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

火山是注入大气的非人为气体和气溶胶的主要来源。这些气体中有很大一部分不是来自真正喷发的火山，而是来自那些悄悄地向大气中冒泡的火山，或者说是“持续脱气”。许多玄武质火山可以以这种方式脱气多年而不会发生大喷发。这些火山经常被监测其气体化学和气体流量以及大量的地球物理参数。这些监测数据包含了关于正在脱气的岩浆地下运动的线索。实际上，气体记录为我们提供了一个了解岩浆房内部运作的窗口，这是任何其他手段都无法达到的。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的机制的基本流体动力学，而且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。这项建议的目的是开发对流过程的流体动力学模型，被认为是负责岩浆脱气，并将这些结果应用到两个最好的监测被动脱气火山在世界上，马萨亚在尼加拉瓜，和斯特龙博利在意大利。流体力学模型建立在布里斯托的初步工作基础上，该工作涉及通过管道与地面相连的地下室中含气岩浆的对流运动。当岩浆上升时，它会失去气体，变得更致密，然后又下沉。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作用的发生及其随时间的演化起着关键的控制作用。这个过程虽然在概念上很简单，但不容易建模，因为它涉及对流和压力相关气体损失之间的相互作用，这两个因素以前没有结合起来。这些模型是通过模拟实验和数学相结合开发的，可用于预测排放气体的成分及其随时间的变化。然后，这些可以与一个监测良好的持续脱气火山的记录进行比较。要做到这一点，我们不仅需要气体流量的记录，还需要所有主要火山气体种类的气体化学成分的记录。由于分析H2O和CO2的困难，拥有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，而且在大气中也很丰富。为了测量这些物种，我们需要一个持续脱气的火山，有一个可进入的火山口，可以在最小的大气干扰下测量气体化学。马萨亚和斯特龙博利是理想的为此目的。我们将通过与意大利项目合作伙伴的合作获得Stromboli的数据，同时我们计划在马萨亚开展3项新的实地活动以收集数据。为了使流体动力学模型直接适用于马萨亚，我们将使用高温高压实验技术，以确定溶解度的主要气体种类，H2O，CO2，SO2和HCl，在一个样本的马萨亚玄武岩。为了限制最初的气体预算的马萨亚岩浆，我们将分析微小的淬火液滴的玄武岩液体，称为熔融包裹体，包含在晶体的橄榄石和斜长石。这两种类型的额外信息，溶解度和初始气体存量，目前还没有为马萨亚，这使得任何脱气过程的建模相当困难。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是如何工作的，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。

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.