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.
Bibliographical noteFunding Information:
The Ascension Island Government, and Administrator Marc Holland, the Ascension Island Heritage Society, Conservation Department and Ascension Island residents, in particular Drew Avery and Holly Connolly, are thanked for their logistical support during field seasons. Richard Hinton, John Craven and Cees-Jan de Hoog at the Edinburgh Ion Microprobe Facility are thanked for their support during our analytical session there. We are grateful to Chris Hayward, Ian Schipper, Bertrand Lezé, Chris Ottley and George Cooper for their laboratory and technical assistance during the course of analyses for this project. The authors wish to thank Margaret Mangan, Brian Jicha and an anonymous reviewer for their prompt and constructive reviews and the efficient handling of this submission. 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 ).
© 2016 Elsevier B.V.