Geochemical Insights Into the Post-Caldera Architecture of the Yellowstone Magma Reservoir

像怀俄明州黄石公园这样的火山系统会产生非常大的、高度爆炸性的喷发，形成巨大的火山口。在过去的两百万年里，黄石公园的火山系统发生了三次形成火山口的喷发。在这些非常大的喷发之间，该系统还产生了许多较小的喷发，这些喷发产生了大量的熔岩流，部分填满了火山口。因此，像黄石这样的火山爆发的潜在危害和破坏性可能差别很大，从全国性甚至全球性的影响到更多的局部危害。火山科学中的一个悬而未决的问题是，是什么因素控制着一座火山在某一次喷发中是产生非常大的爆炸性喷发，还是产生较小的熔岩流。这可能部分取决于火山下面的“管道系统”如何随着时间的推移而变化，以及大型的液体岩浆体是否持续存在，它们是在很长一段时间内缓慢积聚，还是只在喷发前出现。例如，产生一次非常大的喷发是否需要更长的时间来形成一个大的岩浆池？它是否需要比一次较小的喷发更多的岩浆体存在，或者是否在任何时候都有相同数量的岩浆体存在，但更多的岩浆体被触发来产生一次非常大的喷发？在这个项目中，研究人员将使用不同岩浆体的化学“指纹”来确定在黄石公园最近一系列较小的火山喷发中有多少不同的岩浆体被挖掘出来，并将其与最近一次火山口形成火山喷发中确定的岩浆体数量进行比较，以帮助回答这些问题。了解不同类型的喷发是如何产生的，将有助于火山灾害预测和风险管理。该项目还将培养研究生和本科生，为科学队伍做出贡献，而有关研究结果的公众宣传将有助于促进公众对科学的理解。大型硅质体系产生了多种喷发类型和喷发危险，但岩浆储存区结构与喷发类型之间的联系尚不清楚。例如，与较小的喷发相比，形成火山口的喷发是否需要浅层储层中不同的熔体分布和/或不同的触发机制？为了回答这些广泛的问题，该团队建议将火山口内喷发期间岩浆系统的结构与怀俄明州黄石火山口形成火山的现有数据进行比较。对黄石公园（中央高原成员，CPM）最近的火山口内喷发进行的高精度40Ar/39Ar测年显示，它们分为五次喷发，每次喷发最多持续1- 2kyr。这提供了一个机会，可以在岩浆系统的五个快照中检查岩浆体的数量和分布，并将其与已发表的黄石火山口形成火山的工作进行比较。他们将利用238u -230年代的数据，结合化探数据，研究在单个喷发和喷发事件内部和之间，记录的火山灰和锆石晶体的成分多样性。在前人工作的基础上，锆石和硅烷表面成分数据将提供CPM喷发前一段时间内不同岩浆体数量的信息，并限制喷发前不同岩浆体聚集和储存的时间尺度。相比之下，锆石内部通常更古老，在火山喷发之前的年代跨度为10万年，这将提供在更长的时间尺度上，在火山喷发之间存在的成分多样性的衡量标准。研究小组将验证这样一个假设，即岩浆储层的基本结构是由多个成分不同的岩浆体组成的，这些岩浆体位于一个更不均匀的晶体糊状物中，并且在CFE和最近的火山口内喷发之前，该系统的状态是相似的。收集到的数据还将使他们能够评估随着时间的推移，喷发晶体的平均成分是否存在系统性的变化，这可能反映出地幔源和地壳源熔体对该系统的贡献比例的长期变化。将他们的新数据与黄石CFE和其他硅质系统的数据综合起来，将为大型硅质岩浆储层的发育和演化提供更多的见解。更广泛的影响将包括PI的推广，以提高科学素养和公众参与，培训和指导不同的研究生和本科生，加强学术界与美国地质勘探局之间的伙伴关系，并提供将用于公共政策和火山灾害缓解的信息。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Volcanic systems such as Yellowstone in Wyoming produce very large, highly explosive eruptions that form immense craters called calderas. The volcanic system at Yellowstone has had three of these caldera-forming eruptions over the past two million years. In between these very large eruptions the system has also produced many smaller eruptions that were less explosive, which fed large lava flows that partially filled the caldera. As a result, the potential hazards and destructiveness of an eruption at volcanoes such as Yellowstone can vary widely, ranging from nation-wide or even global effects to more local hazards. An open question in volcano science is what factors control whether a volcano produces a very large explosive eruption or a smaller lava flow in a particular eruption. This probably depends partly on how the “plumbing system” beneath volcanoes changes over time, and whether large bodies of mostly-liquid magma are present continuously, whether they build up slowly over long periods of time, or whether they are only present just before an eruption. For example, does generating a very large eruption require a longer time to build a big pool of magma? Does it require more magma bodies to be present than a smaller eruption, or are there the same number of magma bodies present at all times but more of them get triggered to produce a very large eruption? In this project researchers will use chemical “fingerprints” of different magma bodies to identify how many different magma bodies have been tapped in the most recent series of smaller eruptions at Yellowstone, and will compare this to the number of magma bodies that have been identified as part of the most recent caldera-forming eruption to help answer these questions. Understanding how different kinds of eruptions are produced will contribute to volcanic hazard forecasting and risk management. The project will also train graduate and undergraduate students, contributing to the scientific workforce, and public outreach about the results will help contribute to scientific understanding for the general public.Large silicic systems produce a wide range of eruption styles and eruptive hazards, but the connections between the architecture of the magma storage region and the type of eruption produced are not well understood. For example, does a caldera-forming eruption require a different distribution of melts within the shallow reservoir, and/or a different triggering mechanism, than a smaller eruption? As a contribution to answering these broad questions, this team proposes to compare the architecture of the magma system during intra-caldera eruptions to existing data for caldera-forming eruptions at Yellowstone Caldera, Wyoming. New high-precision 40Ar/39Ar dating of the most recent intracaldera eruptions at Yellowstone (the Central Plateau Member, CPM) shows that they were erupted in five episodes, each of which spanned at most 1-2 kyr. This provides an opportunity to examine the number and distribution of magma bodies during each of five snapshots of the magma system, and to compare this with published work on the Yellowstone caldera-forming eruptions. They will investigate compositional diversity recorded in sanidine and zircon crystals both within and between individual eruptions and eruptive episodes using 238U-230Th age data coupled with geochemical data for both sanidine and zircon. Based on previous work, the sanidine and zircon surface compositional data will provide insights into the number of distinct magma bodies present during the period immediately prior to CPM eruptive episodes, as well as constraining the time scale of assembly and storage of different magma bodies prior to eruption. In contrast, zircon interiors are typically older with ages spanning 10s of kyr prior to eruption, which will provide a measure of compositional diversity present on a longer time scale, between eruptive episodes. The team will test the hypothesis that the basic architecture of the magma reservoir consists of multiple compositionally distinct magma bodies within a more heterogeneous crystal mush, and that the state of the system is similar before CFE and recent intracaldera eruptions. The data collected will also allow them to assess whether there are systematic changes in the average composition of erupted crystals over time, which may reflect secular variations in the proportions of mantle-derived and crustal-derived melts contributing to the system. Synthesizing their new data with data for Yellowstone CFE and data for other silicic systems will provide additional insights into the development and evolution of large silicic magma reservoirs. Broader impacts will include outreach by the PI to increase scientific literacy and public engagement, training and mentoring of diverse graduate and undergraduate students, strengthening partnerships between academia and the USGS, and contributing information that will be used to inform public policy and volcanic hazard mitigation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.