In situ observation of the percolation threshold in multiphase magma analogues

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 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 Common.EditSubmissionSteps.Transform.EquationText 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 and high overpressure, we found that a brittle-viscous regime dominates in which brittle rupture allows system spanning coalescence at a low percolation threshold (Common.EditSubmissionSteps.Transform.EquationText via the formation of fracturelike 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 (Common.EditSubmissionSteps.Transform.EquationText 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 characterized by low percolation thresholds ( Common.EditSubmissionSteps.Transform.EquationText . 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.