Collaborative Research: What Do Obsidian Pyroclasts Tell Us? Constraints from Textures, Volatiles, and Experiments

普林尼期的岩浆喷发是沿着火山弧的主要地质灾害之一。在美国境内，55座火山产生这种类型的喷发被美国地质调查局指定为严重威胁，其中包括仅在喀斯喀特山脉中被评为非常高威胁的11座火山（例如，圣海伦山，火山口湖）。普林尼式火山喷发包括火山灰、浮石和气体的爆炸性喷射到大气中，少量的古老岩石和黑曜石火山碎屑。在上升到地表的过程中，岩浆部分地脱气，因为气泡成核、生长和合并形成可渗透的通道，使气体逸出。这种脱气的效率之前，和期间，喷发调制后者的爆炸性：有效的脱气在“开放系统”提示岩浆泡沫崩溃，并可能导致喷出式喷发，而无效的脱气在“封闭系统”可能会导致岩浆碎片和爆炸性喷发。黑曜岩火山碎屑是在普林尼期沉积物中发现的玻璃状、部分脱气的致密岩浆。因为它们是保存岩浆脱气通道的唯一样本，这些碎屑在岩浆脱气和火山爆发的模型中发挥着核心作用。然而，其起源仍然不确定。一个长期持有的原则是，黑曜石火山碎屑是由岩浆泡沫（“先锋”岩浆）在喷发前坍塌形成的气泡贫乏的熔体包裹，然后在爆炸性喷发期间被上升的岩浆挖出。最近对莫诺陨石坑（加州）的黑曜石火成碎屑的结构和化学研究对这一观点提出了质疑，并提出黑曜石火成碎屑是同步喷发的，是由岩浆破碎后火山灰烧结形成的。因此，本研究将测试的假设，1）火成碎屑黑曜石不形成的岩浆泡沫崩溃，但在管道中的灰烧结后破碎的产物，2）纹理和挥发分含量可以用来识别的“先锋”岩浆的遗迹挖掘爆炸喷发，和3）挥发分含量的黑曜石火山碎屑反映的气体成分的喷发混合物。这项研究的结果将允许充分利用黑曜岩火山碎屑的挥发物的独特记录来破译火山爆发的动力学。为了验证不同的假设，这项研究将包括对约公元前5,500年美国火山口湖的Cleetwood喷发，1340年美国北莫诺火山口喷发，以及火山爆发前的短暂爆发。1991年，皮纳图博（菲律宾）。第一项任务是研究黑曜岩火山碎屑的起源，其中同生浮石似乎记录了不同程度的泡沫崩溃。 如果新的模型成立，那么黑曜石火山碎屑将不会是一个连续体的一部分，与denitrifugice。 第二个任务是检查囊泡纹理致密的玻璃碎片，形成在喷发之间的间隙，以确定这些纹理是否可以用来区分“先锋”岩浆喷发前从黑曜石喷发过程中产生的。最后一项任务是调查黑曜岩火山碎屑的挥发性成分和同位素组成的变化，以破译他们的岩浆挥发性预算和脱气途径的方式，超越简单的封闭与开放系统脱气模型的记录。所使用的技术包括傅里叶变换红外光谱、显微拉曼光谱、电子探针和二次离子质谱。囊泡和晶体尺寸和形状分布的详细测量将使用3D高分辨率X射线计算机断层扫描和2D扫描电子显微镜图像的薄切片的组合。自然样品的分析将通过涉及结晶，水合和灰烧结的实验来补充。这些努力将导致更好地了解岩浆在到达地表的过程中所经历的喷发前和喷发时的过程的性质和时间。

Plinian eruptions of silicic magma are among the principal geological hazards along volcanic arcs. Within the United States, fifty-five volcanoes producing such type of eruptions are designated as serious threats by the United States Geological Survey, including eleven rated as very high threats in the Cascades alone (e.g., Mount St Helens, Crater Lake). A Plinian eruption involves the explosive ejection of ash, pumice, and gas into the atmosphere, with smaller amounts of older rock and obsidian pyroclasts. During its ascent to the surface, magma partially degasses as gas bubbles nucleate, grow, and coalesce to form permeable pathways allowing gas to escape. The efficiency of this degassing prior to, and during, the eruption modulates the explosivity of the latter: an efficient degassing in "open-system" prompts the magmatic foam to collapse and may lead to an effusive eruption, whereas inefficient degassing in "closed-system" may cause the magma to fragment and erupt explosively. Obsidian pyroclasts, the subject of this study, are dense pieces of glassy, partially degassed magma found in Plinian deposits. Because they are some of the only samples that preserve the degassing pathway of magma, these clasts play a central role in models for magma degassing and volcanic eruptions. Yet, their origin remains uncertain. A long-held tenet is that obsidian pyroclasts are parcels of bubble-poor melt formed by the collapse of a magmatic foam ("vanguard" magma) prior to the eruption, and then subsequently excavated by the ascending magma during the explosive eruption. Recent textural and chemical studies of obsidian pyroclasts at Mono Craters (California) have questioned this precept and alternatively suggest that obsidian pyroclasts are syn-eruptive and form by sintering of volcanic ash after magma fragmentation. This study will therefore test the hypotheses that 1) pyroclastic obsidians do not form by magmatic foam collapse but are the products of ash sintering in the conduit after fragmentation, 2) textures and volatile contents can be used to identify relics of "vanguard" magma excavated by explosive eruptions, and 3) volatile contents in obsidian pyroclasts reflect the gas composition of the erupting mixture. Results of this study will allow a full utilization of the unique records of volatiles of the obsidian pyroclasts to decipher the dynamics of volcanic eruptions.To test the different hypotheses, this study will include micro-scale textural and chemical analyses performed on explosive products of the ~5,500 BCE Cleetwood eruption of Crater Lake (USA), the 1340 CE North Mono Crater eruption (USA), and the pre-climactic short explosive eruptions of Mt. Pinatubo in 1991 (Philippines). The first task is to examine the origins of obsidian pyroclasts for which co-genetic pumice appear to record variable degrees of foam collapse. If the new model holds, then obsidian pyroclasts will not be part of a continuum with the densest pumices. The second task is to examine vesicle textures of dense glassy fragments that formed during hiatuses between eruptions to establish whether those textures can be used to discriminate "vanguard" magma emplaced before eruption from obsidian produced during eruption. The final task is to investigate variations in volatile contents and isotopic compositions of obsidian pyroclasts to decipher their records of magma volatile budgets and degassing pathways in ways that go beyond the simple closed- versus open-system degassing models. Techniques used include Fourier-Transform Infrared Spectroscopy, micro-Raman spectroscopy, Electron Microprobe, and Secondary Ion Mass Spectrometry. Detailed measurements of vesicle and crystal size and shape distributions will be made using a combination of 3D High-resolution X-ray Computed Tomography and 2D Scanning Electron Microscope images of thin sections. The analyses of natural samples will be complemented by experiments involving crystallization, hydration, and ash sintering. These efforts will lead to a better understanding of the nature and timing of pre- and syn-eruptive processes undergone by magma on its way to the surface.