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The nucleation of H2O bubbles in magmas has been proposed as a trigger for volcanic eruptions. To determine how bubbles nucleate heterogeneously in silicate melts, experiments were carried out in which high-silica rhyolitic melts were hydrated at 740–800°C and 50–175 MPa, decompressed by 20–70 MPa, and held at the lower pressures for ≥10 s before being quenched. The hydration conditions were subliquidus, and all samples contain blocky magnetite + needle-shaped hematite ± plagioclase. Magnetite is abundant at 800°C and high pressures, whereas hematite becomes more abundant at lower temperatures and pressures. Bubbles nucleated in a single event in all samples, with the number density (NT) of bubbles varying between 2 × 107 and 1 × 109 cm−3. At low degrees of supersaturation, one to a few bubbles nucleate on faces of magnetite, but at medium to high degrees of supersaturation, multiple bubbles nucleate on single magnetite grains. On hematite, one to a few bubbles nucleated at the ends of the needle-shaped crystals at medium supersaturations, but formed along their entire lengths at high supersaturations. NT increases as water diffusivity decreases, indicating that the number of bubbles nucleated is influenced by their growth, which depletes the melt with respect to H2O and lowers supersaturation. If volcanic eruptions are triggered by bubble formation in magmas stored in shallow-level magma chambers, then the supersaturations needed for heterogeneous nucleation suggest that only small amounts of crystallization are needed, whereas homogeneous nucleation is unlikely to trigger eruptions. |
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