Collaborative Research: The Dynamics of Rhyolite Lava Eruption and Emplacement Inferred from Micro-Textures, Decompression Experiments, and Numerical Modeling

玻璃状黑曜岩（流纹岩）熔岩是公众最熟悉的火成岩之一，但由于黑曜岩流在历史上没有发生过，所以对这些熔岩在陆地上扩散的速度有多快或爆发持续了多长时间等基本问题没有明确的答案。然而，这些问题的答案可能会记录在黑曜岩的微观结构中，比如被称为微晶岩的小晶体的大小、形状和方向，这些小晶体是在熔岩喷发并从喷口流出时生长的。这种晶体也经常出现在黑曜岩内部的离散带中，可能与流纹岩岩浆流动的方式有关。众所周知，这种晶体的生长是对岩浆喷发的冷却和气体损失的反应，它们的结构会随着冷却和气体析出的速度的变化而有很大的不同。然而，这些纹理还没有被量化为黑曜岩流。对微岩结构分布的实地研究，结合在实验室中再现其生长的实验和分析研究，将用于将微岩结构与喷发动力学联系起来，以确定黑曜岩熔岩在表面挤压和向外流动的速度。这些答案将有助于了解与黑曜岩熔岩有关的危害，黑曜岩熔岩在世界各地和所有构造环境中都有发生，尤其是怀俄明州黄石国家公园的大规模喷发。事实上，如今黄石国家公园的大部分景观都是由覆盖了100平方公里的黑曜岩熔岩形成的，其中一些熔岩是在过去10万年里喷发的。黑曜岩熔岩喷发是未来黄石国家公园最有可能发生的岩浆喷发类型之一，因此了解它们的喷发行为将有助于科学家对下一次喷发做出反应。为了确定微岩结构如何记录黑曜岩熔岩的喷发和流动，将建立一个来自多个熔岩的微岩结构测量的综合数据库，重点关注1)体积相似的多个熔岩，以及2)体积范围大的熔岩。第一组将确定流动之间的共性，而第二组将确定条件如何变化以产生截然不同的输出。这些流纹岩流来自美国境内几个不同的火山中心，分别位于加利福尼亚州、爱达荷州和怀俄明州。将在所有流动中检查微岩的结构数据（类型、数量、大小、方向）和流动带（空间分布、宽度），并通过减压实验将其与岩浆上升和脱气历史联系起来。这些实验不仅可以推断出目标熔岩的上升速率和脱气历史，还可以探索温度、流体成分和晶体含量对流纹岩岩浆结晶动力学的影响等更广泛的问题。确定这些熔岩在地表冷却需要多长时间也是至关重要的。研究人员将采用一种新颖的方法来研究球晶，这是黑曜岩熔岩中常见的微晶辐射团。众所周知，球晶会随着冷却而生长，因此它们的大小、分布和成分变化可以确定黑曜岩熔岩是如何冷却的。球晶生长模型将通过使用高分辨率x射线计算机断层扫描测量球晶的尺寸分布，并使用同步源红外（水）和激光烧蚀ICP-MS（离子）分析球晶周围的多元素组成剖面来开发，这将允许提取样品的冷却历史并将其置于熔岩就位的背景下。

Glassy obsidian (rhyolite) lava is one of the best known igneous rocks to the public, but because obsidian flows have not occurred historically, there are no clear answers to such basic questions as how fast do such lavas spread across the land or how long do such eruptions last. Answers to those questions may, however, be recorded in micro-textures in the obsidian, such as the sizes, shapes, and orientations of small crystals, known as microlites, which grew as the lava erupted and flowed away from the vent. Such crystals also commonly occur in discrete bands within obsidian, probably related to the way rhyolite magma flows. It is known that such crystals grow in response to cooling and gas loss from the erupting magma, and their textures can differ strongly in response to changing rates of cooling and gas exsolution. Those textures have not, however, been quantified for obsidian flows. Field studies of the distributions of microlite textures, in conjunction with experimental and analytical studies reproducing their growth in the laboratory will be used to relate microlite textures and eruption dynamics to determine how fast obsidian lava extrudes at the surface and flow outwards. Those answers will aid in understanding the hazards associated with obsidian lavas, which occur worldwide and in all tectonic environments, with especially large outpourings in Yellowstone National Park, Wyoming. In fact, much of the present-day landscape of Yellowstone National Park is shaped by obsidian lavas that cover 100s of square kilometers, some of which erupted in the past 100,000 years. Obsidian lava eruptions are one of the most likely types of magmatic eruption to occur in the future at Yellowstone National Park, and so understanding their eruptive behavior will aid scientists in responding to the next eruption.To establish how microlite textures record the eruption and flow of obsidian lava, an integrated database of micro-textural measurements from multiple lavas will be established, focused on 1) multiple lavas of similar volume, and 2) lavas that span a large range in volume. The first set will establish commonalities between flows, whereas the second will establish how conditions change to produce greatly different outpourings. Those rhyolite flows come from several distinct volcanic centers within the United States, located in California, Idaho, and Wyoming. Textural data of microlites (types, numbers, sizes, orientations) and flow banding (spatial distribution, widths) will be examined in all flows, and linked to magma ascent and degassing histories through decompression experiments. Those experiments will be designed to not only infer ascent rates and degassing histories of targeted lavas, but also to explore broader questions about the impacts of temperature, fluid composition, and crystal content on crystallization kinetics in rhyolite magma. It will be also critical to establish how long it takes for such lavas to cool at the surface. A novel approach that will be pursued will be to examine spherulites, radiating masses of microlites commonly found in obsidian lava. Spherulites are known to grow in response to cooling, and so their sizes, distributions, and compositional variations can establish how obsidian lava cools. Spherulite growth models will be developed by measuring size distributions of spherulites with high-resolution X-ray Computed Tomography and analyzing multi-element compositional profiles around spherulites with synchrotron-sourced infrared (water) and laser-ablation ICP-MS (cations), which will allow the cooling history of a sample to be extracted and placed into context of lava emplacement.