CAREER: The Effect of Bubbles on Magma Dynamics

自从人类文明出现以来，火山就一直发挥着迷人的作用。火山爆发，除了它们明显的相关危险之外，是地球在时间尺度上的动力学的罕见表现之一，从我们的角度来看，这很容易理解。在过去的几十年里，我们已经认识到溶解的挥发物（气泡）对浅层地壳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火山爆发之前和期间岩浆的演变，严重限制了我们预测火山爆发时间和行为的能力。在这个项目中，我们建议发展新的数值方法，并辅以流体动力学实验，以定量地了解气体的溶解如何影响岩浆的物理性质、它们的化学性质（以及释放到大气中的气体的成分），以及最终气泡如何集体行为并影响地球表面岩浆喷发的行为。岩浆是一种多相系统，由非常粘稠的环境流体（硅酸盐熔体）、晶体组成，有时还含有溶解的（不混溶的）气泡。为了预测岩浆在喷发前和喷发期间是如何演化的，我们需要建立动态模型，使我们能够准确地表示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析出和生长，以模拟它们对化学分化的影响，即二次气相（如硫种和二氧化碳）如何分配到生长的气泡中。火山喷发前气泡中硫的含量对于量化火山喷发对气候的影响具有重要意义（硫是一种影响大气辐射平衡的强力气溶胶）。溶解气相的化学性质及其对未来喷发的影响取决于岩浆中相分离的效率，以及储层中更容易喷发的部分（粘度较低）可能积聚的气泡。我们的第二个任务是研究相分离，气泡-晶体-熔融的物理学，更具体地说，是为了了解岩浆运动是如何被离散气泡和晶体的存在所阻碍或促进的。最后，一旦多相岩浆的流变学得到更好的约束，我们计划利用这些结果研究浮力气泡在分带岩浆储层中的迁移。我们想要回答的问题是，气泡是否容易在晶体含量低或高的区域积聚，以及这种积聚如何影响气泡中气体的化学性质和岩浆的喷发行为。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影响。它将对储存在地壳浅层储层中的岩浆状态提供定量约束，并更好地解释火山喷发期间气泡上升到地表时对岩浆的影响。岩浆系统中硫溶出的预测模型也可以为史前大型爆炸喷发（如多巴火山、塞罗加兰火山）对气候的影响提供线索。提出的研究目标是提供定量工具和本构关系，这些工具和本构关系将广泛用于解释现有数据集（岩浆地球化学，岩浆流变实验），并将这些新知识转移到岩浆动力学模型中。此外，研究多相流体动力学的新数值方法的发展将影响计算流体动力学社区和其他科学和工程领域，其中颗粒悬浮液或气泡乳剂是重要的（例如流体动力学对生物学，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?)