CO2 bubble nucleation upon pressure release in potassium-rich silicate magmas.

We report the first experimental study on CO2 bubble nucleation and growth in silicate magmas in response to pressure release. The starting materials were nominally anhydrous trachyte and trachybasalt glasses that were CO2 saturated at 1200 °C and 300 MPa for at least 48 h under oxidizing conditions. These starting glasses were doped with diverse crystals relevant to the studied melt compositions: olivine, Cr-spinel, clinopyroxene, K-feldspar and leucite. Decompression experiments were performed after an initial equilibration at 1128–1156 °C and 300 MPa for time durations ranging between 15 and 45 h. Pressure was subsequently decreased to a final value of 30 MPa with a decompression rate of 4 MPa/min. Backscattered electron microphotographs of the quenched products were collected using a scanning electron microscope. In all experiments, we observed an event of homogeneous and heterogeneous bubble nucleation, testified by the formation of vesicles with diameters up to 30 μm on the crystal rims as well as in the residual melts. In the samples containing K-feldspar and leucite, another generation of bubbles with diameter up to 130 μm is present in proximity of those crystals, suggesting the occurrence of an earlier heterogeneous nucleation event. CO2 bubble nucleation and growth was investigated by the determination of the wetting angles (ϑ) between the vesicles and the crystal surfaces, which can be used as proxy for the efficiency of the crystals as nucleation sites. It was calculated that the activation energy for bubble nucleation in the studied magmas it is lowered by a factor φ up to approx. 0.64 in presence of crystals. The experimental results show several peculiarities of heterogeneous CO2 bubble nucleation: i) CO2 vesicles form indiscriminately on diverse crystals, in accordance to the small ϑ distribution range between 40° and 70°; ii) CO2 bubbles do not nucleate preferentially on capsule walls; iii) the crystal shapes seem to not to influence bubble nucleation. Our findings are compared with data on heterogeneous H2O bubble nucleation from previous studies to highlight distinctive features. Our new experimental data on crystal wettability in CO2 dominated magmas allow a first assessment of the surface tension through the classical nucleation theory approach. The new findings about CO2 bubble nucleation will be useful to better understand the onset and the intensity of the eruptions in the K-rich magmatic systems.