Azufral (SW Colombia) is a dangerous silicic volcano hosting a crater lake, which serves as an excellent example of an incipient plug disruption through phreatomagmatism. We studied the youngest succession of dilute pyroclastic density currents (PDCs) onlapping the north-eastern crater rim. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy was used to carry out an automated single-particle analysis of fine to extremely fine ash. We were able to obtain fast and accurate chemical analysis and imaging of 15,098 particles within the 250–63-μm and the 63–32-μm size ranges. The 2D form and roughness parameters were determined for 4895 juvenile glassy particles and validated by 3D micro-X ray computer tomography. There are two end members of high (group 1) and low (group 2) roughness juvenile glassy particles. Group 1 comprises high-roughness glass particles with solidity values as low as 0.34 in 2D and 0.33 in 3D, and convexity values as low as 0.33 in 2D and 0.26 in 3D. Group 2 comprises low-roughness glass particles with 2D solidity values > 0.79 and 3D solidity values typically > 0.58. In this group, 2D convexity values are > 0.68 and 3D convexity values are > 0.71. Both end members are mostly discriminated by the 2D Concavity Index (0.14 to 0.77 in group 1 vs. 0.05–0.35 in group 2). The remaining group 3 comprises particles of intermediate roughness values. In this study, we show how an incipient plug developed over a short repose time might be subjected to only a few cycles of vesicle nucleation, collapse and densification, retaining the characteristics of juvenile glass. Each glassy juvenile ash type, defined by a particular morphology, roughness and microtexture can be linked to a density “stratified” conduit model. In Azufral, the capping and conduit lining dense regions and the permeable zones of the incipient plug likely cracked. The newly formed cracks could allow hydraulic forcing caused by external water and induce phreatomagmatic interaction. This interaction favoured the fine fragmentation of the plug while enhancing ongoing magmatic processes. Finally, the variations of bulk componentry provided clues on dilute pyroclastic density current transport and physical fractionation processes by secondary fragmentation, elutriation and interaction with the crater rim.

中文翻译：

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解密安第斯山脉北部 Azufral 火山口湖的硅质岩浆/栓塞碎片：从细到极细灰形态化学的见解

Azufral（哥伦比亚西南部）是一座危险的硅质火山，内有一个火山口湖，它是一个很好的例子，说明了岩浆作用导致的初期岩浆破裂。我们研究了最年轻的稀火山碎屑密度流 (PDC) 与东北火山口边缘重叠的序列。使用扫描电子显微镜结合能量色散 X 射线光谱仪对细到极细的灰分进行自动单粒子分析。我们能够对 250-63-μm 和 63-32-μm 尺寸范围内的 15,098 个颗粒进行快速准确的化学分析和成像。2D 形状和粗糙度参数是针对 4895 个幼年玻璃质颗粒确定的，并通过 3D 微 X 射线计算机断层扫描进行验证。有高（第1组）和低（第2组）粗糙度幼年玻璃状颗粒的两个末端成员。第 1 组包括高粗糙度玻璃颗粒，其 2D 中的硬度值低至 0.34，3D 中的值低至 0.33，而凸度值则低至 2D 中的 0.33 和 3D 中的 0.26。第 2 组包括 2D 密度值 > 0.79 和 3D 密度值通常 > 0.58 的低粗糙度玻璃颗粒。在该组中，2D 凸度值 > 0.68，3D 凸度值 > 0.71。两个末端成员主要通过 2D 凹度指数来区分（第 1 组为 0.14 至 0.77，第 2 组为 0.05-0.35）。剩余的组3包括中等粗糙度值的颗粒。在这项研究中，我们展示了在短休止时间内形成的初始栓塞如何仅经历几个囊泡成核、塌陷和致密化循环，同时保留了幼玻璃的特征。每种玻璃幼灰类型，由特定形态定义的粗糙度和微观纹理可以与密度“分层”管道模型相关联。在 Azufral，覆盖密集区域的封盖和导管以及初始塞的渗透区可能破裂。新形成的裂缝可能允许由外部水引起的水力强迫并诱发潜水岩浆相互作用。这种相互作用有利于塞的精细破碎，同时增强了正在进行的岩浆过程。最后，散装成分的变化为稀火山碎屑密度电流传输和物理分馏过程提供了线索，这些过程是通过二次破碎、淘洗和与火山口边缘的相互作用。密闭区域的封盖和导管衬里以及初始塞的渗透区可能开裂。新形成的裂缝可能允许由外部水引起的水力强迫并诱发潜水岩浆相互作用。这种相互作用有利于塞的精细破碎，同时增强了正在进行的岩浆过程。最后，散装成分的变化为稀火山碎屑密度电流传输和物理分馏过程提供了线索，这些过程是通过二次破碎、淘洗和与火山口边缘的相互作用。密闭区域的封盖和导管衬里以及初始塞的渗透区可能开裂。新形成的裂缝可能允许由外部水引起的水力强迫并诱发潜水岩浆相互作用。这种相互作用有利于塞的精细破碎，同时增强了正在进行的岩浆过程。最后，散装成分的变化为稀火山碎屑密度电流传输和物理分馏过程提供了线索，这些过程是通过二次破碎、淘洗和与火山口边缘的相互作用。