Heterogeneous bubble nucleation in rhyolitic melt: Insights from the Waimihia mixed magma eruption

Bubble nucleation plays a key role in eruption dynamics, yet the relative importance of homogeneous versus heterogeneous nucleation in apparently aphyric rhyolitic melts remains debated. This study investigates bubble nucleation mechanisms using erupted products from the Waimihia eruption of Taupō volcano, which involved magma mixing within a magma chamber. A microlite-free host magma erupted as white pumice, whereas a quenched magma rich in microlites erupted as banded and hybrid gray pumice. Using new and published data, we quantified bubble number densities (BNDs), bubble size distributions (BSDs), and oxide number densities (ONDs). We found that BNDs in microlite-rich samples are slightly higher than those in microlite-poor samples. This observation may reflect the role of Fe-Ti oxides in facilitating heterogeneous bubble nucleation. While the microlite-rich samples have ONDs comparable to BNDs, the microlite-poor samples exhibit nearly constant BNDs ( 1 ⁢ 0 1 5 – 1 ⁢ 0 1 6 m − 3 ) but low ONDs over a wide range ( 1 ⁢ 0 9 – 1 ⁢ 0 1 5 m − 3 ). The very low abundance of Fe-Ti oxides suggests that they are insufficient to provide bubble nucleation sites. Numerical simulations of microlite-rich gray pumice show that non-linear decompression, along with heterogeneous nucleation, yields power-law BSDs similar to those observed. For microlite-free white pumice, heterogeneous nucleation is required to reproduce the observed BSDs under a similar decompression path to the gray pumice. These results suggest that heterogeneous nucleation dominates even in apparently aphyric rhyolitic magma, potentially due to oxide nanolites, silicate crystals, or submicroscopic compositional heterogeneities. Our study emphasizes the importance of integrating textural analyses with vesiculation models to constrain bubble nucleation kinetics.