Rheology of melts from the colli albani volcanic district (Italy): a case study

Abstract

In this study the first viscosity measurements in the glass transition range of melts from highly explosive large-volume eruptions from the Colli Albani Volcanic District (CAVD) are presented. The magmas are ultrapotassic, rich in iron and CaO and characterised by a low silica content (< 45 wt%). Melt compositions range from tephri-phonolitic to foiditic. The Colli Albani eruptions appear anomalous since they produced a large volume of erupted material in spite of their silica undersaturated compositions. The viscosity of the Colli Albani melt changes as the melt composition evolves from the original melt to a country-rock contaminated melt to a crystal-bearing melt with a permanent decrease in liquid viscosity. Conventional estimations of viscosities assume these magmas to have a low viscosity. The presented data show that the melt viscosities are higher than expected. Taking into account further chemical or rheological features of a melt, the investigated CAVD melts are not that striking as assumed in comparison with other large-volume eruptions. Consequently, considering the alkaline-earth to alkaline ratio together with the SiO2 content could provide an alternative when comparing large volume eruptions.

Influence of CO2 on the rheology of melts from the Colli Albani Volcanic District (Italy): foidite to phonolite

“…Geological importance for the PR and PN eruptions Kleest et al (2020) showed that the viscosities of the volatilefree melts of the Pozzolane Rosse and Pozzolane Nere event are higher than expected. They also demonstrated a viscosity decrease for the PR melt caused by assimilation of carbonate wall rock due to an enrichment in Ca 2+ , and an additional reduction in melt viscosity following leucite crystallisation.…”

“…During the storage of the parental PR and PN magmas, crystal formation reaches fractions up to 60 vol% documented by pyroclastic products (Freda et al 2011). Kleest et al (2020) calculated a resulting viscosity increase up to two orders in magnitude for a leucite content of 56 vol% after a model from Caricchi et al (2007). Additionally, carbonate assimilation reaches values of 15 wt% (Iacono-Marziano et al 2007) which clearly exceeds the solubility of CO 2 in the PR melt (Schanofski et al 2019).…”

“…Toplis et al 1997), can be used to force a curve in the fit to the viscosity data. This requires setting the constant A VFT to − 4 (see Kleest et al (2020) for a discussion). Corresponding to the VFT Equation (Eq.…”

“…The viscosity drop decreases with increasing γ and hence with increasing degree of depolymerisation and Angell 2001;Angell et al 2003) and G ∞ = 10 ± 0.5 GPa (G ∞ = shear modulus at infinite frequency) (Dingwell and Webb 1989). For a detailed discussion of A e see Kleest et al (2020) and references therein.…”

Viscosity of evolving magmas: a case study of the Glass House Mountains, Australia

“…The Arrhenian equation should not be extrapolated linearly in temperature. Instead the configurational heat capacity and the Adam-Gibbs equation (Richet 1984) have been used to fit the viscosity data across of the entire temperature range: in order to calculate the temperature dependence of the viscosity at high temperatures; for A AG a constant, taken to be − 4.55 (see Giordano et al 2008;Kleest et al 2020 and references therein); B AG activation energy (J mol −1 ), S conf (T g 12) configuration entropy (J mol −1 K −1 ) at the temperature for which viscosity is 10 12 Pa s, C p conf the difference in ( 9) extrapolated glass heat capacity and relaxed melt heat capacity (J mol −1 K −1 ) at a temperature above the glass transition. This equation describes the viscosity-temperature behaviour of the melt from the temperature range of measurement to higher temperatures.…”