Surface tension of hydrous silicate melts: Constraints on the impact of melt composition

The first step in magma degassing is the nucleation of gas bubbles. The ability of magma to nucleate bubbles is moderated by its surface tension, which is thought to vary with melt composition, temperature, and H2O content. Numerous experimental studies of bubble nucleation in silicate melts have quantified surface tension, but those experiments have been run at different temperatures and used melts with different dissolved H2O contents. The influence of bulk melt composition may thus be masked. In this study, we decompress hydrous silicate melts that range from phono-tephrite to high-silica rhyolite to investigate conditions needed to trigger homogeneous bubble nucleation. Importantly, dissolved water contents are very similar amongst the melts, and all were decompressed at 1150–1200 °C, isolating the influence of melt composition on bubble nucleation. Despite the 25 wt.% range in SiO2 content, both the total pressure drop and the degree of supersaturation needed to trigger bubble nucleation vary a little. Because supersaturation varies little, σ for all melts falls to fall within a relatively narrow range of 0.065 to 0.080 N m− 1. In addition, σ for an even wider range of anhydrous melts is nearly constant, although five times higher. It thus appears that the bulk composition of silicate melt has little impact on σ. It is also known that the variation in σ with temperature is minor, and thus most differences between measured σ values probably result from differences in H2O content.