CO2 diffusion and degassing in volcanic melts

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The eruptive behaviour of a volcano is heavily influenced by the volatiles of the magma (mainly H2O and CO2). During the ascent of a volatile rich magma, bubble nucleation and bubble growth can occur due to the decrease in pressure. Finally, an erupting magma can be fragmented, which results in explosive and dangerous volcanic eruptions. The aim of this research project is to contribute to the understanding of the degassing mechanisms of CO2-rich silicate melts. For that purpose, the mobility of CO2 in silicate melts (diffusion) as well as the degassing of CO2-rich melts will be investigated. Previous studies focussed on diffusion and degassing of H2O. Consequently, there is a need for experimental data on CO2, which represents the second most abundant volatile within magmatic melts. CO2 diffusion will be investigated via diffusion couple experiments at high pressure and high temperatures above the liquidus using an internally heated pressure vessel (IHPV). The melt compositions that will be used for the diffusion experiments are based on natural volcanic melts from Stromboli (basaltic), Popocatépetl (dacitic) and Vesuvius (phonolitic). Since H2O has a strong influence on CO2 diffusion and because it is often present in natural melts, the experiments will be performed using different H2O concentrations in the melts. As a result, the individual influences of different parameters (melt composition, H2O concentration, temperature) will be investigated systematically. The degassing behaviour of CO2-rich melts will be investigated via decompression experiments using the IHPV by simulating a magma ascent with different ascent rates. In order to investigate the degassing of CO2, the experiments will be performed using nominally H2O-free melts. Decompression experiments will be performed using the same basaltic and phonolitic melts that were previously used in diffusion experiments. The experiments will especially shed light on processes of bubble nucleation and bubble growth. The effects of decompression rate and melt composition will be evaluated. Information on diffusion and degassing is important for modelling magma ascent and predicting volcanic eruptions based on volcanic gas emissions. The obtained experimental data will also allow a comparison with the diffusion and degassing behaviour of other volatiles (mainly H2O) for a better understanding of natural volcanic systems.