Fiamme degassing structures and their implications for the post-emplacement temperatures and H2O contents of high-grade ignimbrites
File(s)
Accepted version
Author(s)
Madden-Nadeau, Amber L
Genge, Matthew J
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.
Date Issued
2019-10-15
Date Acceptance
2019-08-06
Citation
Journal of Volcanology and Geothermal Research, 2019, 384, pp.251-262
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 http://creativecommons.org/licenses/by-nc-nd/4.0/
Sponsor
Science and Technology Facilities Council (STFC)
Identifier
https://www.sciencedirect.com/science/article/pii/S0377027319302409?via%3Dihub
Grant Number
ST/N000803/1
Subjects
0402 Geochemistry
0403 Geology
0404 Geophysics
Geochemistry & Geophysics
Publication Status
Published online
Date Publish Online
2019-08-08