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

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    CompositionallyZoned_JVGR_2016 ...
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    Authors
    Chamberlain, Katy J.
    Barclay, Jenni
    Preece, Katie
    Brown, Richard J.
    Davidson, Jon P.
    Affiliation
    University of Durham
    University of East Anglia
    Issue Date
    2016-11-15
    
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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.
    Citation
    Chamberlain, K. J. et al (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 .
    Journal
    Journal of Volcanology and Geothermal Research
    URI
    http://hdl.handle.net/10545/622295
    DOI
    10.1016/j.jvolgeores.2016.08.014
    Additional Links
    http://linkinghub.elsevier.com/retrieve/pii/S0377027316302736
    Type
    Article
    Language
    en
    ISSN
    03770273
    ae974a485f413a2113503eed53cd6c53
    10.1016/j.jvolgeores.2016.08.014
    Scopus Count
    Collections
    School of Environmental Sciences

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