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?

该项目将在纯科学方面取得进展，并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（Lassen Peak）发生了最后一次大规模喷发，它位于连接美国西部城市的许多航空公司的航线下方。 我们的研究汇集了具有各种专业知识的研究人员，以检查喷发前的几个过程，包括喷发触发和岩浆储存。例如，先前的工作显示了几种可能触发火山爆发的机制。一种思路是，当新的岩浆被引入火山下的岩浆库后不久，火山爆发就被触发了，因为新的岩浆与已经存在于浅室中的岩浆混合。或者，新的岩浆输入可能需要冷却，并在喷发发生前部分结晶。这种冷却驱使水进入剩余的未结晶岩浆，如果足够的水集中到岩浆中形成蒸汽相，则会增加岩浆系统内的压力。蒸汽相的膨胀可能使上覆的岩石破裂，从而使火山爆发发生。拉森火山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（1915年在拉森峰，以及大约公元1144年在混沌岩的早期喷发）都显示了喷发前岩浆输入的证据，但这些新的输入在两种情况下表现得非常不同。因此，两次喷发的比较将提供洞察不同的触发机制在拉森岩浆系统的重要性。这项研究将汇集各大学的研究人员和学生，以及来自主要研究机构、国立服务大学和一所文科学院的代表。这将加强许多学生的教育，因为它提供了宝贵的机会，以支持代表性不足的群体（西班牙裔和妇女）在本科和研究生一级，在实验室工作，并与研究人员在地球仪。此外，CSU弗雷斯诺的许多本科生将参与研究，因为研究项目通常被整合到核心课程的实验室要求中。为了解决上述研究问题，我们将在混沌岩和1915年加州拉森火山中心爆发中进行岩浆补给（岩浆的新鲜输入），岩浆混合和喷发之间关系的合作研究。这项研究将结合使用结构研究来评估结晶过程，使用U系列（放射性）和扩散剖面法进行年龄测定，并使用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岩的6个圆顶（表示为A至F），以及拉森峰1915年喷发产物的混合。 在拉森峰，岩浆在喷发之前就已经混合在一起了，而在混沌岩，由于某种原因，混合被抑制了。这种对比是否反映了（a）新鲜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;或（c）热的新鲜岩浆输入与已经居住在岩浆房中的较冷岩浆（从一些先前的岩浆幕中遗留下来）相比的相对量？混沌岩单位是特别感兴趣的，因为圆顶A显示混合的证据很少？因此产生了最极端的长英质和镁铁质成分;所有其他圆顶和1915年的熔岩都落在这些极端之间。因此，穹丘A提供了进入端元岩浆的通道。混沌岩和拉森峰岩套也暴露出有趣的纹理对比，因为混沌岩岩石更具结晶性，它们的包体显示出更多样的淬火纹理，这表明混合和补给之间的关系。关键问题包括：（1）基性岩浆对岩浆房的再充填是否会引发混合（并最终导致喷发），或者在再充填和混合之间是否存在时间差？(2)长英质岩浆的初始侵入和后来的镁铁质补充岩浆的侵入之间的时滞对混合的效力和喷发的时间尺度有什么影响？(3)镁铁质补给/长英质岩浆的比例是否影响混合和/或喷发的时间？(4)岩浆储存的深度和温度会影响混合/喷发过程吗？