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: 2016
• 资助国家: 加拿大
• 起止时间: 2016-01-01 至 2017-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=610699
• 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，这些挥发物溶解形成气泡。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，这可能导致爆炸性火山爆发。因此，从原则上讲，气体逃逸有可能化解火山爆发。然而，在现实中，这种联系更为复杂，与岩浆上升速率相比，这种气体逃逸的效率和时间尺度在确定实际喷发方式方面至关重要。这项研究的主要目的是量化的时间尺度和效率的过程中，气体逃逸及其贡献的火山排放。 当主要挥发性组分溶出时，对气相具有亲和力的微量元素也将分配到其中。一个例子是氡，一种稀有气体和铀-238衰变链中的短寿命放射性核素。随着时间的推移，氡从岩浆中通过气体逃逸的持续损失导致其子体核素210 Pb相对于其母核226 Ra在脱气岩浆中的不足，以及在气体被输送通过的岩浆中的210 Pb过量。这种不平衡表明，气体逃逸是快速的，岩浆脱气发生在喷发前的几年到几十年。这需要有效的气体逸出机制，即使在粘性的岩浆中。气泡的生长和膨胀可导致气体通过聚结和破裂逸出。这些途径是短暂的，典型的寿命从几秒钟（泡沫破裂的情况下）到几小时（破裂的情况下），以及几年到几十年（泡沫熔体中气泡上升的情况下）。这些路径的效率及其对火山排放的贡献是非常不同的。在岩浆上升的过程中，元素会不断地在气体和熔体之间重新分配，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而变化，而且还随气体和熔体的相对上升速率而变化。而气体通常不保存或保存不良，补充淬火熔体，火山玻璃，保存在其化学异质性的气体-熔体相互作用的记录。因此，这些玻璃提供了岩浆上升和脱气的机制和动力学的记录。在这项研究计划中，我们将开发工具来读取这些记录。 作为该项目的一部分，火山岩中的化学和同位素变化将以米（现场）到微米（实验室）的比例绘制。特别关注的是金属和210 Pb-226 Ra的分布。为了从这种化学异质性中检索时间尺度信息，将对气体-熔体相互作用期间不同元素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不断变化的熔体和气体化学。这样的模型将把表面排放与岩浆上升和气体逸出率联系起来，这对准确评估火山危险至关重要，并有助于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，包括金属释放和浓缩到潜在的成矿流体中，玻璃工业以及类似的工业脱气过程，如冶炼和废物焚烧。对金属流动性及其对玻璃和熔体性质的依赖性的理解可用于在工业过程中保留或隔离金属并限制其排放。