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.
Highlights
Rheology of magma has an enormous influence on the eruptive style of volcanoes as well as mass transfer in magmas or crystallisation and degassing processes in cooling melts
The first viscosity measurements of melt compositions from both the Pozzolane Rosse (PR) and Pozzolane Nere (PN) event of the Colli Albani Volcanic District (CAVD) are presented to study their rheological features at sub-liquidus conditions in the glass transition range
Five different compositions are investigated: the K-foidite of the Pozzolane Rosse eruption (PR) with the composition taken from Freda et al (2011), and another K-foidite that is calculated with the original PR composition minus 7 wt% CaO to represent the viscosity of the melt before its assimilation of the carbonate wall rocks (PR-CaO)
Summary
Rheology of magma has an enormous influence on the eruptive style of volcanoes as well as mass transfer in magmas or crystallisation and degassing processes in cooling melts. As shown by Freda et al (2011), a way to illustrate the relationship between the erupted magma volume (a proxy for Volcanic Explosivity Index) and composition is to plot dense-rock equivalent (DRE) vs S iO2 wt% (Fig. 1). On this plot, the magmas of Pozzolane Rosse and Pozzolane Nere, characterised by a low silica content, constitute two striking exceptions. The magmas of Pozzolane Rosse and Pozzolane Nere, characterised by a low silica content, constitute two striking exceptions These events belong to the Colli Albani Volcanic District (CAVD) located about 20 km south of Rome.
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