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

该项目将在纯科学方面取得进展，在公共安全和教育方面产生更广泛的影响。拉森火山中心上一次大喷发是在1915年的拉森峰，它位于连接美国西部城市的许多航空公司的航道下方。因此，了解拉森峰火山喷发的方式和原因将在未来的危险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操作过程，包括喷发触发因素和岩浆储存。例如，先前的工作显示了几种甚至可能触发喷发的机制。一种思路是，火山喷发是在新的岩浆被引入火山下面的岩浆库后不久触发的，因为新的岩浆与浅房间中已经存在的岩浆混合在一起。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。这种冷却将水驱动到剩余的未结晶岩浆中，如果有足够的水集中到岩浆中形成气相，则会增加岩浆系统内的压力。气相的膨胀可能会使上覆岩石破裂，从而导致喷发。拉森为这些观点提供了一个重要的试验场，因为最近的两次喷发(1915年拉森峰喷发，以及更早的约公元1144年混沌岩喷发)都显示了喷发前岩浆输入的证据，但这些新的输入在这两个案例中的作用非常不同。因此，对这两次喷发的比较将有助于深入了解不同触发机制在拉森岩浆系统中的重要性。这项研究将汇集来自不同大学的研究人员和学生，来自主要研究机构、州立服务大学和一所文科学院的代表。这将加强对许多学生的教育，因为它提供了宝贵的机会，在本科生和研究生阶段支持代表性不足的群体(拉美裔和妇女)，在实验室工作，并与全球各地的研究人员合作。此外，加州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，我们将对加州拉森火山中心的混沌岩和1915年喷发中的岩浆补充(新的岩浆输入)、岩浆混合和喷发之间的关系进行合作研究。这项研究将结合纹理研究来评估结晶过程，使用U系列(辐射测量法)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划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(标为A到F)，以及拉森峰1915年混合较好的喷发产物。在拉森峰，岩浆在喷发前紧密混合，而在混沌岩，由于某种原因，混合被抑制。这种对比是否反映了(A)新的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)热的新鲜岩浆输入的相对数量与已经存在于洞穴中的较冷的岩浆(先前的一些岩浆事件遗留下来的)相比？混沌岩单位特别令人感兴趣，因为穹顶A几乎没有混合的证据，因此产生了最极端的长英质和镁铁质成分；所有其他穹顶和1915年的熔岩都属于这两个极端。因此，穹顶A提供了进入端员岩浆的途径。Chaos Crags和Lassen Peak套件也暴露了有趣的纹理对比，因为Chaos Crags岩石更具结晶，它们的包体显示出更多种类的淬火纹理，这表明混合和补给之间的关系。关键问题包括：(1)镁铁质岩浆充填是否触发混合(并最终喷发)，或者补给和混合之间是否存在时间滞后？(2)长英质岩浆初始侵入和后期镁铁质补给岩浆侵入之间的时差对混合的效果和喷发的时间尺度有何影响？(3)镁铁质补给/留存长英质岩浆的比率是否影响混合和/或喷发的时间？(4)岩浆储存的深度和温度是否影响混合/喷发过程？

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?