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

GSTDTAP

项目编号	1725186
	Collaborative Research: What Do Obsidian Pyroclasts Tell Us? Constraints from Textures, Volatiles, and Experiments
	James Gardner
主持机构	University of Texas at Austin
项目开始年	2017
	2017-07-01
项目结束日期	2020-06-30
资助机构	US-NSF
项目类别	Standard Grant
项目经费	282058(USD)
国家	美国
语种	英语
英文摘要	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.
文献类型	项目
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/71232
专题	环境与发展全球科技态势
推荐引用方式 GB/T 7714	James Gardner.Collaborative Research: What Do Obsidian Pyroclasts Tell Us? Constraints from Textures, Volatiles, and Experiments.2017.