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

Christin Kleest,Sharon L Webb

doi:10.1007/s00410-021-01838-w

Christin Kleest, Sharon L Webb

Open Access

https://doi.org/10.1007/s00410-021-01838-w

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Abstract

In this study, the influence of CO2 on the rheology of silica poor and K-rich melts from highly explosive eruptions from the Colli Albani Volcanic District (Italy) (CAVD) is measured for the first time. The investigated melts range from foidite to tephri-phonolite to tephrite from the CAVD to a phonolite from the Vesuvius (Italy) with CO2 concentrations up to 0.50 wt%. Viscosity and calorimetric measurements are performed in the glass transition range Tg between 600 and 780 °C. Although nominally anhydrous, the investigated melts contain H2O concentrations up to 0.23 wt%. The data exhibit a decrease in viscosity of approx. 100.40 Pa s for the phonolitic composition with ~ 0.07 wt% CO2 and a Tg reduced by approx. 14 °C. For the tephritic composition, Tg is approx. 5 °C lower and has a viscosity reduced by 100.25 Pa s for the sample containing ~ 0.5 wt% CO2. Calorimetric measurements of the tephri-phonolite show lowered onset of Tg by approx. 6 °C for the melt with ~ 0.11 wt% CO2 and Tg of the foidite appears not to be influenced by a CO2 concentration of ~ 0.37 wt% CO2. However, these tephri-phonolitic and foiditic melts foamed during calorimetric measurements preventing a reliable measurement. It would appear that most of this overall drop in viscosity is caused by the small amounts of H2O in the melts with CO2 slightly reducing the viscosity or having no effect on viscosity. Additionally, it is shown that the reduction in viscosity decreases with an increasing degree of the depolymerisation for the investigated melts. Consequently, the explosive style of the CAVD eruptions is mainly caused by crystals and bubbles which form and rise during magma storage and ascent which increases the magma viscosity whereas the CO2 in the melt slightly reduces the viscosity.

Highlights

Rheology of melts plays a key role in magmatic processes
The present study investigates the influence of C O2 on rheological properties of three synthetic equivalents of the Colli Albani Volcanic District (CAVD) and a fourth sample equivalent to the “white pumice” phonolite from the Vesuvius AD 79 eruption (WPVe series) (Iacono Marziano et al 2007)
The presented study concerns the viscosities of C O2-bearing phonolitic, tephri-phonolitic, tephritic and foiditic melt compositions

Summary

Introduction

Rheology of melts plays a key role in magmatic processes. It influences mass transfer, crystallisation processes, nucleation, forming and exsolution of bubbles, the speed of ascent in the volcanic vent and the eruptive style. Viscosity studies on a K-rich silicate melt by Bourgue and Richet (2001) exhibit a reducing effect of CO2 on the viscosity within the first 3 wt% CO2 added They measured a linearly decreasing density of the melt with increasing C O2 concentration at room temperature. Morizet et al (2007) inferred viscosity from calorimetric measurements of the glass transition temperature Tg of CO2 bearing jadeitic and phonolitic melts. Seifert et al (2013) did experiments on the relaxation times for basaltic, phonolitic and rhyolitic glasses and showed a reduction in relaxation time with increasing CO2 content for the basaltic melt but no changes for the other compositions As a consequence, they deduce CO2 decreases the viscosity of basaltic melts but does not affect the other SiO2 rich melts. A study by Robert et al (2015) on a remelted natural basalt confirms the effect of CO2 slightly reducing viscosity in the presence of up to 1 wt% H2O

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