 |
| Titel |
How do crystal-rich magmas outgas? |
| VerfasserIn |
Julie Oppenheimer, Katharine V. Cashman, Alison C. Rust, Bjornar Sandnes |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250095078
|
| Publikation (Nr.) |
 EGU/EGU2014-10519.pdf |
|
|
|
|
|
| Zusammenfassung |
| Crystals can occupy ~0 to 100% of the total magma volume, but their role in outgassing
remains poorly understood. In particular, the upper half of this spectrum – when the particles
touch – involves complex flow behaviours that inevitably affect the geometry and rate of gas
migration.
We use analogue experiments to examine the role of high particle concentrations on
outgassing mechanisms. Mixtures of sugar syrup and glass beads are squeezed
between two glass plates to allow observations in 2D. The experiments are performed
horizontally, so buoyancy does not intervene, and the suspensions are allowed to
expand laterally. Gas flow regimes are mapped out for two sets of experiments: foams
generated by chemical reactions, and single air bubbles injected into the particle
suspension.
Chemically induced bubble nucleation and growth throughout the suspension gradually
generated a foam and allowed observations of bubble growth and migration as the foam
developed. High particle fractions, close to the random maximum packing, reduced foam
expansion (i.e. promoted outgassing). In the early phases of the experiments, they caused a
flushing of bubbles from the system which did not occur at low crystal contents. High particle
fractions also led to melt segregation and phase re-arrangements, eventually focusing gas
escape through connected channels.
A more in-depth study of particle-bubble interactions was carried out for single bubbles
expanding in a mush. These show a clear change in behaviour close to the limit for loose
maximum packing of dry beads, determined experimentally. At concentrations below loose
packing, gas expands in a fingering pattern, characterized by a steady advance of widening
lobes. This transits to a “pseudo-fracturing” regime at or near loose packing, whereby gas
advances at a point, often in an episodic manner, and outgases with little to no bulk
expansion. However, before they can degas, pseudo-fractures typically build up larger internal
gas pressures. As with foams, phase re-arrangements appear central to this change in
behaviour: pseudo-fracture penetration causes a compaction of particles in the vicinity
of the gas, and segregation of syrup toward the edges of the experiment. Further
increasing the crystal fraction causes rigid or locked particle networks, and bubbles
grow through filter pressing, segregating the liquid from the particle phase without
dislocating the particle network. These regime transitions are dominantly controlled by
particle fractions, while other factors such as viscosity and gas flow rate play minor
roles.
We hypothesize that particle networks that form at such high solid fractions provide
further resistance to bubble growth. In doing so, however, they offer an alternative: bubbles
can grow through local particle re-arrangements, with minimal disruption to the rest of the
suspension. To conclude, degassing in crystalline magmas involve complex underlying
dynamics: although high particle concentrations increase magma viscosity, which could
promote bubble overpressure and fragmentation, with sufficient time crystals may promote
phase re-arrangements that favour open system degassing and therefore limit bulk expansion. |
|
|
|
|
|
|