Violence and Volume: an experimental study of CO2 driven eruptions of Colli Albani
Final Report Abstract
The viscosity of the melts of Colli Albani has been found to be higher than expected for foiditic composition melts. The main reason for this is the contamination of the melts with CaO and MgO from the limestone in which the magma chamber sits. The information from a TAS diagram is not adequate to allow a realistic estimation of expected melt viscosity. The entire composition of the melt (SiO2, alkali and alkaline-earth as well as aluminium oxide content) needs to be used in estimating silicate melt viscosities. Thus, as the Pozzolane Rosso and Nere magmas are high viscosity melts it is not unusual that they are also high DRE (Dense Rock Equivalent) volcanic events. It was found that CO2 has very little effect on the viscosity of these melts. The CO2 was introduced into the melts at high pressure. The oxygen fugacity experienced inside the highpressure chamber results in a different Fe2+/Fetot than that occurring in melts synthesized in air. Therefore, the same composition melts were synthesized at pressure, but without added CO2. In this way, the effect of adding CO2 could be separated from the effect of changing of iron-speciation on the melt viscosity. Taking the melts to high pressure also results in an increase in the water content of the melt. The effect of the presence of water on the melt viscosity overshadowed the effect of CO2. It was not possible to add more than 0.5 wt% CO2 to the present melts, but the data available indicates that an increased amount of CO2 would decrease melt viscosity slightly. Melts were synthesized in a gas-mixing furnace in order to achieve a range of Fe2+/Fetot from 0.35 to 0.85. This change in iron speciation led to a 0.5 to 2.0 order of magnitude decrease in viscosity with increasing Fe2+/Fetot for melts with ~6.0 to 7.5 wt% FeO. This effect on viscosity is, however, relative to the iron content of the magma. Modelling of the viscosity of magmas as they rise through the vents to the surface should include the effects of changing iron-speciation as well as the effect of decreasing amounts of volatiles (e.g. H2O, CO2) and the presence of bubbles, and possibly crystals.
Publications
- Rheology of melts from the Colli Albani volcanic district (Italy): a case study, Contributions to Mineralogy and Petrology, 175, 2020
Kleest C, SL Webb & S Fanara
(See online at https://doi.org/10.1007/s00410-020-01720-1) - Influence of CO2 on the rheology of melts from the Colli Albani Volcanic District (Italy): foidite to phonolite, Contributions to Mineralogy and Petrology, 176, 92, 2021
Kleest C & SL Webb
(See online at https://doi.org/10.1007/s00410-021-01838-w) - Influence of Fe2+/Fetot on the viscosity of melts from the Colli Albani Volcanic District (Italy) – foidite to phonolite, Journal of Volcanology and Geothermal Research, 431, 2022
Kleest C & SL Webb
(See online at https://doi.org/10.1016/j.jvolgeores.2022.107649)
