Collaborative Research: Elucidating the Role of Titanomagnetite in Vesiculation of Silicic Magmas

合作研究：阐明钛磁铁矿在硅质岩浆泡化中的作用

• 批准号: 1839230
• 负责人: Julia Hammer
• 金额: $ 37.31万
• 依托单位: University of Hawaii
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-04-01 至 2024-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1839230&HistoricalAwards=false
• 关键词: Collaborative; Research; Elucidating; Role; Titanomagnetite

The formation of bubbles (or volatile exsolution) from silicate melt is the most important phase transformation occurring in magmas en route to the surface during an explosive eruption. As dissolved magmatic species H2O and CO2 exsolve and produce a separate vapor phase, magma buoyancy and magma ascent rate increase. Bubble expansion driven by decompression accelerates vesiculation, further increasing magma ascent rate, and thereby the potential for increasing conduit overpressure. Thus, the processes governing volatile exsolution and outgassing, and particularly the timing of initial vapor phase nucleation with respect to depth in the conduit, control a magma's explosive potential. A key unresolved aspect of eruption of crystal-poor silicic magmas is whether bubble nucleation occurs homogeneously (in the bulk fluid) or heterogeneously (aided by a substrate). The distinction is important because homogeneous bubble nucleation requires substantively larger pressure overstepping, or supersaturation than heterogeneous bubble nucleation. Magnetite crystals are known to enhance the rate of bubble nucleation by providing energetically-favorable substrates, and yet failure to detect a sufficiently high abundance of these crystals in natural pumice has supported the inference that bubble nucleation occurs homogenously. However when numerical ascent models are supplied with measured bubble number densities and executed specifying homogeneous bubble nucleation, the predicted decompression and ascent rates are not only inconsistent with independent assessments but also present physical paradoxes. This study explores the possibility that nanometer sized magnetite particles are present in magmas prior to eruption, and that these crystals trigger bubble nucleation at much lower supersaturation values (i.e., at greater depth in the plumbing system) than would occur in their absence. This proposal provides educational opportunities for undergraduate and graduate students, and highlights participation of under-represented minorities at the University of Hawaii and Montclair State University (NJ). Professional development for Natural Science educators will be provided through a series of workshops focusing on local geology, volcanology, and Earth processes operating over short and long time scales.This project addresses the question: do bubbles nucleate homogeneously or heterogeneously in explosively erupting rhyolite magma? The objectives of the work to be undertaken are to ascertain whether plinian rhyolite pumices contain nano-scale titanomagnetite crystals in numerical abundances that rival those of bubbles and to quantify the potential influence of titanomagnetite crystals in bubble nucleation. Addressing these issues is critical for understanding eruptive processes at a fundamental level and is of practical significance in the numerical modeling of conduit flow dynamics and thus assessment of volcanic hazard. The work plan centers around two tasks: (a) applying techniques of rock magnetism to characterize the number density and size distribution of titanomagnetite crystals in nominally aphyric pumice clasts spanning a range in bubble interconnectivity; and (b) performing novel laboratory crystallization experiments utilizing fO2 modulation to isolate the influence of titanomagnetite from other variables. Magnetic characterization (resolving the magnetic domain states, particle number densities, and size distributions of submicron titanomagnetite crystals) is promising because it lowers the detection limit of sparse Fe-Ti oxides by several orders of magnitude relative to traditional electron microprobe imaging. Pairwise measurement of clast permeability, to be obtained for a suite of natural rhyolites spanning a range in eruption parameters and textural attributes, will facilitate determination of magnetite-bubble nucleation chronology. In an experimental task, magnetite stability will be manipulated using ambient fO2 in order to generate rhyolite magma containing populations of magnetite crystals that span large ranges in number density and volume fraction. A subsequent vesiculation step will evaluate the quantitative control of magnetite nanoparticle number density on the bubble-formation process.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

硅酸盐熔体气泡（或挥发性出溶）的形成是岩浆在爆炸喷发过程中最重要的相变过程。当溶解的岩浆物质H2O和CO2出溶并产生单独的气相时，岩浆浮力和岩浆上升速率增加。减压驱动的气泡膨胀加速了囊泡形成，进一步增加了岩浆上升速率，从而增加了管道超压的可能性。因此，控制挥发性出溶和脱气的过程，特别是相对于导管中的深度的初始气相成核的时间，控制岩浆的爆炸潜力。贫结晶岩浆喷发的一个关键未解决的方面是气泡成核是否均匀（在大量流体中）或不均匀（由衬底辅助）发生。这种区别是重要的，因为均匀气泡成核比非均匀气泡成核需要实质上更大的压力超越或过饱和。 已知磁铁矿晶体通过提供能量有利的基质来提高气泡成核的速率，但未能在天然浮石中检测到足够高丰度的这些晶体，这支持了气泡成核均匀发生的推断。然而，当数值上升模型提供测量的气泡数密度和执行指定均匀的气泡成核，预测的减压和上升速率不仅不一致的独立评估，但也提出了物理悖论。这项研究探讨了纳米尺寸的磁铁矿颗粒在喷发前存在于岩浆中的可能性，以及这些晶体在低得多的过饱和值（即，在管道系统中更大的深度处）比在没有它们的情况下发生的情况。该提案为本科生和研究生提供了教育机会，并强调了代表性不足的少数群体在夏威夷大学和蒙特克莱尔州立大学（新泽西州）的参与。自然科学教育工作者的专业发展将通过一系列的讲习班提供，重点是当地地质学，火山学，和地球过程在短期和长期的时间尺度上运作。这个项目解决的问题：泡沫成核均匀或不均匀的流纹岩岩浆爆炸喷发？要进行的工作的目标是确定是否含有纳米级的钛磁铁矿晶体的数量丰度的竞争对手的气泡和量化的潜在影响的钛磁铁矿晶体在气泡成核。解决这些问题是至关重要的了解喷发过程中的基础水平，并在管道流动动力学的数值模拟，从而评估火山灾害具有实际意义。工作计划围绕两个任务：（a）应用岩石磁学技术，以表征名义上无定形浮石碎屑中的钛磁铁矿晶体的数量密度和尺寸分布，这些碎屑跨越了气泡互连性的范围;（B）利用fO 2调制进行新颖的实验室结晶实验，以将钛磁铁矿的影响与其他变量隔离开来。磁特性（解决磁畴状态，粒子数密度，和亚微米钛磁铁矿晶体的尺寸分布）是有前途的，因为它降低了几个数量级的相对于传统的电子探针成像稀疏的Fe-Ti氧化物的检测限。碎屑渗透率的成对测量，获得一套自然rhythmia跨越范围内的喷发参数和纹理属性，将有助于确定磁铁矿气泡成核年表。在一项实验任务中，磁铁矿的稳定性将被操纵使用环境fO 2，以产生流纹岩岩浆包含人口的磁铁矿晶体，跨越大范围的数量密度和体积分数。随后的囊泡化步骤将评估对气泡形成过程中磁铁矿纳米颗粒数密度的定量控制。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

项目成果

Evaluating the Role of Titanomagnetite in Bubble Nucleation: Novel Applications of Low Temperature Magnetic Analysis and Textural Characterization of Rhyolite Pumice and Obsidian From Glass Mountain, California

• DOI: 10.1029/2023gc011338
• 发表时间: 2024-04-01
• 期刊: GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS
• 影响因子: 3.5
• 作者: McCartney，Kelly N.;Hammer，Julia E.;Giachetti，Thomas
• 通讯作者: Giachetti，Thomas