Collaborative Research: Timescales of Crystal Residence and Magma Mixing at Mount Hood, Oregon

知识价值。岩浆系统研究的一个重要组成部分是能够将岩浆的热、物理和成分演化置于一个时间背景下。近年来，人们对限制火成岩过程的速率越来越感兴趣，同时也逐渐认识到，岩浆系统中的晶体往往记录了岩浆过程的复杂历史，而这些历史被岩浆的液体部分所掩盖。三种有效的技术用于限制中、硅岩浆系统的结晶和晶体停留的时间尺度，包括结构研究、晶体化学梯度扩散弛豫研究和铀系晶体定年。然而，应用这些技术的努力也揭示了晶体储存和岩浆停留的表观时间尺度的巨大变化（高达四个数量级）。虽然这可能反映了不同类型岩浆系统之间晶体停留时间的自然变化，但每种技术记录的结果似乎也存在系统差异，这意味着每种技术可能对不同的信号做出反应并记录不同的信号。因此，该项目有两个目标：(1)量化爆发前晶体停留的特征时间尺度，并研究这些时间尺度在单个火山上是否随时间而变化；(2)应用和比较三种不同的技术来估计同一套岩石样品的岩浆时间尺度。为此，本项目研究了俄勒冈州胡德山晶体居住和岩浆混合的时间尺度。现有的数据表明，在现代大厦的约500 ka的生命周期中，长英质和镁质岩浆之间的混合非常一致，从各自的端部得到的晶体种群很容易被识别，重要的是，初步结果表明，来自这些种群的晶体在火山爆发前的停留时间在整个火山期间基本保持不变。年代的历史。这一假设将通过测量预混合晶体停留时间和估计混合和喷发之间的时间来检验。提出了利用结构定量、扩散弛豫和238U-230Th-226Ra定年等方法定量胡德山结晶、岩浆储存和岩浆混合的时间尺度。很少有研究在相同的样本组上直接比较这两种技术，也没有研究同时应用了这三种技术。直接比较将提高估计岩浆时间尺度的可信度，并提供一种可用于其他研究的方法。此外，这三种技术相互作用提供的信息比单独使用任何一种技术提供的信息都要多。更广泛的影响。这项研究将为了解中等成分硅质火山的行为提供有价值的见解，并可能对了解此类火山对附近交通走廊（哥伦比亚河）和大都市地区（波特兰）的潜在危害具有重要意义。该项目将促进俄勒冈州立大学和加州大学戴维斯分校以及达勒姆大学（杜格尔·杰拉姆——岩浆结构量化专家）和美国地质勘测局的科学家之间的合作。它将促进一名女PI和一名有前途的女研究生的职业发展。杰拉姆将在科瓦利斯教授一个非正式的研讨会，他的访问将被安排，以便允许该提案支持的研究生以及北加州和太平洋西北地区的其他学生参加。该提案还将支持继续向科瓦利斯地区的本科生和K-12学生展示基本火山学原理的推广工作。

Intellectual Merit. An essential component of studies of magmatic systems is the ability to place the thermal, physical, and compositional evolution of magmas within a temporal context. In recent years there has been increased interest in constraining the rates of igneous processes and also an emerging awareness that crystals in magmatic systems often record complex histories of magmatic processes that are obscured in the liquid fractions of magmas. Three effective techniques used to constrain the timescales of crystallization and crystal residence in intermediate and silicic magma systems include textural studies, studies of diffusional relaxation of chemical gradients in crystals, and uranium-series crystal dating. However, efforts to apply these techniques have also revealed large variations (up to four orders of magnitude) in the apparent timescales of crystal storage and magma residence. Although this likely reflects natural variations in crystal residence times between different types of magmatic systems, there also appear to be systematic differences in the results recorded by each technique, which implies that each technique may respond to and record somewhat different signals. Accordingly, this project has two goals: (1) to quantify the characteristic timescales of crystal residence prior to eruption and to investigate whether these timescales vary over time at a single volcano, and (2) to apply and compare three different techniques for estimating magmatic timescales on the same suite of rock samples. To this end, this project addresses timescales of crystal residence and magma mixing at Mount Hood, Oregon. Existing data indicate that mixing between felsic and mafic magmas there has been remarkably consistent throughout the ~500 ka life of the modern edifice, Crystal populations derived from the respective endmembers are readily recognized, and importantly, initial results suggest that the pre-eruptive residence times of crystals from each of these populations have remained broadly constant throughout the volcano?s history. This hypothesis will be tested by measuring pre-mixing crystal residence times, and estimating the time elapsed between mixing and eruption. It is proposed to quantify the timescales of crystal and magma storage and magma mixing at Mount Hood by applying textural quantification (CSD), diffusional relaxation and 238U-230Th-226Ra dating. There have been few studies that directly compare any two of these techniques on the same sample suites, and none that have applied all three. Direct comparison will provide an increased level of confidence in estimates of magmatic timescales and provide a methodology that can be used for other studies. Moreover, the three techniques interact to provide more information than would be provided by using any one technique in isolation. Broader Impacts. This study will provide valuable insight into the behavior of intermediate composition silicic volcanoes and likely will have valuable implications for understanding the hazard potential of such volcanoes to nearby transportation corridors (Columbia River) and metropolitan regions (Portland). The project will promote collaboration between scientists from Oregon State and UC-Davis, as well as from Durham Univ. (Dougal Jerram - an expert in quantification of magmatic textures) and the USGS. It will advance the careers of a female PI and a promising female graduate student. Jerram will teach an informal workshop while in Corvallis, and his visit will be scheduled so as to allow for participation by the graduate students supported by this proposal in addition to others in the Northern California and Pacific Northwest region. The proposal will also support continued outreach efforts to demonstrate basic volcanologic principles to undergraduate and K-12 students in the Corvallis area.