Surface tension and bubble nucleation in phonolite magmas

Explosive Plinian eruptions tap a wide range of magma compositions, including highly alkaline phonolite magma. Such eruptions are driven by volatiles exsolving from those magmas, and so determining how gas bubbles form is important to understanding those eruptions. Nucleation of bubbles in silicate melts is dictated strongly by the surface tension (σ) of the melt, and so this study focuses on determining σ for phonolite melts. Cylinders cored from a sodium-rich phonolite were hydrated with 5.37 ± 0.09 wt.% dissolved water at 150 MPa and 1150 °C, and then decompressed at either 1150 °C or 875 °C. Bubbles nucleated at 1150 °C only when pressure dropped below 95 MPa, in number densities of 104–5 cm−3. At 875 °C bubbles nucleated only when pressure dropped below 53 MPa, in number densities of 104–7 cm−3. Depending on whether the pressure within critical bubble nuclei equals the saturation pressure or a variable lower pressure, the observed number densities and nucleation rates imply that surface tension for Na-rich phonolite is 0.061 or 0.068 N m−1 at 875 °C and 0.052 or 0.066 N m−1 at 1150 °C. Importantly, temperature has little or slightly negative impact on σ, in contrast to the thermal impact on σ of rhyolite melts. Regardless of pressure assumption, the inferred surface tensions indicate that Na-rich phonolite can become highly super-saturated with water before bubbles nucleate, which could cause them to explosively degas and erupt, similar to highly viscous rhyolites.