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The mechanics of shallow magma reservoir outgassing

Parmigiani, A., Degruyter, Wim, Leclaire, S., Huber, C. and Bachmann, O. 2017. The mechanics of shallow magma reservoir outgassing. Geochemistry Geophysics Geosystems 18 (8) , pp. 2887-2905. 10.1002/2017GC006912

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Abstract

Magma degassing fundamentally controls the Earth's volatile cycles. The large amount of gas expelled into the atmosphere during volcanic eruptions (i.e. volcanic outgassing) is the most obvious display of magmatic volatile release. However, owing to the large intrusive:extrusive ratio, and considering the paucity of volatiles left in intrusive rocks after final solidification, volcanic outgassing likely constitutes only a small fraction of the overall mass of magmatic volatiles released to the Earth's surface. Therefore, as most magmas stall on their way to the surface, outgassing of uneruptible, crystal-rich magma storage regions will play a dominant role in closing the balance of volatile element cycling between the mantle and the surface. We use a numerical approach to study the migration of a magmatic volatile phase (MVP) in crystal-rich magma bodies (“mush zones”) at the pore-scale. Our results suggest that buoyancy driven outgassing is efficient over crystal volume fractions between 0.4 and 0.7 (for mm-sized crystals). We parameterize our pore-scale results for MVP migration in a thermo-mechanical magma reservoir model to study outgassing under dynamical conditions where cooling controls the evolution of the proportion of crystal, gas and melt phases and to investigate the role of the reservoir size and the temperature-dependent visco-elastic response of the crust on outgassing efficiency. We find that buoyancy-driven outgassing allows for a maximum of 40-50% volatiles to leave the reservoir over the 0.4-0.7 crystal volume fractions, implying that a significant amount of outgassing must occur at high crystal content (>0.7) through veining and/or capillary fracturing.

Item Type: Article
Date Type: Published Online
Status: Published
Schools: Earth and Ocean Sciences
Uncontrolled Keywords: Outgassing; gas migration in mushes; multiscale modelling
Publisher: American Geophysical Union
ISSN: 1525-2027
Date of First Compliant Deposit: 6 July 2017
Date of Acceptance: 23 June 2017
Last Modified: 30 Jul 2019 09:01
URI: http://orca.cf.ac.uk/id/eprint/102095

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