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

玻璃状黑曜石（流纹岩）熔岩是公众最熟悉的火成岩之一，但由于黑曜石流动在历史上没有发生过，因此对于这些熔岩在陆地上传播的速度有多快或这种喷发持续多久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石的微观结构中，例如被称为微晶石的小晶体的大小，形状和方向，这些晶体随着熔岩喷发并从喷口流出而生长。 这种晶体也通常出现在黑曜石中的离散带中，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冷却和喷发岩浆中气体损失的反应，它们的质地会随着冷却和气体出溶速率的变化而变化。 然而，黑曜石流的这些纹理还没有量化。 微晶纹理分布的实地研究，结合实验和分析研究，再现其在实验室中的增长将被用来关联微晶纹理和喷发动力学，以确定如何快速黑曜石熔岩挤出表面和向外流动。 这些答案将有助于理解与黑曜石熔岩相关的危害，黑曜石熔岩在世界各地和所有构造环境中都有发生，尤其是在怀俄明州的黄石国家公园。 事实上，黄石国家公园现今的大部分景观都是由黑曜石熔岩形成的，这些熔岩覆盖了100平方公里，其中一些是在过去的10万年里爆发的。 黑曜石熔岩喷发是黄石国家公园未来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应对下一次喷发。为了确定微晶石纹理如何记录黑曜石熔岩的喷发和流动，将建立多个熔岩微观结构测量的综合数据库，集中在1）相似体积的多个熔岩，和2）体积跨度大的熔岩。 第一组将建立流动之间的共性，而第二组将建立条件如何变化，以产生巨大的不同的流出。 这些流纹岩流来自美国境内的几个不同的火山中心，位于加州、爱达荷州和怀俄明州。 将在所有流动中检查微粒（类型，数量，大小，方向）和流动条带（空间分布，宽度）的纹理数据，并通过减压实验与岩浆上升和脱气历史联系起来。 这些实验的目的不仅是推断上升速率和脱气历史的目标熔岩，但也探索更广泛的问题，温度，流体成分和晶体含量的影响，在流纹岩岩浆结晶动力学。 确定这些熔岩在地表冷却需要多长时间也很关键。 一种新的方法，将追求将是检查球晶，辐射质量的微晶通常发现在黑曜石熔岩。 已知球粒会随着冷却而生长，因此它们的大小、分布和成分变化可以确定黑曜石熔岩如何冷却。 球晶生长模型将通过高分辨率X射线计算机断层扫描测量球晶的尺寸分布，并通过同步辐射源红外（水）和激光消融ICP-MS（阳离子）分析球晶周围的多元素成分分布来开发，这将允许提取样品的冷却历史并将其置于熔岩侵位的背景下。

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.