Magma degassing: Defusing volcanic eruptions?

岩浆脱气：缓解火山喷发？

• 批准号: RGPIN-2014-03882
• 负责人: Berlo, Kim
• 金额: $ 2.11万
• 依托单位: McGill University
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2014
• 资助国家: 加拿大
• 起止时间: 2014-01-01 至 2015-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=566147
• 关键词: Magma; degassing; Defusing; volcanic; eruptions

At depth, magma contains dissolved volatiles, such as H2O and CO2. As magma rises to the surface, these volatiles exsolve to form gas bubbles. Unless this gas escapes from the melt, pressure builds up during magma ascent and this can lead to an explosive volcanic eruption. In principle, gas escape thus has the potential to defuse explosive volcanic eruptions. However, in reality, the link is more complex and the efficiency and timescales over which this gas escapes compared to the magma ascent rate are crucial in determining the actual eruption style. The main aim of this study is to quantify the timescales and efficiency of the processes by which gas escapes and their contribution to volcanic emissions. When the main volatile components exsolve, trace elements with an affinity for the gas phase will also partition into this. An example is radon, a noble gas and short-lived radionuclide in the Uranium-238 decay chain. Over time, continuous loss of radon from the magma through gas escape results in deficits of its daughter nuclide 210Pb relative to its parent 226Ra in degassed magma, and in excesses of 210Pb in the magma through which the gas is transported. Such disequilibria suggest that gas escape is fast and magma degassing occurs over periods of years to decades prior to an eruption. This requires efficient gas escape mechanisms even in viscous, silicic magma. Growth and expansion of bubbles can lead to gas escape through coalescence and fracturing. These pathways are transient with typical lifespans from seconds, in the case of foam collapse, to hours for fractures, and years to decades for bubble ascent in silicic melt. The efficiency of these pathways, and their contribution to volcanic emissions is very different. Elements will be continuously redistributed between gas and melt during magma ascent, but in a dynamic system there may not be sufficient time to attain equilibrium partitioning for all elements. This implies that the gas composition changes not only with depth, but also with the relative ascent rates of gas and melt. Whereas gas is typically not or poorly preserved, the complementary quenched melts, volcanic glass, do preserve a record of gas-melt interaction in their chemical heterogeneity. Theses glasses thus provide a record of the mechanisms and kinetics of magma ascent and degassing. In this research program we will develop the tools to read this record. As part of this project chemical and isotopic variation in volcanic rocks will be mapped on a scale of meters (in the field) to micro-meters (in the lab). Particular focus will be on the distribution of the metals and 210Pb-226Ra. To retrieve timescale information from this chemical heterogeneity, experiments will be conducted on the behaviour and mobility of different elements during gas-melt interaction. These fundamental parameters will be used to set up a reactive transport model that simulates the evolving melt and gas chemistry during magma ascent. Such a model would tie surface emissions to magma ascent and gas escape rates, which is critical for accurate hazard assessment at volcanoes and can help identify potentially explosive events ahead of time. However the mobility of metals has many applications beyond volcanology, including the release and concentration of metals into potential ore-forming fluids, glass industry, and analogous industrial degassing processes such as smelting and waste incineration. An understanding of metal mobility and its dependence on glass and melt properties can be used to retain, or segregate metals in industrial processes and limit their emission.

在深处，岩浆含有溶解的挥发物，如H2O和CO2。当岩浆上升到地表时，这些挥发物逸出形成气泡。除非这种气体从熔体中逸出，否则在岩浆上升过程中压力会增加，这可能会导致火山爆发。因此，从原则上讲，气体泄漏有可能化解火山爆发。然而，在现实中，联系要复杂得多，与岩浆上升速率相比，这些气体逃逸的效率和时间尺度对于确定实际的喷发方式至关重要。这项研究的主要目的是量化气体逃逸过程的时间尺度和效率及其对火山排放的贡献。当主要挥发性成分析出时，与气相有亲和力的痕量元素也会分配到这一部分。铀-238衰变链中的一种惰性气体和短命放射性核素--氡就是一个例子。随着时间的推移，通过气体逸出从岩浆中不断损失的氡导致其子体核素210Pb在脱气岩浆中相对于母体226Ra的亏损，并导致在输送气体的岩浆中过量的210Pb。这种不平衡表明，气体逸出速度很快，岩浆在喷发前几年到几十年内发生脱气。这需要有效的气体逃逸机制，即使是在粘稠的硅质岩浆中也是如此。气泡的生长和膨胀可通过合并和压裂导致气体逃逸。这些路径是短暂的，典型的寿命从泡沫崩溃的几秒钟到破裂的几个小时，以及硅质熔体中气泡上升的几年到几十年。这些途径的效率及其对火山排放的贡献是非常不同的。在岩浆上升过程中，元素将在气体和熔体之间不断地重新分配，但在一个动态系统中，可能没有足够的时间来达到所有元素的平衡分配。这意味着气体成分不仅随深度变化，而且随气体和熔体的相对上升速率而变化。虽然天然气通常没有保存或保存得很差，但互补的淬火熔体火山玻璃确实保存了气体-熔体相互作用的记录，因为它们的化学不均质性。因此，这些玻璃记录了岩浆上升和脱气的机制和动力学。在这个研究项目中，我们将开发工具来读取这一记录。作为该项目的一部分，火山岩中的化学和同位素变化将按米(野外)到微米(实验室)的比例绘制地图。特别关注的是金属和210Pb-226Ra的分布。为了从这种化学非均质性中提取时间尺度信息，将对不同元素在气体-熔体相互作用过程中的行为和流动性进行实验。这些基本参数将被用来建立一个反应输运模型，模拟岩浆上升过程中熔体和气体化学的演化。这样的模型将把地表排放与岩浆上升和气体逃逸速度联系起来，这对于准确的火山危险评估至关重要，可以帮助提前识别潜在的爆炸事件。然而，金属的流动性在火山学之外还有许多应用，包括将金属释放和浓缩成潜在的成矿流体、玻璃工业以及类似的工业脱气过程，如熔炼和废物焚烧。了解金属的流动性及其对玻璃和熔体性质的依赖可用于在工业过程中保留或分离金属并限制其排放。