Modeling the Viscosity of Anhydrous and Hydrous Volcanic Melts. (30th August 2021)

• Record Type: Journal Article
• Authors: Langhammer, D.; Di Genova, D.; Steinle‐Neumann, G.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 8(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-08-30
• Subjects: volcanoes -- magmas -- glasses -- viscosity -- crystal -- DSC; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2021GC009918
• Languages: English
• ISSNs: 1525-2027

Abstract

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 ( T g ) model for the effects of water. The equation uses the viscosity at infinite temperature η ∞, T g, 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. Plain Language Summary: How violently a volcano erupts strongly depends on the viscosity of the ascending magma. Temperature and the amount of dissolved water in the magma significantly impact viscosity. Therefore, models that predict it as a function of these parameters are of great interest and can be calibrated by measured data. We find a model that performs comparably to or better than other published ones. One model parameter describes the viscosity at infinite temperature; we investigate whether this is a constant value for all melts, and find this to be a justifiable assumption that leads to accurate predictions. Finally, we explore the possibility of deriving viscosity via differential scanning calorimetry (DSC). This approach avoids or significantly reduces melt crystallization which is a possible consequence of using more common measurement methods. By combining the constant value for the infinite temperature viscosity with DSC‐derived data, our model can accurately predict viscosity at any relevant temperature. Key Points: We provide a temperature‐ and water‐dependent viscosity model for volcanic melts that performs better or comparable to literature models Anhydrous data are fit to explore the constant infinite temperature viscosity. Polymerized melts skew results toward lower values Differential scanning calorimetry data enable extrapolation of viscosity to any temperature using a constant value at infinite temperature