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

智力优势。岩浆系统研究的一个重要组成部分是能够将岩浆的热，物理和成分演化的时间背景下。近年来，人们对限制岩浆作用速率的兴趣越来越大，也逐渐认识到岩浆系统中的晶体往往记录了岩浆作用的复杂历史，而这些历史在岩浆的液体部分中被掩盖了。三个有效的技术用于限制结晶和晶体驻留在中间和中间岩浆系统的时间尺度，包括纹理研究，研究扩散弛豫的化学梯度在晶体中，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驻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性差异，这意味着每种技术可能会响应并记录不同的信号。因此，该项目有两个目标：（1）量化喷发前晶体驻留的特征时间尺度，并调查这些时间尺度是否随时间在单个火山中变化，以及（2）应用和比较三种不同的技术来估计同一套岩石样本的岩浆时间尺度。为此，该项目地址的晶体居住和岩浆混合在俄勒冈州胡德山的时间尺度。现有的数据表明，长英质和镁铁质岩浆之间的混合有显着一致的整个~500 ka的生活的现代建筑物，晶体人口来自各自的端员很容易认识到，重要的是，初步结果表明，从这些人口中的每一个晶体喷发前的停留时间一直保持大致恒定的整个火山？的历史。这一假设将通过测量预混合晶体停留时间，并估计混合和喷发之间的时间进行测试。建议通过应用结构量化（CSD）、扩散弛豫和238 U-230 Th-226 Ra定年来量化胡德山晶体和岩浆储存以及岩浆混合的时间尺度。很少有研究直接比较这些技术中的任何两种在相同的样本套件，没有应用所有三个。直接比较将提高岩浆时间尺度估计的可信度，并提供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。 这项研究将提供有价值的洞察力的行为，中间组成的火山，并可能会有价值的影响，了解潜在的危险，这样的火山附近的交通走廊（哥伦比亚河）和大都市地区（波特兰）。该项目将促进来自俄勒冈州和加州大学戴维斯分校以及达勒姆大学（Dougal Jerram -岩浆结构量化专家）和美国地质勘探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生涯。杰拉姆将教一个非正式的研讨会，而在科瓦利斯，他的访问将被安排，以便让研究生的参与，除了其他人在北方加州和太平洋西北地区支持这个建议。该提案还将支持继续开展外联工作，向科瓦利斯地区的本科生和K-12学生展示基本的火山学原理。

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.