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

火山是向大气中注入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真正喷发的火山，而是来自火山悄悄地将气体冒泡到大气中，或“持续脱气”。许多玄武岩火山可以以这种方式脱气多年而不发生大喷发。人们经常监测这些火山的气体化学和气体通量，以及大量的地球物理参数。这些监测数据包含了正在脱气的岩浆在地下运动的线索。实际上，气体记录为我们提供了一扇通往岩浆房内部工作的窗口，这是其他任何方法都无法到达的。然而，在地下岩浆动力学和岩浆气体记录之间建立联系并非易事。它不仅需要了解岩浆释放气体机制的基本流体动力学，而且还需要了解在地表测量到的不同气体种类的压力依赖性溶解度。这项提议的目的是建立对流过程的流体动力学模型，该模型被认为是岩浆脱气的原因，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。流体力学模型是建立在布里斯托尔的初步研究基础上的，研究的是含气岩浆在一个通过管道与地表相连的地下洞穴内的对流运动。岩浆上升时失去气体，密度增大，然后下沉。上升的气态岩浆和下沉的脱气岩浆之间的相互作用对脱气如何发生及其随时间的演变起着关键的控制作用。这个过程，虽然在概念上很简单，但并不容易建模，因为它涉及对流和压力相关气体损失之间的相互作用，这两者以前没有结合起来。这些模型是通过模拟实验和数学相结合开发的，可以用来预测所排放气体的组成以及它如何随时间变化。然后可以将这些数据与监测良好的持续脱气火山的记录进行比较。要做到这一点，我们不仅需要气体通量的记录，还需要所有主要火山气体种类的气体化学记录。由于分析H2O和CO2这两种不仅是最重要的火山气体，而且在大气中含量丰富的气体的困难，能获得这类数据的火山相对较少。为了测量这些物种，我们需要一个持续脱气的火山，有一个可以接近的火山口，在最小的大气干扰下可以测量气体化学。Masaya和Stromboli是理想的选择。来自Stromboli的数据将通过我们与意大利项目合作伙伴的合作获得，同时我们计划在Masaya进行3次新的实地活动来收集数据。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主要气体种类H2O， CO2， SO2和HCl在Masaya玄武岩样品中的溶解度。为了限制Masaya岩浆的初始气体收支，我们将分析玄武岩液体的微小淬火液滴，称为熔融包裹体，包含在橄榄石和斜长石晶体中。Masaya目前还没有这两种类型的附加信息，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。这项研究的最终目的是更好地了解火山是如何工作的，特别强调如何从即将发生的火山危险的角度解释气体化学及其随时间的演变。

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.