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，影响大面积地区，对气候产生全球影响，或溢出，产生相对较短的熔岩，只有局部影响。该项目力求查明地球表面喷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察表明，这两种喷发类型可能同时发生在同一个喷口位置（图1）。这些观察表明，爆炸性喷发经常会穿透裂缝网络，否则会流出熔岩，并留下裂缝开口和闭合的焊接残余物，然后是筏式流下。重要的是，这些排气和焊接过程表明，喷发系统是开放的-和关闭-到大气中。这具有广泛的物理和地球化学影响，但人们对此知之甚少。该项目的主要目标是：（1）将各种规模的联合收割机实地观察与新的实验室实验相结合，以重建控制通过火山裂缝脱气的物理过程的范围;（2）使用火山玻璃中的氩和铁作为地球化学标志，以表明火山管道系统最上部在喷发期间与空气混合的开放程度。当裂隙网络开放时，大气可以进入最上层的岩浆通道，改变铁的氧化态，并扩散到熔体中。这些过程还未被充分探索，但将为浅层火山系统“化学开放”的深度提供关键见解。这些信息将有助于更好地提出通风口的开放和密封，目前过于简化的实验和数值研究。

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.