only in the last 4 years, 60 studies 21,23,[31][32][33][34][35][36] have been published reporting or inferring the presence of nanocrystals in natural and synthetic samples (Fig. 1).…”