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

普林斯时期的硅质岩浆喷发是沿火山弧的主要地质灾害之一。在美国境内，55座产生这种类型喷发的火山被美国地质调查局列为严重威胁，其中11座火山仅在喀斯喀特地区就被评为非常高的威胁(例如，圣海伦斯火山、火山口湖)。普林尼安火山喷发包括将火山灰、浮石和气体爆炸性地喷发到大气中，并伴随着少量的较老岩石和黑铁矿火山碎屑。在上升到地表的过程中，岩浆会随着气泡的成核、长大和聚集形成可渗透的通道而部分脱气，从而使气体得以逃逸。在喷发前和喷发期间，这种脱气的效率调节了后者的爆炸性：“开放系统”中的有效脱气会促使岩浆泡沫坍塌，并可能导致喷发，而“封闭系统”中的低效脱气可能会导致岩浆碎裂和爆炸。这项研究的主题是黑麻岩，是在普林尼亚矿床中发现的致密的玻璃状、部分脱气的岩浆碎片。因为它们是仅有的保存岩浆脱气途径的样本之一，这些碎屑在岩浆脱气和火山喷发的模型中发挥着核心作用。然而，它们的来源仍不清楚。长期以来的信条是，黑铁矿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稀少的熔体块，然后在爆炸喷发期间被上升的岩浆挖掘。最近对莫诺陨石坑(加利福尼亚)的黑铁矿火山碎屑的结构和化学研究对这一观点提出了质疑，并提出黑铁矿火山碎屑是同喷发的，是由岩浆破碎后的火山灰烧结形成的。因此，这项研究将检验如下假设：1)火山碎屑黑岩不是由岩浆泡沫坍塌形成的，而是碎裂后在管道中灰烬烧结的产物；2)结构和挥发分含量可以用来识别爆炸喷发挖掘出的“先锋”岩浆遗迹；3)黑麻岩中的挥发分含量反映了喷发混合物的气体成分。为了验证不同的假设，这项研究将包括对公元前5500年的克利特伍德火山口湖喷发、1340年的北莫诺陨石坑喷发和高潮前的山顶短暂爆发的爆炸产物进行的微尺度结构和化学分析。1991年，皮纳图博(菲律宾)。第一项任务是研究黑铁矿火山碎屑的起源，对于这些火山碎屑，其共生浮石似乎记录了不同程度的泡沫坍塌。如果新的模型成立，那么黑铁矿火山碎屑将不会是浮岩密度最大的连续体的一部分。第二项任务是检查在两次喷发之间的间歇期形成的致密玻璃碎片的泡泡结构，以确定这些结构是否可以用来区分喷发前侵位的“先锋”岩浆和喷发期间产生的黑铁矿。最后一项任务是调查黑麻岩火山碎屑的挥发分含量和同位素组成的变化，以破译它们记录的岩浆挥发预算和脱气途径，其方式超越了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仪。将结合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对囊泡和晶体大小和形状分布进行详细测量。对天然样品的分析将由结晶、水化和火山灰烧结等实验补充。这些努力将有助于更好地理解岩浆在到达地表的过程中经历的喷发前和喷发同步过程的性质和时间。

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.