Degassing mechanisms during silicic eruptions

硅质火山可以产生地球上最具破坏性的喷发。所涉及的高粘度岩浆可能会爆发，影响大面积并对全球气候产生影响，或者是喷涌，产生相对较短的熔岩，只产生局部影响。该项目旨在确定地球表面爆发行为的根本差异的主要原因。最近对大型流纹岩喷发的详细观测表明，这两种喷发方式可能同时发生在同一火山口位置（图1）。这些观测结果表明，爆炸性喷发经常穿过断裂网络，穿透其他熔岩，并留下一个焊接的残余物，它们的开口和关闭，然后顺着气流向下流动。重要的是，这些排气和焊接过程表明，喷发系统对大气的开放和关闭是可变的。这具有广泛的物理和地球化学意义，但人们对其了解甚少。本项目的主要目标是：(1)将不同尺度的野外观测与新颖的实验室实验相结合，重建控制硅火山裂缝脱气的物理过程范围；(2)利用火山玻璃中的氩和铁作为火山喷发时硅酸火山管道系统最上部与空气混合的开放程度的地球化学标志。当裂缝网络打开时，大气可以进入最上层的岩浆通道，改变铁的氧化状态，并扩散到熔体中。这些过程尚未得到充分探索，但将为了解浅层火山系统“化学开放”的深度提供关键见解。这些信息将有助于更好地对目前过于简化的通风口开启和密封进行实验和数值研究。

Silicic volcanoes can produce the most devastating eruptions on Earth. The high-viscosity magmas involved may erupt explosively, affecting large areas and producing a global impact on climate, or effusively, producing relatively short-range lavas, with only local impacts. This project seeks to identify the principal causes for that fundamental difference in eruptive behaviour at the Earth's surface. Recent detailed observations of large rhyolitic eruptions demonstrate that these two eruption styles may occur simultaneously at the same vent location (Figure 1). These observations show that explosive eruptions often penetrate otherwise effusing lavas through fracture networks, and leave behind a welded remnant of their opening and closing, that is then rafted down-flow. Importantly, these venting and welding processes demonstrate that the erupting system is variably open- and closed- to the atmosphere. This has a wide range of physical and geochemical implications and yet is only poorly understood. The main goals of this project are to: (1) combine field observations at a range of scales, with novel laboratory experiments to reconstruct the range of physical processes controlling degassing through fractures at silicic volcanoes; and (2) use argon and iron in volcanic glass as geochemical markers of the degree to which the uppermost parts of the silicic volcanic plumbing system is open to mixing with air during eruption. While fracture networks are open, atmospheric air can enter the uppermost magmatic conduit, and change the oxidation state of iron, as well as diffusing into the melt. These processes are underexplored, but will provide key insights into the depths to which the shallow volcanic system is 'chemically open'. This information will help to better pose experimental and numerical studies of vent opening and sealing, which are currently over-simplified.