The effect of iron and alkali on the nanocrystal-free viscosity of volcanic melts: A combined Raman spectroscopy and DSC study

The iron coordination, its oxidation state (Fe2+/Fetot.), and alkali ratio [Na/(Na + K)] greatly influence the structure and thus the viscosity of volcanic melts, which is known to play a key role in the dynamics of volcanic eruptions. Furthermore, it has been recently reported that volcanic melts can contain iron-bearing nanocrystals and this makes it difficult to isolate and quantify the chemical contribution to the viscosity of magmas. Here, we present Raman spectroscopic and differential scanning calorimetry (DSC) data on nanocrystal-free peralkaline rhyolites with different Fe2+/Fetot. (0.15–0.84) and Na/(Na + K) (0–1) molar ratios. Raman spectra are used to infer the structural changes occurring with varying iron oxidation state and alkali content, whereas the combination of Raman spectroscopy and DSC measurements allow the characterization of the anhydrous nanocrystal-free viscosity as a function of temperature. Results suggest that at similar and high Fe2+/ Fetot. ratio the Raman spectral feature controlled by the iron coordination changes with Na/(Na + K). Conversely, the change of alkali content at a fixed Fe2+/Fetot. ratio results in a variation of the spectral feature that reflects the size distribution of rings of tetrahedra in the melt structure. We further discuss the implications of our findings for magma transport and estimate that the viscosity of anhydrous peralkaline rhyolites at the eruptive temperature of 750 ◦C can increase up to 3.5 log units when Fe2+/Fetot. and Na/(Na + K) ratios decrease contemporaneously from 0.84 to 0.15 and from 1 to 0, respectively. Finally, the comparison of our viscosity data with those from the literature suggests that the DSC-approach presented and adopted in this study is independent on chemical composition and thus can be used also to retrieve the effect of nanocrystals on the viscosity of volcanic melts. The results presented here have profound implications for the modelling of magma viscosity.