Chamberlain, K.J., Barclay, J., Preece, K., Brown, R.J. and Davidson, J.P. (2016) Origin and evolution of silicic magmas at ocean islands: Perspectives from a zoned fall deposit on Ascension Island, South Atlantic. Journal of Volcanology and Geothermal Research, 327, 349 - 360. (doi: 10.1016/j.jvolgeores.2016.08.014)
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Abstract
Ascension Island, in the south Atlantic is a composite ocean island volcano with a wide variety of eruptive styles and magmatic compositions evident in its ~ 1 million year subaerial history. In this paper, new observations of a unique zoned fall deposit on the island are presented; the deposit gradationally changes from trachytic pumice at the base, through to trachy-basaltic andesite scoria at the top of the deposit. The key features of the eruptive deposits are described and are coupled with whole rock {XRF} data, major and trace element analyses of phenocrysts, groundmass glass and melt inclusions from samples of the compositionally-zoned fall deposit to analyse the processes leading up to and driving the explosive eruption. Closed system crystal fractionation is the dominant control on compositional zonation, with the fractionating assemblage dominated by plagioclase feldspar and olivine. This fractionation from the trachy-basaltic andesite magma occurred at pressures of ~ 250 MPa. There is no evidence for multiple stages of evolution involving changing magmatic conditions or the addition of new magmatic pulses preserved within the crystal cargo. Volatile concentrations range from 0.5 to 4.0 wt. {H2O} and progressively increase in the more-evolved units, suggesting crystal fractionation concentrated volatiles into the melt phase, eventually causing internal overpressure of the system and eruption of the single compositionally-zoned magma body. Melt inclusion data combined with Fe-Ti oxide modelling suggests that the oxygen fugacity of Ascension Island magmas is not affected by degree of evolution, which concentrates {H2O} into the liquid phase, and thus the two systems are decoupled on Ascension, similar to that observed in Iceland. This detailed study of the zoned fall deposit on Ascension Island highlights the relatively closed-system evolution of felsic magmas at Ascension Island, in contrast to many other ocean islands, such as Tenerife and Iceland.
| Item Type: | Articles |
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| Additional Information: | This project was funded by a Leverhulme Trust Research Project Grant (RPG-2013-042), with the second field season supported by a Gloyne Outdoor Geological Research award from the Geological Society of London. Ion microprobe time was funded by the Natural Environment Research Council Grant (IMF561/0515). |
| Status: | Published |
| Refereed: | Yes |
| Glasgow Author(s) Enlighten ID: | Preece, Dr Katie |
| Authors: | Chamberlain, K.J., Barclay, J., Preece, K., Brown, R.J., and Davidson, J.P. |
| College/School: | College of Science and Engineering > Scottish Universities Environmental Research Centre |
| Journal Name: | Journal of Volcanology and Geothermal Research |
| Publisher: | Elsevier |
| ISSN: | 0377-0273 |
| Published Online: | 20 August 2016 |
| Copyright Holders: | Copyright © 2016 Elsevier B.V. |
| First Published: | First published in Journal of Volcanology and Geothermal Research 327: 349-360 |
| Publisher Policy: | Reproduced in accordance with the publisher copyright policy |
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