Collaborative research: How variable is magma decompression rate during a single eruption?

了解控制火山喷发方式转变的过程是火山学中主要悬而未决的问题之一，对减灾工作具有重要意义。协同岩浆减压率被认为是火山喷发方式的主要控制因素，部分原因是它在调节气泡形成和运移动力学方面的作用。近年来，研究人员发展了一些技术，利用快速熄灭的火山温体中的化学梯度来限制岩浆的减压速率；然而，适合这些技术应用的样本的稀缺性严重限制了统计可靠数据集的获取，这些数据集可用于探索单次喷发过程中岩浆减压率的时间变化。一种新开发的岩浆减压“时钟”利用了普遍橄榄石晶体中的水浓度梯度，现在可以用来克服这一限制。我们建议将这一新的岩浆减压时钟应用于加利福尼亚Lassen火山国家公园Cinder Cone喷发岩浆减压速率的时间变化研究。保存异常完好的Cinder Cone的tephra序列记录了从夏威夷/斯特龙堡式到斯特龙堡式/暴力斯特龙堡式的喷发风格转变，为研究减压速率对喷发风格的控制作用提供了一个很好的自然实验室。拉森地区是美国最活跃的火山地区之一，对加州人、国家公园游客以及往返繁忙的旧金山湾区的航班构成了重大威胁。拟议工作的结果将与加利福尼亚火山观测站、喀斯喀特火山观测站和拉森火山国家公园共享，它们负责管理加利福尼亚的火山监测基础设施和火山灾害缓解工作。本文将在1666 C.E.煤渣锥喷发矿床的协同温度变化和结构变化的背景下研究岩浆减压速率的变化。我们将采用三种独立的技术来估计岩浆减压速率：(1)橄榄石中水扩散计时法；(2)气泡数密度；(3)微岩数密度。此外，我们将利用含橄榄石的熔体包裹体中的MgO分带来限制岩浆冷却速率。我们将建立一个统计上可靠的减压速率和冷却速率估计数据集，该数据集将用于评估（首次在玄武岩系统中）在喷发风格过渡期间减压速率和冷却速率的临时解决变化。这种方法将允许以前所未有的细节研究喷发前管道中压力-温度-时间条件的演变，从而增加我们对玄武岩火山爆发爆炸性控制的理解。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。

Understanding the processes that govern transitions in volcanic eruptive style is one of the major outstanding questions in volcanology, with important implications for hazard mitigation efforts. Syneruptive magma decompression rate is thought to be a dominant control on volcanic eruptive style, due in part to its role in modulating the dynamics of bubble formation and migration. In recent years, techniques have been developed that use chemical gradients in rapidly quenched volcanic tephra to constrain magma decompression rates; however, the rarity of suitable samples for the application of these techniques has severely limited the acquisition of statistically robust datasets that could be used to explore temporal variations in magma decompression rate over the course of a single eruption. A newly developed magma decompression “clock” that exploits water concentration gradients in prevalent olivine crystals can now be used to overcome this limitation. We propose to apply this new magma decompression clock to study temporal variations in the decompression rate of magma erupted at Cinder Cone, Lassen Volcanic National Park, CA. The exceptionally well-preserved tephra sequence from Cinder Cone captures a transition in eruptive style from Hawaiian/Strombolian to Strombolian/Violent Strombolian, providing an excellent natural laboratory for studying the role of decompression rate as a control on eruptive style. The Lassen region is one of the most active volcanic regions in the US and poses a significant hazard to Californians, National Park visitors, and flights to and from the busy San Francisco Bay Area. The results of the proposed work will be shared with the California Volcano Observatory, the Cascade Volcano Observatory, and Lassen Volcanic National Park, which manage volcano-monitoring infrastructure and volcanic hazard mitigation efforts in California.In the proposed work, variations in magma decompression rate will be studied in the context of syneruptive temperature changes and textural variations in the 1666 C.E. Cinder Cone eruptive deposit. We will apply three independent techniques to estimate magma decompression rates: (1) water-in-olivine diffusion chronometry; (2) bubble number density; and (3) microlite number density. Additionally, we will constrain magma cooling rates using MgO zonation in olivine-hosted melt inclusions. We will build a statistically robust dataset of decompression rate and cooling rate estimates that will be used to assess (for the first time in a basaltic system) temporally resolved variations in decompression rate and cooling rate across a transition in eruptive style. This approach will allow the evolution of pressure-temperature- time conditions in the conduit just prior to eruption to be studied in unprecedented detail, thereby increasing our understanding of the controls on the explosivity of basaltic volcanic eruptions.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.