Excess volatiles supplied by mingling of mafic magma at an andesite arc volcano

Edmonds, M. and Aiuppa, A. and Humphreys, M. C. S. and Moretti, R. and Giudice, G. and Martin, R. S. and Herd, R. A. and Christopher, T. (2010) Excess volatiles supplied by mingling of mafic magma at an andesite arc volcano. Geochemistry Geophysics Geosystems, 11. 16 PP.. DOI https://doi.org/10.1029/2009GC002781

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We present the results of a study of volcanic gases at Soufrière Hills Volcano, Montserrat, which includes the first spectroscopic measurements of the major gas species CO2 and H2S at this volcano using a Multisensor Gas Analyzer System (MultiGAS) sensor. The fluxes of CO2 and H2S were 640–2750 t/d and 84–266 t/d, respectively, during July 2008, during a prolonged eruptive pause. The flux of CO2 is similar to estimates for the entire arc from previous geochemical studies, while the measured H2S flux significantly alters our interpretation of the sulphur budget for this volcano. The fluxes of both sulphur and carbon show considerable excesses over that which can be supplied by degassing of erupted magma. We demonstrate, using thermodynamic models and published constraints on preeruptive volatile concentrations, that the gas composition and fluxes are best modeled by mixing between (1) gases derived from isobaric quenching of mafic magma against cooler andesite magma at depth and (2) gases derived from shallower rhyolitic interstitial melt within the porpyritic andesite. The escape of deep-derived gases requires pervasive permeability or vapor advection extending to several kilometers depth in the conduit and magma storage system. These results provide more compelling evidence for both the contribution of unerupted mafic magma to the volatile budget of this andesitic arc volcano and the importance of the intruding mafic magma in sustaining the eruption. From a broader perspective, this study illustrates the importance and role of underplating mafic magmas in arc settings. These magmas play an important role in triggering and sustaining eruptions and contribute in a highly significant way to the volatile budget of arc volcanoes.

Item Type: Article
Uncontrolled Keywords: 2010AREP; IA61;
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title: Geochemistry Geophysics Geosystems
Volume: 11
Page Range: 16 PP.
Identification Number: https://doi.org/10.1029/2009GC002781
Depositing User: Sarah Humbert
Date Deposited: 21 Jul 2010 09:26
Last Modified: 23 Jul 2013 09:57
URI: http://eprints.esc.cam.ac.uk/id/eprint/1477

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