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

概述：坎帕尼亚Ignimbrite是伴随着坎皮Flegrei火山口崩塌形成的稀释火山碎屑密度流形成的。这些气流到达了离火山口80多公里的地方，沉降物覆盖了350万平方公里，距离火山口至少2500公里。要产生明显超过一公里厚的电流需要大量的气体，但这些气体从何而来呢？研究小组最近的工作发现，坎帕尼亚伊格尼姆布里特火山的喷发温度在580摄氏度以上，这表明洋流在运输过程中没有冷却太多，这反过来意味着大气气体的夹带最小，喷发持续时间很短。岩浆气体可能很重要，但如果这是唯一的来源，那么喷发前就需要非常高水平的溶解气体。过热的热液水（今天在5公里深处估计为500摄氏度）可能是另一个气源。我们提出1)详细的野外调查，以更好地表征坎帕尼亚-伊格尼布里岩地层，以便将远端和近端沉积物联系起来，并确定火山事件的相对时间；2)对坎帕期菱灰石产品进行了详细的岩石学研究，特别是通过傅里叶变换红外光谱（FTIR）、离子探针和电子探针对熔体包裹体和磷灰石微晶进行了完整的挥发性分析（H2O、CO2、S、F和Cl）；3)利用研究不同应变速率和残余含水量下的玻璃化转变温度的技术，对坎帕期火成岩喷发产物的冷却速率进行了实验研究。智力优势：我们建议测试持续稀释的PDCs可能是由岩浆和/或热液系统的气体驱动的假设，而不是主要来自纳入柱和流的大气气体。这将通过将实地观察和测量与内部岩浆过程联系起来，扩大我们预测火山灾害的能力。坎帕系Ignimbrite有近端和中/远端沉积，很难进行对比。地球化学和磁学技术的最新进展用于将远端沉降物联系起来，将应用于火成岩和近端沉积物。将使用最近开发的方法来分析玻璃化转变温度和流变学变化来追踪碎屑的冷却历史。开放体系脱气过程中挥发物浓度的反计算是一项新技术，它将揭示坎帕系伊格姆布里岩岩浆历史的许多信息。了解热液流体在增压中的潜在作用？火山喷发需要重新考虑当前的模型和对数据集的解释，因为热液水可能不会出现在许多气体岩石学测量中。更广泛的影响：那不勒斯地区是世界上火山风险最高的地区之一，大约有400万人生活在受坎帕尼亚伊格尼姆布里特影响的地区（不包括生活在放射性尘埃区的人——大约1亿人）。了解导致如此高流动性洋流的岩浆/热液条件，将有助于更好地评估当前条件变化带来的风险。我们将与维苏威亚天文台的人员合作，因此我们的结果将纳入他们的风险评估。这个NSF/NERC项目将促进美国、英国和意大利之间的重要国际合作，并将以博士/硕士学生和博士后研究人员的形式纳入代表性不足的群体的教育。

Overview:The Campanian Ignimbrite was emplaced from a dilute pyroclastic density current that accompanied caldera collapse of the Campi Flegrei. The currents reached over 80 km from the vent area and the fallout covers 3.5 million km2, reaching at least 2500 km from the vent. Abundant gas is needed to produce a current that was apparently more than a kilometer thick, but where did this gas come from? Recent work by the proposing team found that emplacement temperatures of the Campanian Ignimbrite were above 580oC, indicating that the current did not cool off very much during transport, which in turn implies minimal entrainment of atmospheric gases and a very short eruption duration. Magmatic gases are likely important, but very high levels of dissolved gas prior to eruption would be needed if that is the only source. Superheated hydrothermal water (estimated at 500oC at 5 km depth today) may be an additional gas source. We propose 1) detailed field investigations to better characterize the Campanian Ignimbrite stratigraphy, in order to link distal and proximal deposits and to define the relative timing of volcanic events; 2) a detailed petrologic study of Campanian Ignimbrite products, notably by performing 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; and 3) an experimental investigation of the cooling rates of Campanian Ignimbrite eruptive products, using a technique that studies the glass transition temperature atvarying strain rates and residual water contents.Intellectual Merit:We propose to test the hypothesis that sustained dilute PDCs may be driven by gas sourced from the magmatic and/or hydrothermal systems, rather than dominantly from atmospheric gases incorporated into the column and flow. This would extend our ability to forecast volcanic hazards by linking field observations and measurements to internal magmatic processes. The Campanian Ignimbrite has proximal and medial/distal deposits that are difficult to correlate. Recent advances in geochemical and magnetic techniques used to correlate the distal fallout will be applied to the ignimbrite and proximal deposits. Cooling histories of clasts will be traced using recently developed methods to analyze glass transition temperatures and rheological changes. Back-calculating volatile concentrations during open-system degassing is a new technique that should reveal much about the Campanian Ignimbrite magma history. Understanding the potential role of hydrothermal fluids in ?supercharging? an eruption would require some re-thinking of current models and interpretations of datasets, as hydrothermal water may not appear in many petrologic measurements of gases.Broader Impacts:The Naples area has one of the highest volcanic risks in the world, with about 4 million people living in the area that was affected by the Campanian Ignimbrite (ignoring the people living in the fallout zone - >100 million). Understanding the magmatic/hydrothermal conditions that led to such a high-mobility current would allow a better assessment of the risks as current conditions change. We will be working with personnel from the Osservatorio Vesuviano, so our results will be incorporated into their risk assessments. This proposed NSF/NERC project will foster a significant international collaboration between the US, UK, and Italy and will incorporate the education of underrepresented groups in the form of PhD/MS students and post-doctoral researchers.