Fiamme degassing structures and their implications for the post-emplacement temperatures and H2O contents of high-grade ignimbrites

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Title: Fiamme degassing structures and their implications for the post-emplacement temperatures and H2O contents of high-grade ignimbrites
Authors: Madden-Nadeau, AL
Genge, MJ
Item Type: Journal Article
Abstract: A new process in post-emplacement ignimbrite degassing is proposed from observations of unusual structures in a high-grade ignimbrite in Sardinia. The structures consist of fiamme decorated above and below by vesicles, termed here Fiamme Degassing Structures (FDS). The relative volumes of vesicles and fiamme have a positive, linear correlation, and the ratio between the volume of vesicles above and below fiamme (asymmetry ratio) is <1 for a significant number of structures. The positive relationship between the volume of fiamme and vesicles implies that the vesicles were produced by the degassing of fiamme post-emplacement. An equal volume of vesicles would be expected above and below fiamme under simple degassing under compression, contrary to observation in many FDS. The low asymmetry ratio is, therefore, related to a secondary process; vesicle loss and capture is proposed as an explanation. A numerical model of vesicle migration within rhyolitic ignimbrites is used to show that vesicles can rise further than the average separation distance between the fiamme, allowing them to be trapped by overlying fiamme. Sufficient migration of vesicles to result in trapping, however, requires a restricted range of H2O contents and temperatures on emplacement (e.g > 1 wt% at <860 °C), explaining the rare occurrence of FDS. The H2O contents required to form these structures post-emplacement are relatively high for pyroclasts, suggesting they are indicators of unusually large supersaturation in water resulting from rapid excavation rate. These structures also illustrate that interaction of volatiles with fiamme occurs during post-emplacement degassing; FDS may act as baffles that channel volatiles into fumarole columns, slowing volatile loss.
Issue Date: 15-Oct-2019
Date of Acceptance: 6-Aug-2019
ISSN: 0377-0273
Publisher: Elsevier BV
Start Page: 251
End Page: 262
Journal / Book Title: Journal of Volcanology and Geothermal Research
Volume: 384
Copyright Statement: © 2019 Elsevier Ltd. All rights reserved. This manuscript is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Licence
Sponsor/Funder: Science and Technology Facilities Council (STFC)
Funder's Grant Number: ST/N000803/1
Keywords: 0402 Geochemistry
0403 Geology
0404 Geophysics
Geochemistry & Geophysics
Publication Status: Published online
Embargo Date: 2020-08-08
Online Publication Date: 2019-08-08
Appears in Collections:Earth Science and Engineering

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