Room to Grow, Room to BLOW! the Influence of Crystals on Bubble-Driven Inflation and Outgassing of Dacitic Magmas
Abstract
Dacitic magmas can erupt effusively or explosively, depending on their ability to release overpressurized gas during ascent in volcanic conduits . Crystals can aid in heterogeneous nucleation of bubbles, facilitating degassing of volatiles from the melt, but may hinder escape of gas (outgassing) from the three-phase magma, thereby playing a crucial role in controlling the switch between effusive and explosive eruptive behavior. We studied degassing and outgassing from synthetic dacite glass samples containing 4.2 wt.% dissolved H2O and different aliquots of quartz crystals ( ϕ = 0, 0.5, 0.6, 0.7 and 0.8, respectively) . Cylindrical cores of ~5 mm diameter and ~10 mm length were heated in a parallel-plate viscometer and held at between 650 and 850°C, at room pressure. The matrix glass transition is ~483°C, above which temperature the samples initially expand (liquid thermal expansion), then shrink (viscous deformation), then expand again at a much faster rate (bubble nucleation and growth). We used X-ray micro-computed tomography to gather 2D and 3D data to identify nucleation sites and methods of outgassing, and evaluate the bubble volume fraction of each sample . Sample expansion decreased systematically with increasing crystal content, from ~41-55% ( ϕ = 0), ~19-54% ( ϕ = 0.5), ~12-16% ( ϕ = 0.6), ~1-7% ( ϕ = 0.7), and ~0-5% ( ϕ = 0.8) . Total porosity of each sample at 750 °C showed a relative increase from ~ 49% ( ϕ = 0) to ~63% ( ϕ = 0.5), followed by an abrupt decrease to ~49% ( ϕ = 0.6), ~33% ( ϕ = 0.7), and ~28% ( ϕ = 0.8). Isolated porosity varied from ~ 38% ( ϕ = 0), ~51% ( ϕ = 0.5), ~29% ( ϕ = 0.6), ~12% ( ϕ = 0.7), and ~1% ( ϕ = 0.8). The data suggests that low crystallinity facilitates efficient forms of outgassing, such as bubble coalescence, but this is hindered for crystal fractions of 0.5 and 0.6. Higher crystal fractions ( ϕ = 0.7 and 0.8) , whose growth was much less than anticipated, experienced brittle behavior with outgassing occurring through pore-pressure-induced fracturing in the crystals and melt, stalling expansion of the system. Consequently, magmas with moderate to high crystal fractions (~0.5-0.6) have the longest outgassing timescale, with neither bubble coalescence nor brittle fracture being particularly effective. Therefore, these crystal fractions likely characterize magmas with the greatest explosive potential.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51I0178H
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 1060 Planetary geochemistry;
- GEOCHEMISTRY;
- 8430 Volcanic gases;
- VOLCANOLOGY;
- 8450 Planetary volcanism;
- VOLCANOLOGY