The impact of dissolved fluorine on bubble nucleation in hydrous rhyolite melts

Surface tension of hydrous rhyolitic melt is high enough that large degrees of supersaturation are needed to homogeneously nucleate H2O bubbles during eruptive magma ascent. This study examines whether dissolved fluorine lowers surface tension of hydrous rhyolite, and thus lowers the supersaturation required for bubble nucleation. Fluorine was targeted because it, like H2O, changes melt properties and is highly soluble, unlike all other common magmatic volatiles. Rhyolite melts were saturated at Ps = 245 MPa with H2O fluid that contained F, generating rhyolite with 6.7 ± 0.4 wt.% H2O and 1.1–1.3 wt.% F. When these melts were decompressed rapidly to Pf = 149–202 MPa and quenched after 60 s, bubbles nucleated at supersaturations of ΔP = Ps − Pf ≥52 MPa, and reached bubble number densities of NB = 1012–13 m−3 at ΔP = 78–101 MPa. In comparison, rhyolite saturated with 6.34 ± 0.09 wt.% H2O, but only 0.25 wt.% F, did not nucleate bubbles until ΔP ≥ 100–116 MPa, and even then, at significantly lower NB (<1010 m−3). Numerical modeling of bubble nucleation and growth was used to estimate the values of surface tension required to generate the observed values of NB. Slight differences in melt compositions (i.e., alkalinity and H2O content), H2O diffusivity, or melt viscosity cannot explain the observed differences in NB. Instead, surface tension of F–rich rhyolite must be lower by approximately 4% than that of F–poor rhyolite. This difference in surface tension is significant and, for example, exceeds that found between hydrous basaltic andesite and hydrous rhyolite. These results suggest that is likely that surface tension for F–rich magmas, such as topaz rhyolite, is significantly lower than for F–poor magmas.