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.

从硅酸盐熔体中形成气泡(或挥发性出溶)是在爆炸喷发期间岩浆到达地表的过程中发生的最重要的相变。当溶解的岩浆物种H2O和CO2析出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气泡的形成，进一步增加了岩浆上升的速度，从而增加了管道超压的可能性。因此，控制挥发性出溶和出气的过程，特别是关于管道深度的初始气相成核的时间，控制着岩浆的爆炸潜力。贫晶硅质岩浆喷发的一个关键未解决的方面是气泡成核是均匀地(在主体流体中)还是不均匀地(在基质的辅助下)发生的。这一区别很重要，因为均相气泡成核需要比非均相气泡成核更大的压力超越或过饱和度。众所周知，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度，然而在天然浮石中未能检测到足够高的这些晶体的丰度，这支持了气泡成核是均匀发生的推断。然而，当数值上升模型提供测量的气泡数密度，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，而且存在物理悖论。这项研究探索了这样一种可能性，即纳米尺寸的磁铁矿颗粒在喷发前存在于岩浆中，这些晶体在比它们不存在的情况下触发气泡成核的过饱和度值要低得多(即管道系统中更深的地方)。这项提案为本科生和研究生提供了教育机会，并突出了夏威夷大学和蒙特克莱尔州立大学(新泽西州)代表人数不足的少数族裔的参与。自然科学教育工作者的专业发展将通过一系列研讨会提供，重点是当地地质、火山学和地球过程在短时间和长时间范围内的运行。这个项目解决的问题是：在喷发的流纹岩岩浆中，气泡是均匀成核还是异质成核？将开展的工作的目标是确定脉状流纹岩浮岩是否含有纳米级钛磁铁矿晶体，其数值丰度可与气泡相媲美，并量化钛磁铁矿晶体在气泡成核过程中的潜在影响。解决这些问题对于从根本上理解火山喷发过程至关重要，对管道流动动力学的数值模拟以及火山灾害的评估具有实际意义。该工作计划围绕两项任务展开：(A)应用岩石磁学技术，在一定范围内的气泡相互连通范围内，表征名义上的浮石碎屑中钛磁铁矿晶体的数密度和尺寸分布；以及(B)利用fO2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。磁性表征(解决亚微米级钛磁铁矿晶体的磁畴状态、粒子数密度和尺寸分布)是很有前途的，因为它比传统的电子探针成像降低了稀疏Fe-Ti氧化物的探测极限几个数量级。对一组天然流纹岩的碎屑渗透率进行成对测量，跨越喷发参数和结构属性的范围，将有助于确定磁铁矿-气泡成核年代学。在一项实验任务中，将使用环境中的fO2来操纵磁铁矿的稳定性，以产生包含大量磁铁矿晶体的流纹岩岩浆，这些流纹岩晶体的数密度和体积分数范围很大。随后的发泡步骤将评估磁铁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的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。