CAREER: The Effect of Bubbles on Magma Dynamics

自文明诞生以来，火山就一直具有魅力。喷发，除了其明显的相关危险，是地球在时间尺度上的动力学的罕见表达之一，从我们的立场很容易理解。在过去的几十年里，我们的社区已经认识到挥发物(气泡)对浅层地壳中岩浆的化学和物理演化以及喷发动力学的重要性。火山过程的复杂性，以及我们无法直接观察到火山喷发之前和期间岩浆是如何演化的，这严重限制了我们预测火山喷发的时间和行为的能力。在这个项目中，我们建议开发新的数值方法，并辅之以流体力学实验，以提供对气体出溶如何影响岩浆的物理性质、它们的化学成分(以及释放到大气中的气体的组成)以及最终气泡如何集体行为并影响地球表面喷发的岩浆行为的定量了解。岩浆是由非常粘性的环境流体(硅酸盐熔体)、晶体组成的多相系统，有时还由出溶(不混溶)的气泡补充。为了预测岩浆在喷发前和喷发期间如何演化，我们需要开发动力学模型，使我们能够准确地描述这三个阶段在不同条件下的相互作用。在这个项目中，我们计划首先研究富水气泡的出溶和生长，以模拟它们对化学分化的影响，即硫物种和二氧化碳等次生气相如何分配成不断增长的气泡。喷发前进入气泡的硫磺含量对于量化喷发对气候的影响意义重大(硫磺是一种影响大气辐射平衡的强效气溶胶)。出溶气相的化学成分及其在未来喷发中的参与程度取决于岩浆中相分离的效率，以及气泡在储集层中更有可能喷发的部分(粘度较低)的可能积聚。我们的第二个任务是研究相分离，气泡-晶体-熔化的物理学，更具体地说，了解离散气泡和晶体的存在是如何阻碍或促进岩浆运动的。最后，一旦多相岩浆的流变性得到更好的约束，我们计划利用这些结果，研究浮力气泡如何在带状岩浆储集层中迁移。我们要回答的问题是，气泡是否容易聚集在晶体含量低或高的区域，以及这种聚集如何影响气泡中气体的化学和岩浆的喷发行为。拟议的研究结果将对物理火山学、岩石学、地球化学和流体动力学产生广泛的影响。它将提供对储存在地壳浅层储集层中的岩浆状态的定量限制，还将更好地解释喷发期间气泡对岩浆上升到地表的影响。硫磺在岩浆系统中出溶的预测模型还可以提供有关史前大型爆炸性喷发(如Toba、Cerro Galan)对气候影响的线索。拟议研究的目标是提供可广泛用于解释现有数据集(岩浆地球化学、岩浆流变学实验)的量化工具和本构关系，并将这一新知识转化为岩浆动力学模型。此外，研究多相流体动力学的新的数值方法的发展将影响计算流体动力学社区和科学和工程中的其他领域，其中颗粒悬浮或气泡乳化是重要的(例如，流体动力学对生物、食品加工的影响？)

Volcanoes have exerted a fascination since the dawn of civilization. Eruptions, besides their obvious associated hazards, are one of the rare expressions of the dynamics of the Earth over timescales that are easy to comprehend from our standpoint. Over the last decades, our community has recognized the importance of exsolved volatiles (gas bubbles) on the chemical and physical evolution of magmas in the shallow crust and on eruption dynamics. The complexity of volcanic processes and our inability to observe directly how magmas evolve prior and during volcanic eruptions has seriously limited our ability to predict the timing and behavior of volcanic eruptions. With this project, we propose the development of new numerical methods, complemented with fluid dynamics experiments to provide a quantitative understanding of how the exsolution of gases affects the physical properties of magmas, their chemistry (and the composition of gases released to the atmosphere) and ultimately how bubbles behave collectively and affect the behavior of magmas erupting at the Earth surface.Magmas are multiphase systems composed of a very viscous ambient fluid (silicate melt), crystals and sometimes complemented by exsolved (immiscible) gas bubbles. In order to predict how magmas evolve before and during eruptions, we need to develop dynamic models that allow us to accurately represent the interplay between these three phases under various conditions. In this project, we plan to study first the exsolution and growth of water-rich bubbles to model their effect on chemical differentiation, i.e. how secondary gas phases such as sulfur species and CO2 partition into growing bubbles. The amount of sulfur incorporated into bubbles before an eruption is significant to quantify the impact of eruptions on climate (sulfur is a potent aerosol that affects the radiative balance in the atmosphere). The chemistry of the exsolved gas phase and its participation in a future eruption depend on the efficiency of phase separation in magmas and the possible accumulation of gas bubbles in parts of the reservoir that are more likely to erupt (lower viscosity). Our second task is to study the physics of phase separation, bubble-crystals-melt, more specifically to understand how the magma motion is impeded or facilitated by the presence of discrete bubbles and crystals. Finally, once the rheology of multiphase magmas is better constrained, we plan to use these results and study how buoyant gas bubbles migrate in zoned magma reservoirs. The question we aim to answer is whether bubbles are prone to accumulate in regions of low or high crystal content and how this accumulation affects the chemistry of the gas in the bubbles and the eruptive behavior of the magma.The results of the proposed research will have broad implications for physical volcanology, petrology, geochemistry and fluid dynamics. It will provide quantitative constraints on the state of magmas stored in shallow crustal reservoirs and also provide a better account of the effect of bubbles on magmas as they ascend to the surface during eruptions. Predictive models for the exsolution of sulfur in magmatic systems can also provide clues as to the climate impact of pre-historical large explosive eruptions (e.g. Toba, Cerro Galan). The goal of the proposed research is to provide quantitative tools and constitutive relations that will be widely available to interpret existing datasets (geochemistry of magmas, rheological experiments on magmas) and transfer this new knowledge to models of magma dynamics. Additionally, the development of new numerical methods to study multiphase fluid dynamics will impact the Computational Fluid Dynamics community and other fields in science and engineering, where particle suspensions or bubble emulsions are important (e.g. effect of fluid dynamics on biology, food processing?)