Infrared Spectroscopy of Silicic Glasses and Melts: Deriving Volcano-Scale Processes from Laboratory-Scale Measurements

活跃的硅质火山通常会产生危险的穹顶和流动，它们要么以富含气体的熔岩形式喷发，含有一系列气泡和玻璃成分，要么以更密集的碎石堆和块状物的形式喷发，比如正在进行的山体喷发。圣海伦斯火山。前者可以在穹顶挤压和危险的爆炸性喷发之间交替。后者通常不那么危险，但如果足够大，仍然可以灾难性地坍塌，形成致命的块状和火山灰流。为了评估与活动的熔岩穹顶有关的危险，需要许多信息，包括穹顶的类型/成分/温度。然而，直接采集手部样本通常风险太大(就像在Mt.2004年圣海伦斯火山喷发)。监测活动熔岩穹顶成分和温度的远程技术是更可取的，最适合这些测量的数据在热红外(TIR)波长区。TIR数据可以从空间或野外仪器收集，这些仪器位于离活动喷口安全的地方(几公里)。TIR波长对发射的热量和这些穹顶的矿物成分都很敏感。然而，为了定量地了解来自天然熔岩穹顶的TIR信号，需要以实验室为基础的关键数据。研究人员建议在实验室使用根据先前的资助开发的微型熔炉收集天然玻璃和熔体材料的数据，以便通过采用热相机(FLIR)捕获多光谱数据来将这项研究扩展到该领域，该数据将模拟从太空收集的数据。所提出的研究对TIR光谱/遥感数据分析以及火山灾害监测/减灾的实际应用具有重要的理论意义。实验室结果将与实地和卫星TIR数据进行比较，并将提供有助于开发下一代实地监测工具的信息。该项目将支持富有成效的国际合作，并资助一名博士水平的研究生和几名本科生。具体地说，建议进行一项后续研究，包括实验室和基于现场的任务，以表征由基本Si-(Al)-O单元的振动产生的硅酸盐TIR发射数据。在第一项任务中，他们将使用他们现有的实验室FTIR光谱仪和最近制造的微炉来首次系统地表征硅酸盐玻璃和熔体的诊断TIR吸收带位置/形状。具体地说，他们将专注于实验室研究中的三种状态：(1)高于固相线和玻璃转变温度的样品，(2)冷却时最初在熔岩和矿物熔体上形成的玻璃结壳，以及(3)矿物和天然硅质样品的最终间隙基质玻璃。他们将收集实验室光谱仪的整个TIR光谱范围(5-25微米或2000-400厘米-1)，但集中在地球“S”大气窗区域(8-12微米区域或1250-830厘米-1)，以便直接与卫星和野外收集的数据进行比较。8-12微米区域也是硅酸盐矿物和玻璃中强吸收带(主要是Si-O和Al-O)的位置。这项拟议的研究将促进我们对红外光谱、分子尺度玻璃和熔体结构以及活动和非活动熔岩穹顶上的表面过程的理解。然而，为了使TIR光谱成为一种准确的监测工具，必须更好地了解影响活动穹顶表面发射TIR能量的因素。具体地说，TIR发射受到冷却/冷却玻璃结壳的形成、玻璃基质的结构和百分比、其他涂层(如升华物)以及中间大气的影响。这项拟议研究的第二项任务是基于实地的。该公司计划为P.I.S TIR宽带相机购买波长滤光片，将其转换为场基光谱仪。从这台改装后的仪器收集的数据将允许将实验室结果应用于不活跃的硅质穹顶的现场数据。这将是第一次以这种方式使用这种相机，希望它最终将导致建造一种坚固耐用的监测仪器，能够部署在偏远的火山上，用于实时监测和得出熔岩穹顶的基本物理性质(例如，表面气泡、斑晶成分和百分比、玻璃成分和百分比以及温度)。

Active silicic volcanoes commonly produce hazardous domes and flows that are erupted either as gas-rich lavas with a range of bubble and glass contents, or as denser piles of rubble and blocks like the ongoing eruption of Mt. St. Helens volcano. The former style can alternate between dome extrusion and hazardous explosive eruptions. The latter style is commonly less hazardous, but if large enough can still collapse catastrophically forming deadly block and ash flows. To assess the hazards associated with active lava domes, many pieces of information are needed including the dome's type/composition/temperature. However, it is typically too risky to collect hand samples directly (as was the case at during the first few weeks of the Mt. St. Helens eruption in 2004). Remote techniques for monitoring active lava dome composition and temperature are more desirable and the data best-suited for these measurements are in the thermal infrared (TIR) wavelength region. TIR data can be collected from space or field instruments, which are located safely away (several kilometers) from the active vent. TIR wavelengths are sensitive to both the emitted heat as well as the mineral composition of these domes. In order to quantitatively understand the TIR signal from natural lava domes, critical laboratory-based data are needed however. The investigators are proposing to collect data of natural glassy and molten materials in the laboratory using a micro-scale furnace developed under a previous grant in order to extend this research to the field by adapting a thermal camera (FLIR) to capture multispectral data, which will simulate the data collected from space. The research proposed has important theoretical implications for TIR spectroscopy/remote sensing data analysis as well as practical applications for volcanic hazard monitoring/mitigation. The laboratory results will be compared to field and satellite TIR data and will provide information that will aid development of the next-generation of field-based monitoring tools. This project will support a fruitful international collaboration and fund both a Ph.D.-level graduate student as well as a several undergraduate students.Specifically, it is proposed to do a follow-on research study comprised of laboratory and field-based tasks to characterize silicate TIR emission data produced by vibrations of the fundamental Si-(Al)-O units. In the first task, they will use their existing laboratory FTIR spectrometer and the recently-fabricated micro-furnace to provide the first systematic characterization of the diagnostic TIR absorption band positions/shapes of silicate glasses and melts. Specifically, they will focus on three states in the laboratory studies: (1) samples above the solidus and the glass transition temperatures, (2) the glassy crusts that initially form on lava and mineral melts upon cooling, and (3) the final interstitial matrix glass of mineral and natural silicic samples. They will collect the full TIR spectral range of the laboratory spectrometer (5-25 micrometer or 2000-400 cm-1), but concentrate on the region of the Earth''s atmospheric window (8-12 micrometer region or 1250-830 cm-1) in order to compare the data directly to those collected by satellite and from the field. The 8-12 micrometer region is also the location of the strong absorption bands (dominantly Si-O and also Al-O) in the silicate minerals and glasses. The proposed research will advance our understanding of infrared spectroscopy, molecular-scale glass and melt structure, and surface processes on both active and inactive lava domes. However, for TIR spectroscopy to be an accurate monitoring tool, the factors that affect the emitted TIR energy from active dome surfaces must be better-understood. Specifically, TIR emission is influenced by the formation of cooled/cooling glassy crusts, the structure and percentage of glassy matrix, other coatings such as sublimates, as well as the intervening atmosphere. The second task of this proposed research is field-based. It is planned to purchase wavelength filters for the P.I.'s TIR broadband camera that will convert it into a field-based spectrometer. The data collected from this modified instrument will allow an application of the laboratory results to field data of inactive silicic domes. This will be the first time such a camera will be used in this way and the hope is that it will lead to eventual construction of a rugged monitoring instrument capable of deployment on remote volcanoes and used for monitoring and derivation of fundamental physical properties of the lava dome (e.g., surface vesicularity, phenocryst composition and percentage, glass composition and percentage, and temperature) in real time.