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: 1839313
• 负责人: Stefanie Brachfeld
• 金额: $ 11.32万
• 依托单位: Montclair State University
• 依托单位国家: 美国
• 项目类别: Standard 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=1839313&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.

硅酸盐熔体的气泡（或挥发性）的形成是爆炸性喷发期间岩浆到表面途中发生的最重要的相变。随着溶解的岩浆物种H2O和二氧化碳的Exsolve并产生单独的蒸气相，岩浆浮力和岩浆上升速率也会增加。由减压驱动的气泡膨胀会加速囊泡，进一步提高岩浆上升速度，从而增加导管过压的潜力。因此，控制挥发性的溶出和挤压的过程，尤其是初始蒸气相成核相对于导管深度的时间，控制岩浆的爆炸潜力。晶体贫乏的硅质岩浆喷发的关键方面是气泡成核是同质发生（在散装流体中）还是异质（由底物辅助）。区别很重要，因为均匀的气泡成核需要比异质气泡成核更大的压力过高或过饱和。 已知磁铁矿晶体通过提供能量有利的底物来提高气泡成核的速率，但未能检测到天然浮石中这些晶体的足够高的丰度支持了气泡成核均具有同质发生的推论。但是，当数值上升模型提供测量的气泡数密度并执行指定均匀的气泡成核时，预测的减压和上升速率不仅与独立的评估不一致，而且还具有物理悖论。这项研究探讨了在喷发之前存在纳米尺寸的磁铁矿颗粒，并且这些晶体在超饱和值（即，在水管系统的深度更高）下触发了气泡成核的可能性，而不是在缺乏时发生的。该建议为本科和研究生提供了教育机会，并强调了代表不足的少数民族在夏威夷大学和蒙特克莱尔州立大学（NJ）的参与。自然科学教育者的专业发展将通过一系列专注于当地地质，火山学和地球过程的研讨会，以短期和长时间的规模运行。该项目解决了一个问题：在爆炸性地爆发了弹性的流质岩浆中，气泡是否同质或异质化？要实现的工作的目标是确定plinian ryolite pumices是否在数值丰度中包含纳米级钛磁铁矿晶体，这些钛含量与气泡的数字丰度相媲美，并量化了气泡成核中钛磁铁矿晶体的潜在影响。解决这些问题对于理解基本层面的喷发过程至关重要，并且在导管流动动力学的数值建模以及对火山危害的评估中具有实际意义。工作计划围绕两个任务：（a）应用岩石磁化技术来表征名义上有声性浮石中钛磁铁矿晶体的数量密度和尺寸分布，跨越了气泡互连范围的范围； （b）使用FO2调制进行新的实验室结晶实验，以将钛磁铁矿的影响与其他变量隔离。磁性表征（解决磁域状态，颗粒数密度和亚微米钛铁矿晶体的尺寸分布）是有希望的，因为它相对于传统的电子微探针，稀疏Fe-Ti氧化物的检测极限降低了几个数量级。跨碎屑渗透性的成对测量，将获得一套跨越喷发参数和纹理属性范围的天然流纹岩的套件，将有助于确定磁铁矿 - 泡沫成核的年代学。在实验任务中，将使用环境FO2来操纵磁铁矿稳定性，以生成含有磁铁矿晶体种群的流纹岩岩浆，这些磁铁矿晶体的数量密度和体积分数范围很大。随后的囊泡步骤将评估磁铁矿纳米颗粒数密度在气泡形成过程中的定量控制。该奖项反映了NSF的法定任务，并被认为是值得通过基金会的智力优点和更广泛影响的审查标准通过评估来获得支持的。