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

由硅酸盐熔体形成的气泡（或挥发性析出物）是在火山爆发过程中岩浆向地表流动过程中发生的最重要的相变。随着溶蚀的岩浆物质H2O和CO2的析出并产生单独的气相，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泡化，进一步提高了岩浆上升速率，从而增加了管道超压的可能性。因此，控制挥发性析出和放气的过程，特别是相对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。缺乏晶体的硅质岩浆喷发的一个尚未解决的关键问题是，气泡成核是均匀发生（在散装流体中）还是非均匀发生（在基底的帮助下）。这种区别很重要，因为均匀气泡成核比非均匀气泡成核需要更大的超压或过饱和。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率，然而，在天然浮石中未能检测到足够高的磁铁矿晶体丰度，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指定均匀气泡成核时，预测的减压和上升率不仅与独立评估不一致，而且存在物理悖论。这项研究探索了纳米级磁铁矿颗粒在喷发前存在于岩浆中的可能性，并且这些晶体在低得多的过饱和值（即，在管道系统中更深的地方）触发气泡成核，而不是在没有它们的情况下发生。该提案为本科生和研究生提供了教育机会，并强调了夏威夷大学和蒙特克莱尔州立大学（NJ）中未被充分代表的少数民族的参与。自然科学教育工作者的专业发展将通过一系列讲习班提供，重点是当地地质学、火山学和地球过程的短期和长期运作。这个项目解决了这样一个问题：气泡在爆炸喷发的流纹岩岩浆中是均匀成核还是非均匀成核？这项工作的目标是确定普林尼流纹岩浮石是否含有纳米级钛磁铁矿晶体，其数值丰度可与气泡的丰度相媲美，并量化钛磁铁矿晶体对气泡成核的潜在影响。解决这些问题对于从根本上理解火山喷发过程至关重要，并且对于管道流动动力学的数值模拟以及火山危险性的评估具有实际意义。工作计划主要围绕两项任务展开：(a)应用岩石磁学技术表征在气泡互联性范围内名义上的浮石碎屑中钛磁铁矿晶体的数量、密度和尺寸分布；(b)利用fO2调制进行新的实验室结晶实验，以从其他变量中分离出钛磁铁矿的影响。磁性表征（解析亚微米钛磁铁矿晶体的磁畴状态、粒子数密度和尺寸分布）是有希望的，因为相对于传统的电子探针成像，它将稀疏的Fe-Ti氧化物的检测极限降低了几个数量级。对一组具有不同喷发参数和结构属性的天然流纹岩的岩屑渗透率进行两两测量，将有助于确定磁铁矿泡成核年代学。在一项实验任务中，将使用环境fO2来控制磁铁矿的稳定性，以产生含有磁铁矿晶体种群的流纹岩岩浆，这些磁铁矿晶体在数量密度和体积分数上都有很大的范围。随后的起泡步骤将评估磁铁矿纳米颗粒数量密度对气泡形成过程的定量控制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.