Abstract

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.

Highlights

  • Permeability controls the efficiency with which exsolved volatiles can escape from the magma and are released to the atmosphere or conduit walls before and during volcanic eruptions (Jaupart and Allègre 1991)

  • Our results allow us to separate vesiculation processes into those which resemble classic bubble growth, and those which resemble fracture-driven processes. Our results unify those regimes under the conceptual framework in which the former is typical of low melt viscosity (< 106 Pa s), or low gas overpressure systems (0.1 MPa), while the latter is typical of high viscosity (> 106 Pa s), or high gas overpressure systems (1–4 MPa)

  • & When our data is combined with data from previous work (Lindoo et al 2017), percolation and outgassing in crystalrich magmas can be explained by variations in crystallinity, melt viscosity and gas overpressure

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Summary

Introduction

Permeability controls the efficiency with which exsolved volatiles can escape from the magma and are released to the atmosphere or conduit walls before and during volcanic eruptions (Jaupart and Allègre 1991). Magma permeability evolution in a volcanic conduit is affected by processes such as bubble coalescence (Eichelberger et al 1986; Lindoo et al 2017), brittle fracturing (Tuffen and Dingwell 2005; Kushnir et al 2017; Lamur et al 2017), compaction (Westrich and Eichelberger 1994; Michaut et al 2009; Heap et al 2015; Gonnermann et al 2017) or granular densification in particle-filled fractures and veins (Okumura and Sasaki 2014; Kendrick et al 2016) This evolution of magma permeability can show a hysteresis with vesiculation followed by outgassing and compaction or fragmentation followed by welding (e.g. Rust and Cashman 2004; Wright et al 2009; Michaut et al 2009; Okumura et al 2013). The percolation threshold ΦC is defined as the critical porosity at which the transition from impermeable to permeable magma occurs and represents the divide between a chemically and physically closed system and one that is open

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