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

玻璃状黑岩(流纹岩)熔岩是公众最熟悉的火成岩之一，但由于黑岩流动在历史上从未发生过，对于这样的熔岩在陆地上扩散的速度有多快，或者这样的喷发持续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被记录在黑铁矿的微观结构中，比如被称为微石的小晶体的大小、形状和取向，这些晶体随着熔岩喷发和从喷口流出而生长。这种晶体也通常出现在黑铁矿中的离散带中，可能与流纹岩岩浆流动的方式有关。众所周知，这种晶体是随着岩浆喷发的冷却和气体损失而生长的，它们的结构可能会随着冷却和气体出溶速度的变化而明显不同。然而，这些纹理还没有被量化为黑铁矿流动。对微晶结构分布的现场研究，结合实验室中再现其生长的实验和分析研究，将被用来将微晶结构与喷发动力学联系起来，以确定黑岩熔岩在表面挤出和向外流动的速度有多快。这些答案将有助于理解与黑岩熔岩相关的危害，这种熔岩在世界各地和所有构造环境中都存在，尤其是怀俄明州黄石国家公园的大量喷发。事实上，黄石国家公园今天的大部分景观都是由占地100平方公里的黑岩熔岩塑造的，其中一些喷发于过去10万年内。黑岩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对下一次喷发做出反应。为了确定微晶结构如何记录黑岩熔岩的喷发和流动，将建立一个综合的数据库，其中包括多个熔岩的微观结构测量，重点是1)相似体积的多个熔岩，以及2)跨越大范围体积的熔岩。第一套方案将建立流动之间的共性，而第二套方案将确定条件如何变化以产生截然不同的涌出。这些流纹岩流来自美国境内位于加利福尼亚州、爱达荷州和怀俄明州的几个不同的火山中心。将检查所有流动中的微岩结构数据(类型、数量、大小、取向)和流带(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。这些实验不仅旨在推断目标熔岩的上升速率和脱气历史，而且还将探索更广泛的问题，即温度、流体组成和晶体含量对流纹岩岩浆结晶动力学的影响。确定这些熔岩在地表冷却需要多长时间也是至关重要的。将采用一种新的方法来研究球晶，球晶是一种散布在黑岩熔岩中常见的微晶的物质。众所周知，球粒陨石会随着冷却而增长，因此它们的大小、分布和成分变化可以确定黑麻岩熔岩的冷却方式。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，并使用同步辐射红外(水)和激光消融电感耦合等离子体质谱(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中。

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.