NSFGEO-NERC: Physical and Chemical Constraints on Large-volume Pyroclastic Blasts: The Campanian Ignimbrite Eruption, Italy

火山碎屑密度流，火山灰和浮石(也称为火山碎屑)的混合物，以及高速移动的气体，是自然界中一些最具破坏性的力量。4万年前从意大利那不勒斯附近的坎皮·弗莱格雷喷发的坎帕尼亚伊格尼布利岩，是地球上最年轻的大型火山碎屑密度流之一的沉积物。洋流到达了距离喷口80多公里的地方，摧毁了目前居住着400多万人的地区。这些洋流的传统模型假设它们有几十到100米厚，但我们过去的工作表明，坎帕尼亚洋流以千米厚的水流穿过地形，超过了1.5倍于这个高度的山脊。这种巨大的流动性使得这种“扩大的”火山碎屑密度流变得更加危险，损害也更难预测。这种稀薄的水流需要大量的气体，比喷发前溶解在岩浆中的气体多得多。另外两个可能的来源是空气的夹带和火山内热水系统的水的参与。该项目将确定参与洋流的岩浆、空气和热液系统中的气体量。这些信息将被用来更好地预测哪些火山可能产生高度流动的水流，并设计考虑这种流动性的危险地图。两名本科生、两名研究生、一名博士后研究员和一名教授将在北亚利桑那大学从事这一项目，一名研究生和一名教授将在牛津大学工作。三名意大利研究人员也将在这个项目中合作。该结果可用于危险评估和响应计划，包括美国海军在那不勒斯的支持活动。最近对坎帕尼亚Ignimbrite(CI)的热磁化研究发现，即使在距离喷口80公里的地方，侵位温度也在580摄氏度以上。这限制了可能包含多少空气，但为了计算准确的数量，需要更好地了解岩浆温度，以及是否有大量其他气体来源，如岩浆气体和热液。该项目将包括1)野外调查，以更好地描述CI地层学，连接远端和近端沉积，并确定火山事件的相对时间；2)通过傅立叶变换红外光谱、离子探针和电子探针对熔体包裹体和磷灰石微斑晶进行完整的挥发性分析(水、二氧化碳、S、F和氯)。反算开放系统脱气过程中的挥发分浓度是一种新技术，应该能揭示许多有关寒武纪Ignimbrite岩浆历史的信息；3)研究不同应变率和残余水含量下的玻璃转变温度，以更好地确定侵位温度。目前对数据集的模型和解释可能需要修改，以包括热液流体在喷发中“增压”的潜在作用，因为热液水并不出现在许多气体的岩石学测量中。包含喷发热液系统潜在贡献的爆炸性喷发模型将有助于更准确地定义火山灾害，并可能阐明爆发火山碎屑密度洋流喷发的可能原因。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。

Pyroclastic density currents, mixtures of volcanic ash and pumice (also known as pyroclasts) and gas that move at high rates of speed, are some of the most destructive forces in nature. The Campanian Ignimbrite, which erupted from the Campi Flegrei near Naples, Italy, 40,000 years ago, is the deposit from one of the youngest truly large pyroclastic density currents on Earth. The currents reached over 80 km from the vent and devastated an area currently occupied by more than 4 million people. Traditional models for these currents assume they are tens to perhaps 100 meters thick, but our past work shows the Campanian current moved as a kilometer-thick flow across the landscape, overtopping ridges 1.5 times that height. This great mobility makes this sort of "expanded" pyroclastic density current much more hazardous and the damage less predictable. This dilute current required an enormous amount of gas, more than can be explained by the amount dissolved in the magma prior to eruption. Two other possible sources are the entrainment of air and the involvement of water from a hot hydrothermal system within the volcano. This project will determine the amounts of gas from the magma, air, and hydrothermal system involved in the current. This information will be used to make better predictions of which volcanoes might produce highly mobile currents and design hazards maps that take this mobility into account. Two undergraduate students, two graduate students, a post-doctoral researcher, and a professor will work on this project at Northern Arizona University and a graduate student and a professor will work at Oxford University. Three Italian researchers will also collaborate in this project. The results may be used for hazards assessments and response plans, including for the US Naval Support Activity in Naples.A recent thermal magnetization study of the Campanian Ignimbrite (CI) found the temperature of emplacement was above 580 oC even at 80 km from the vent. This limits how much air could have been incorporated, but, to calculate precise quantities, a better knowledge of the magmatic temperature and whether other sources of gas, such as magmatic gas and hydrothermal fluids, were significant, is needed. This project will include 1) field investigations to better characterize the CI stratigraphy and link distal and proximal deposits and define the relative timing of volcanic events; 2) complete volatile analysis (H2O, CO2, S, F and Cl) of melt inclusions and apatite microphenocrysts via Fourier Transform Infrared (FTIR) spectroscopy, ion probe, and electron microprobe. Back-calculating volatile concentrations during open-system degassing is a new technique that should reveal much about the Campanian Ignimbrite magma history; and 3) a study of the glass transition temperature at varying strain rates and residual water contents to better define the emplacement temperature. Current models and interpretations of datasets may need to be modified to include the potential role of hydrothermal fluids in "supercharging" an eruption, as hydrothermal water does not appear in many petrologic measurements of gases. A model for explosive eruptions that includes the potential contribution of erupted hydrothermal systems will help define volcanic hazards more accurately and may elucidate a possible cause for paroxysmal pyroclastic density current-producing 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.