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In this study, the influence of CO 2  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 CO 2  concentrations up to 0.50 wt%. Viscosity and calorimetric measurements are performed in the glass transition range  T  g  between 600 and 780 °C. Although nominally anhydrous, the investigated melts contain H 2 O concentrations up to 0.23 wt%. The data exhibit a decrease in viscosity of approx. 10 0.40  Pa s for the phonolitic composition with ~ 0.07 wt% CO 2  and a  T  g  reduced by approx. 14 °C. For the tephritic composition,  T  g  is approx. 5 °C lower and has a viscosity reduced by 10 0.25  Pa s for the sample containing ~ 0.5 wt% CO 2 . Calorimetric measurements of the tephri-phonolite show lowered onset of  T  g  by approx. 6 °C for the melt with ~ 0.11 wt% CO 2  and  T  g  of the foidite appears not to be influenced by a CO 2  concentration of ~ 0.37 wt% CO 2 . 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 H 2 O in the melts with CO 2  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 CO 2  in the melt slightly reduces the viscosity.

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