Abstract Nanometer‐scale titanomagnetite crystals have been detected in nominally aphyric rhyolite pumice, but whether they are numerous enough to impact bubble nucleation in explosive silicic volcanism was unresolved. This study examines sub‐micron crystals using rock magnetic techniques, Rhyolite‐MELTS modeling, and physical characterization. We analyzed pumice from four eruptions spanning wide ranges in intensity, storage depth, and bubble number density (1016 to 1013 m−3 liquid): 1060 CE Glass Mountain, 1912 CE Novarupta, 232 CE Taupo, and 0.45 Ma Pudahuel. Calculations assuming monospecific assemblages of 10 and 1,000 nm cubic particles yield titanomagnetite number densities of 1021 to 1013 m−3 dense rock equivalent, respectively. In all cases, titanomagnetite is thermodynamically stable at pre‐eruptive storage conditions and magnetic susceptibility (χLF) is independent of vesicularity and permeability, indicating that crystals likely formed prior to vesiculation. The existence of nm‐scale Fe‐Ti oxides in four diverse cases suggests that heterogeneous bubble nucleation is a general feature of explosive rhyolite volcanism.
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Last updated: October 16, 2025 04:11 AM
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