Colombier M, Wadsworth F B, Scheu B, Vasseur J, Dobson K J, Cáceres F, Allabar A, Marone F, Schlepütz C M, Dingwell D B
1Earth and Environmental Sciences, Ludwig-Maximilians-Universität, Theresienstr. 41, 80333 Munich, Germany.
2Department of Earth Sciences, Durham University, Durham, DH1 3LE UK.
Bull Volcanol. 2020;82(4):32. doi: 10.1007/s00445-020-1370-1. Epub 2020 Mar 4.
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 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 Pa s) and high overpressure (~ 1-4 MPa), we found that a regime dominates in which brittle rupture allows system-spanning coalescence at a low percolation threshold ( ~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 ( > 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 ( = 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.
岩浆在上升过程中会发生气泡化，形成复杂的相互连通的孔隙网络，这些网络可作为排气通道，随后会瘪缩或压实成火山栓。同样，穹顶形成喷发期间的管道内破碎事件会短暂地形成开放系统，之后焊接会导致孔隙封闭。渗流阈值是封闭系统和开放系统排气动力学之间的一级转变。在此，我们使用时间分辨的同步加速器源X射线断层扫描技术对经历开合循环的合成岩浆进行成像，以确定与浅部岩浆上升相关的一系列熔体结晶度、粘度和超压下的渗流阈值。在气泡化过程中，我们观察到对于相同的初始总体结晶度，根据熔体粘度和气体超压会出现不同的渗流状态。在高粘度（>10帕斯卡·秒）和高超压（约1 - 4兆帕斯卡）下，我们发现一种状态占主导地位，即脆性破裂允许在低渗流阈值（约0.17）下通过形成类似裂缝的气泡链实现系统跨度的聚结。渗流之后是排气和气泡坍塌，导致气泡网络致密化和隔离，从而在连通性与孔隙度的演化中产生滞后现象。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，我们观察到一种渗流阈值高得多（>0.37）的粘性状态。最后，我们的结果还表明，无晶体和含晶体岩浆类似物的烧结具有低渗流阈值（=0.04 - 0.10）的特征。我们得出结论，晶体的存在会降低气泡化过程中的渗流阈值，并可能在武尔卡诺式和斯特龙博利式喷发的初始阶段促进富含晶体的浅部岩浆排气。
