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

硅岩浆的普林尼期喷发是沿火山弧的主要地质灾害之一。在美国境内，产生这种类型喷发的55座火山被美国地质调查局指定为严重威胁，其中仅喀斯喀特山脉就有11座火山被评为高度威胁（如圣海伦斯火山、火山口湖）。普林尼火山喷发包括将火山灰、浮石和气体爆炸性地喷射到大气中，同时还有少量较老的岩石和黑曜岩火山碎屑。在岩浆上升到地表的过程中，随着气泡成核、生长和合并形成可渗透的通道，岩浆会部分脱气，从而使气体逸出。在喷发之前和喷发期间，这种脱气的效率调节了后者的爆炸性：在“开放系统”中，有效的脱气会促使岩浆泡沫坍塌，并可能导致喷涌式喷发，而在“封闭系统”中，低效的脱气可能导致岩浆碎裂并爆炸性喷发。本研究的主题黑曜岩火山碎屑是在普林尼期矿床中发现的致密的玻璃状、部分脱气的岩浆。因为它们是唯一保存岩浆脱气路径的样本，这些碎屑在岩浆脱气和火山爆发模型中起着核心作用。然而，它们的起源仍然不确定。长期以来，人们一直认为黑曜岩火山碎屑是由岩浆泡沫（“先锋”岩浆）在喷发前坍塌形成的无气泡熔体，随后在爆发期间被上升的岩浆挖掘出来。最近对Mono陨石坑（加利福尼亚）黑曜岩火山碎屑的结构和化学研究对这一说法提出了质疑，并表明黑曜岩火山碎屑是同喷发的，是岩浆破碎后火山灰烧结形成的。因此，本研究将验证以下假设：1)火山碎屑黑曜岩不是岩浆泡沫坍塌形成的，而是管道破碎后灰烬烧结的产物；2)结构和挥发物含量可用于识别爆炸喷发所挖掘的“先锋”岩浆遗迹；3)黑曜岩火山碎屑的挥发物含量反映了喷发混合物的气体组成。该研究结果将允许充分利用黑曜岩火山碎屑的独特挥发物记录来破译火山爆发的动力学。为了验证不同的假设，本研究将包括对约5500 BCE克利特伍德火山口湖喷发（美国），1340 CE北莫诺火山口喷发（美国）和1991年皮纳图博火山（菲律宾）的气候前短暂爆发的爆炸产物进行微观尺度的结构和化学分析。第一个任务是检查黑曜岩火山碎屑的起源，其中共生浮石似乎记录了不同程度的泡沫崩塌。如果新的模型成立，那么黑曜岩火山碎屑将不会是最密集浮石的连续体的一部分。第二项任务是检查在两次喷发之间的间歇期形成的密集玻璃碎片的囊泡结构，以确定这些结构是否可以用来区分喷发前的“前卫”岩浆和喷发期间产生的黑曜岩。最后的任务是研究黑曜岩火山碎屑的挥发性含量和同位素组成的变化，以破译其岩浆挥发收支和脱气途径的记录，以超越简单的封闭与开放系统脱气模型。使用的技术包括傅里叶变换红外光谱、微拉曼光谱、电子探针和二次离子质谱。将使用3D高分辨率x射线计算机断层扫描和2D扫描电子显微镜薄片图像的组合来详细测量囊泡和晶体的大小和形状分布。对天然样品的分析将辅以结晶、水化和灰分烧结等实验。这些努力将使我们更好地了解岩浆在向地表流动的过程中所经历的前喷发和同期喷发过程的性质和时间。

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.