Origin and evolution of silicic magmas at ocean islands: Perspectives from a zoned fall deposit on Ascension Island, South Atlantic.

Journal article


Chamberlain, Katy J., Barclay, Jenni, Preece, Katie, Brown, Richard J. and Davidson, Jon 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. https://doi.org/10.1016/j.jvolgeores.2016.08.014
AuthorsChamberlain, Katy J., Barclay, Jenni, Preece, Katie, Brown, Richard J. and Davidson, Jon P.
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.

KeywordsMagmatism; Magma chamber processes; Ascension Island; Fractional crystallisation; Geology; Volcanology
Year2016
JournalJournal of Volcanology and Geothermal Research
ISSN03770273
Digital Object Identifier (DOI)https://doi.org/10.1016/j.jvolgeores.2016.08.014
Web address (URL)http://hdl.handle.net/10545/622295
hdl:10545/622295
Publication dates15 Nov 2016
Publication process dates
Deposited13 Mar 2018, 12:35
Accepted18 Aug 2016
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Archived with thanks to Journal of Volcanology and Geothermal Research

ContributorsUniversity of Durham and University of East Anglia
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