Collaborative Research: Recharge, Mixing and Eruption Triggering Mechanisms at Chaos Crags and 1915 Eruptions, Lassen Volcanic Center, California

合作研究：混沌峭壁和 1915 年火山喷发的补给、混合和喷发触发机制，拉森火山中心，加利福尼亚州

• 批准号: 1250323
• 负责人: Keith Putirka
• 金额: $ 10.54万
• 依托单位: California State University-Fresno Foundation
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-01-01 至 2015-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1250323&HistoricalAwards=false
• 关键词: Collaborative; Research; Recharge; Mixing; Eruption

This project will produce advances in pure science with broader impacts in public safety and education. The Lassen volcanic center had its last major eruption in 1915 at Lassen Peak, and it is located beneath many of the flight paths of airlines that connect cities in the western U.S. An understanding of how and why volcanic eruptions are triggered at Lassen Peak thus will play a key role in future hazards assessments. Our study brings together researchers with a wide variety of expertise to examine several processes operating prior to eruption, including eruption triggers and magma storage. For example, prior work shows several mechanisms by which an eruption even may be triggered. One line of thought is that eruptions are triggered soon after new magma is introduced into a magma reservoir beneath a volcano, as new magmas mix with magmas already present in a shallow chamber. Alternatively, new magma inputs may need to cool, and partially crystallize before an eruption occurs. This cooling drives water into the remaining uncrystallized magma, which increases pressure within the magmatic system if enough water is concentrated into the magma to form a vapor phase. Expansion of the vapor phase may crack the overlying rock and allow an eruption to occur. Lassen provides an important testing ground for these ideas because the two most recent eruptive episodes (1915 at Lassen Peak, and the earlier eruption at approximately 1144 AD at Chaos Crags) both show evidence for pre-eruption magma inputs, but those fresh inputs acted very differently in the two cases. Thus a comparison of the two eruptions will provide insight into the importance of different triggering mechanisms in the Lassen magma system. This study will bring together researchers and students across the spectrum of universities, with representatives from major research institutions, state service universities and a liberal arts college. This will enhance the education of many students, as it provides valuable opportunities to support under-represented groups (Hispanics and women) at both the undergraduate and graduate level, to work in laboratories and with researchers across the globe. In addition, many undergraduates at CSU Fresno will participate in the research as research projects are routinely integrated into the laboratory requirements for core courses.To attack the research problems outlined above, we will perform a collaborative study of the relationships between magma recharge (fresh inputs of magma), magma mixing and eruption, in the Chaos Crags and 1915 eruptions of the Lassen Volcanic Center, California. The study will use a combination of textural studies to assess crystallization processes, U-series (radiometric) and diffusion profile methods for age dating, and mineral-melt and fluid inclusion studies to delimit pressures and temperatures of magma storage. The specific targets of investigation are the 6 domes of Chaos Crags (denoted as A to F), and the more well-mixed 1915 eruptive products at Lassen Peak. At Lassen Peak, magmas mixed intimately prior to eruption, while at Chaos Crags, mixing was for some reason inhibited. Does this contrast reflect (a) the timing between fresh magma inputs and eruption; (b) contrasts in temperatures and depths of magma storage/interaction or; (c) the relative amounts of hot fresh magma inputs compared to the cooler magmas that already inhabit the chamber (left-over from some prior magmatic episode)? The Chaos Crags units are of special interest because Dome A shows little evidence of mixing?and so yields the most extreme felsic and mafic compositions; all other domes, and the 1915 lavas, fall between these extremes. Dome A thus provides access to end-member magmas. The Chaos Crags and Lassen Peak suites also expose interesting textural contrasts as the Chaos Crags rocks are more crystalline, and their enclaves show a greater variety of quenching textures, which suggests a relationship between mixing and recharge. Key questions include: (1) Does recharge of a chamber with mafic magma trigger mixing (and eventually, eruption), or is there a time lag between recharge and mixing? (2) What effect does a time lag between the initial intrusion of felsic magma and the later intrusion of mafic recharge magma have on the efficacy of mixing and the time scale of eruption? (3) Does the ratio of mafic recharge/resident felsic magma affect mixing and/or the timing of eruption? (4) Do the depths and temperatures of magma storage affect the mixing/eruption process?

该项目将带来纯科学方面的进步，对公共安全和教育产生更广泛的影响。拉森火山中心于 1915 年在拉森峰进行了最后一次大规模喷发，该火山中心位于连接美国西部城市的许多航空公司的航线下方。因此，了解拉森峰如何以及为何引发火山喷发将发挥重要作用在未来的危害评估中发挥关键作用。 我们的研究汇集了具有广泛专业知识的研究人员，以检查喷发前的几个运行过程，包括喷发触发因素和岩浆储存。例如，先前的工作显示了甚至可能引发火山喷发的几种机制。一种想法是，新的岩浆被引入火山下方的岩浆库后不久就会引发喷发，因为新的岩浆与浅室中已经存在的岩浆混合。或者，新的岩浆输入可能需要冷却，并在喷发发生之前部分结晶。这种冷却将水驱入剩余的未结晶岩浆中，如果有足够的水浓缩到岩浆中形成气相，就会增加岩浆系统内的压力。气相的膨胀可能会使上覆的岩石破裂并导致喷发。拉森为这些想法提供了一个重要的试验场，因为最近的两次喷发事件（1915 年的拉森峰，以及大约公元 1144 年混沌峭壁的早期喷发）都显示了喷发前岩浆输入的证据，但这些新的输入表现得非常好。两种情况有所不同。因此，对两次喷发的比较将有助于深入了解拉森岩浆系统中不同触发机制的重要性。这项研究将汇集各大学的研究人员和学生，其中包括来自主要研究机构、公立服务大学和文理学院的代表。这将加强许多学生的教育，因为它提供了宝贵的机会来支持本科和研究生阶段代表性不足的群体（西班牙裔和女性），在实验室工作并与全球研究人员一起工作。此外，科罗拉多州立大学弗雷斯诺分校的许多本科生将参与这项研究，因为研究项目通常会纳入核心课程的实验室要求中。为了解决上述研究问题，我们将对岩浆补给（新鲜输入）之间的关系进行合作研究岩浆）、岩浆混合和喷发，在混沌峭壁和 1915 年加利福尼亚州拉森火山中心的喷发中。该研究将结合使用结构研究来评估结晶过程、U系列（辐射测量）和扩散剖面方法来进行年龄测定，以及矿物熔体和流体包裹体研究来界定岩浆储存的压力和温度。调查的具体目标是混沌峭壁的 6 个圆顶（用 A 到 F 表示），以及拉森峰 1915 年喷发产物的混合更均匀。 在拉森峰，岩浆在喷发前充分混合，而在混沌峭壁，混合由于某种原因受到抑制。这种对比是否反映了 (a) 新岩浆输入和喷发之间的时间？ (b) 岩浆储存/相互作用的温度和深度的对比； (c) 与已经存在于室内的较冷岩浆（先前岩浆事件留下的）相比，新鲜热岩浆输入的相对量？ Chaos Crags 单位特别令人感兴趣，因为 Dome A 几乎没有显示出混合的证据，因此产生了最极端的长英质和镁铁质成分；所有其他圆顶以及 1915 年的熔岩都处于这两个极端之间。因此，穹顶 A 提供了接触端元岩浆的通道。混沌峭壁和拉森峰套房也暴露出有趣的纹理对比，因为混沌峭壁的岩石更加结晶，并且它们的飞地显示出更多种类的淬火纹理，这表明混合和补给之间存在关系。关键问题包括：(1) 镁铁质岩浆对岩浆室的补给是否会引发混合（并最终引发喷发），或者补给和混合之间是否存在时间滞后？ (2) 长英质岩浆最初侵入与镁铁质充填岩浆后期侵入之间的时间差对混合效果和喷发时间尺度有何影响？ (3) 镁铁质补给/常驻长英质岩浆的比例是否影响混合和/或喷发时间？ (4) 岩浆储存深度和温度是否影响混合/喷发过程？