Magma Dynamics at Persistently Degassing Basaltic Volcanoes: A Novel Approach to Linking Volcanic Gases and Magmatic Volatiles within a Physical Model

玄武岩火山持续脱气的岩浆动力学：一种在物理模型中连接火山气体和岩浆挥发物的新方法

• 批准号: NE/F005342/1
• 负责人: Clive Oppenheimer
• 金额: $ 10.93万
• 依托单位: University of Cambridge
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FF005342%2F1
• 关键词: Magma; Dynamics; Persistently; Degassing; Basaltic

Volcanoes are the principal source of non-anthropogenic gases and aerosols injected into the atmosphere. A significant proportion of this gas comes not from volcanoes that are actually erupting, but from volcanoes that are quietly bubbling their gas to the atmosphere, or 'degassing persistently'. Many basaltic volcanoes can degas in this way for many years without a major eruption. Such volcanoes are often monitored for their gas chemistry and gas flux as well as a host a host of geophysical parameters. These monitoring data contain clues as to the underground movement of the magma that is degassing. In effect, the gas record affords us a window into the inner workings of magma chambers, which are not accessible by any other means. Making the link, however, between subterranean magma dynamics and magma gas records is not straightforward. It requires not only an understanding of the fundamental fluid dynamics of the mechanisms by which magma releases its gas, but also knowledge on the pressure-dependent solubility of the different gas species measured at the surface. The aim of this proposal is to develop fluid dynamical models of the convective processes thought to be responsible for magma degassing and to apply these results to two of the best monitored passively degassing volcanoes in the world, Masaya in Nicaragua, and Stromboli in Italy. The fluid mechanical models build on preliminary work at Bristol concerning the convective motion of gas-bearing magma within an underground chamber connected to the surface by a pipe. As the magma ascends it loses gas, becomes denser and sinks back down. The interaction between ascending gassy magma and sinking degassed magma exerts a key control on how degassing occurs and how it evolves with time. The process, although conceptually quite straightforward, is not easy to model because it involves interplay between convection and pressure-dependent gas loss, which have not previously been combined. The models, which are developed through a combination of analogue experiments and mathematics, can be used to make predictions about the composition of the gas emitted and how it varies with time. These can then be compared to the record from a well-monitored persistently degassing volcano. To do this we require a record not just of gas flux, but also of gas chemistry, for all of the major volcanic gas species. There are relatively few volcanoes at which such data are available because of the difficulty of analysing H2O and CO2, which are not only the most important volcanic gases, but are also abundant in the atmosphere. In order to measure these species we require a persistently degassing volcano with an accessible crater across which gas chemistry can be measured with minimal atmospheric interference. Masaya and Stromboli are ideal for that purpose. Data from Stromboli will be acquired though our collaboration with Project Partners in Italy, while we plan 3 new field campaigns to collect data at Masaya. In order that the fluid dynamical models are directly applicable to Masaya we will use high temperature and pressure experimental techniques to determine the solubility of the principal gas species, H2O, CO2, SO2 and HCl, in a sample of Masaya basalt. In order to constrain the initial gas budget of the Masaya magma we will analyse tiny quenched droplets of basalt liquid, known as melt inclusions, contained in crystals of olivine and plagioclase. These two types of additional information, solubility and initial gas inventory, are not currently available for Masaya, which makes any modelling of the degassing process rather difficult. The project brings together experts in volcanic and experimental petrology, volcano monitoring, gas chemistry and fluid mechanics. The ultimate objective of this research is a better understanding of how volcanoes work, with particular emphasis on how to interpret gas chemistry and its evolution with time from the point of view of impending volcanic hazard.

火山是注入大气中的非人为气体和气溶胶的主要来源。这种气体的很大一部分不是来自实际喷发的火山，而是来自那些正在悄悄地将气体冒泡到大气中或“持续排气”的火山。许多玄武岩火山可以通过这种方式脱气多年而不会发生大规模喷发。通常对此类火山的气体化学和气体通量以及大量地球物理参数进行监测。 These monitoring data contain clues as to the underground movement of the magma that is degassing.实际上，气体记录为我们提供了一扇了解岩浆室内部运作的窗口，而这是通过任何其他方式无法进入的。然而，在地下岩浆动力学和岩浆气体记录之间建立联系并不简单。它不仅需要了解岩浆释放气体机制的基本流体动力学，还需要了解在地表测量的不同气体种类的压力依赖性溶解度。该提案的目的是开发被认为负责岩浆脱气的对流过程的流体动力学模型，并将这些结果应用于世界上监测最好的两座被动脱气火山：尼加拉瓜的马萨亚火山和意大利的斯特龙博利火山。流体力学模型建立在布里斯托尔的初步工作基础上，该工作涉及通过管道与地面相连的地下室内含气岩浆的对流运动。当岩浆上升时，它会失去气体，变得更稠密并下沉。上升的含气岩浆和下沉的脱气岩浆之间的相互作用对脱气如何发生以及如何随时间演变发挥关键控制作用。该过程虽然在概念上非常简单，但并不容易建模，因为它涉及对流和压力相关的气体损失之间的相互作用，而这两者之前尚未结合起来。这些模型是通过模拟实验和数学相结合而开发的，可用于预测排放气体的成分及其随时间的变化。然后可以将这些记录与监控良好的持续排气火山的记录进行比较。为此，我们不仅需要记录所有主要火山气体种类的气体通量，还需要记录气体化学。由于分析 H2O 和 CO2 很困难，可获得此类数据的火山相对较少，而 H2O 和 CO2 不仅是最重要的火山气体，而且在大气中也含量丰富。为了测量这些物种，我们需要一座持续脱气的火山，并有一个可进入的火山口，可以在最小的大气干扰下测量气体化学。 Masaya 和 Stromboli 是实现此目的的理想选择。我们将通过与意大利项目合作伙伴的合作获取斯特龙博利的数据，同时我们计划开展 3 个新的实地活动以在马萨亚收集数据。为了使流体动力学模型直接适用于 Masaya，我们将使用高温高压实验技术来确定 Masaya 玄武岩样品中主要气体种类（H2O、CO2、SO2 和 HCl）的溶解度。为了限制马萨亚岩浆的初始气体预算，我们将分析橄榄石和斜长石晶体中含有的微小玄武岩液体淬火液滴，称为熔体包裹体。 Masaya 目前无法获得这两类附加信息（溶解度和初始气体库存），这使得脱气过程的任何建模都相当困难。该项目汇集了火山和实验岩石学、火山监测、气体化学和流体力学方面的专家。这项研究的最终目标是更好地了解火山的工作原理，特别强调如何从即将发生的火山灾害的角度解释气体化学及其随时间的演变。

项目成果

High-resolution size distributions and emission fluxes of trace elements from Masaya volcano, Nicaragua

尼加拉瓜马萨亚火山微量元素的高分辨率尺寸分布和排放通量

• DOI: 10.1029/2012jb009487
• 发表时间: 2012
• 期刊: Solid Earth
• 影响因子: 3.4
• 作者: Martin R