In situ observation of the percolation threshold in multiphase magma analogues

Autor: Colombier, M., Wadsworth, F. B., Scheu, B., Vasseur, J., Dobson, K. J., Cáceres, F., Allabar, A., Marone, F., Schlepütz, C. M., Dingwell, D. B., Earth and Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany, Centre for Advanced Study, Ludwig-Maximilians-Universität, Munich, Germany, Department of Earth Sciences, Durham University, Durham, UK, Department of Geosciences, University of Tübingen, Tübingen, Germany, Swiss Light Source, Paul Scherer Institute, Villigen PSI, Switzerland
Rok vydání: 2020
Předmět:
Zdroj: Bulletin of Volcanology
Bulletin of volcanology, 2020, Vol.82(4), pp.32 [Peer Reviewed Journal]
ISSN: 1432-0819
0258-8900
DOI: 10.1007/s00445-020-1370-1
Popis: Magmas vesiculate during ascent, producing complex interconnected pore networks, which can act as outgassing pathways and then deflate or compact to volcanic plugs. Similarly, in-conduit fragmentation events during dome-forming eruptions create open systems transiently, before welding causes pore sealing. The percolation threshold is the first-order transition between closed- and open-system degassing dynamics. Here, we use time-resolved, synchrotron-source X-ray tomography to image synthetic magmas that go through cycles of opening and closing, to constrain the percolation threshold ΦC at a range of melt crystallinity, viscosity and overpressure pertinent to shallow magma ascent. During vesiculation, we observed different percolative regimes for the same initial bulk crystallinity depending on melt viscosity and gas overpressure. At high viscosity (> 106 Pa s) and high overpressure (~ 1–4 MPa), we found that a brittle-viscous regime dominates in which brittle rupture allows system-spanning coalescence at a low percolation threshold (ΦC~0.17) via the formation of fracture-like bubble chains. Percolation was followed by outgassing and bubble collapse causing densification and isolation of the bubble network, resulting in a hysteresis in the evolution of connectivity with porosity. At low melt viscosity and overpressure, we observed a viscous regime with much higher percolation threshold (ΦC > 0.37) due to spherical bubble growth and lower degree of crystal connection. Finally, our results also show that sintering of crystal-free and crystal-bearing magma analogues is characterised by low percolation thresholds (ΦC = 0.04 – 0.10). We conclude that the presence of crystals lowers the percolation threshold during vesiculation and may promote outgassing in shallow, crystal-rich magma at initial stages of Vulcanian and Strombolian eruptions.
Paul Scherrer Institut http://dx.doi.org/10.13039/501100004219
European Research Council http://dx.doi.org/10.13039/501100000781
NERC
Deutsche Forschungsgemeinschaft
Databáze: OpenAIRE
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