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

像怀俄明州的黄石火山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，称为火山口。在过去的两百万年里，黄石公园的火山系统已经有过三次这样的火山口形成喷发。在这些非常大的喷发之间，这个系统还产生了许多不那么爆炸性的较小的喷发，这些喷发提供了大量的熔岩流，部分填满了火山口。因此，黄石火山等火山喷发的潜在危险和破坏性可能差异很大，从全国性甚至全球性影响到更局部的危害。火山科学中的一个悬而未决的问题是，是什么因素控制着火山在一次特定的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动。这可能在一定程度上取决于火山下方的“管道系统”如何随时间变化，以及大量以液态为主的岩浆是否持续存在，它们是否在很长一段时间内缓慢积累，或者它们是否只在喷发前存在。例如，产生一次非常大的喷发是否需要更长的时间来建立一个巨大的岩浆池？与规模较小的喷发相比，它是否需要更多的岩浆体存在，或者是否始终存在相同数量的岩浆体，但更多的岩浆体被触发以产生非常大的喷发？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喷发中有多少不同的岩浆体被挖掘出来，并将这一数字与被确认为最近一次火山口形成喷发的岩浆体的数量进行比较，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。该项目还将培训研究生和本科生，为科学工作者做出贡献，对结果的公众宣传将有助于公众对科学的理解。大型硅质系统产生广泛的喷发风格和喷发灾害，但岩浆储存区的建筑和所产生的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集层内不同的熔体分布，和/或不同的触发机制？作为对回答这些广泛问题的贡献，这个团队建议将火山口内喷发期间的岩浆体系结构与怀俄明州黄石火山口形成火山口的现有数据进行比较。新的高精度40Ar/39Ar测年表明，黄石公园(中央高原成员，CPM)最近一次火山口内喷发分五次喷发，每一次最多跨越1-2KYR。这提供了一个机会，在岩浆系统的五张快照中的每一张中检查岩浆体的数量和分布，并将其与已发表的关于黄石火山口形成喷发的工作进行比较。他们将利用辉长岩和锆石的238U-230th年龄数据，结合辉长岩和锆石的地球化学数据，研究在各个喷发和喷发幕内部和之间记录的辉长岩和锆石晶体中的成分多样性。在以前工作的基础上，闪长岩和锆石表面成分数据将提供对CPM喷发前一段时间内存在的不同岩浆体数量的洞察，并限制不同岩浆体在喷发前组装和储存的时间范围。相比之下，锆石内部的年龄通常较老，喷发前的年龄跨度为10 KYR，这将提供一种在较长时间尺度上存在的成分多样性的衡量标准，即在喷发时期之间。该团队将测试一种假设，即岩浆库的基本结构由多个成分不同的岩浆体组成，这些岩浆体位于更不均匀的晶体泥浆中，并且在CFE之前和最近的火山口内喷发之前，系统的状态是相似的。收集的数据还将使它们能够评估喷发晶体的平均成分是否随着时间的推移而发生系统变化，这可能反映出构成该系统的地幔和地壳来源的熔体比例的长期变化。将他们的新数据与黄石CFE的数据和其他硅质系统的数据相结合，将为大型硅质岩浆储集层的发展和演化提供更多的见解。更广泛的影响将包括PI为提高科学素养和公众参与度而进行的外联，对不同的研究生和本科生的培训和指导，加强学术界和USGS之间的伙伴关系，以及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NSF的法定使命，并通过使用基金会的智力优势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.