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 Φ C 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 (> 10 6 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 ( Φ C ~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 ( Φ C > 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 ( Φ C = 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.

中文翻译：

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多相岩浆类似物中渗流阈值的原位观测

岩浆在上升过程中形成囊泡，产生复杂的相互连接的孔隙网络，可以作为排气通道，然后收缩或压实成火山塞。类似地，圆顶形成喷发期间的管道内碎裂事件会在焊接导致孔隙密封之前暂时创建开放系统。渗透阈值是封闭系统和开放系统脱气动力学之间的一阶过渡。在这里，我们使用时间分辨的同步辐射源 X 射线断层扫描对经历打开和关闭循环的合成岩浆进行成像，以将渗透阈值 Φ C 限制在与浅层岩浆上升有关的熔体结晶度、粘度和超压范围内. 在成泡过程中，我们观察到相同初始体积结晶度的不同渗流方式，具体取决于熔体粘度和气体超压。在高粘度 (> 10 6 Pa s) 和高超压 (~ 1–4 MPa) 下，我们发现脆性-粘性状态占主导地位，其中脆性破裂允许在低渗透阈值 (Φ C ~0.17) 下跨系统合并通过形成裂缝状气泡链。渗流之后是排气和气泡坍塌，导致气泡网络的致密化和隔离，导致连通性与孔隙度的演变滞后。在低熔体粘度和超压下，由于球形气泡生长和较低的晶体连接程度，我们观察到具有更高渗透阈值 (Φ C > 0.37) 的粘性状态。最后，我们的结果还表明，无晶体和含晶体的岩浆类似物的烧结具有低渗流阈值 (Φ C = 0.04 – 0.10) 的特点。