The generation of felsic melts (through open or closed system processes) within ocean island volcanoes has been a key area of study since their identification. At Ascension Island in the south Atlantic, explosively erupted felsic melts have, to date, demonstrated a marked absence of signs of magma mixing and crustal assimilation. Here we present the first observations of a fall deposit from Ascension Island recording both macro- and micro-scale evidence for magma mingling. Geochemical analyses of mineral and glass phases, coupled with volatile concentrations of melt inclusions highlight the role of lower-crustal partial melting to produce rhyolitic magmas. Glass textures and the lack of zoning in major mineral phases indicate that mingling with a mafic melt occurred shortly prior to eruption. These inferences of a deep rhyolite production zone, coupled with rapid ascent rates highlight the challenges in forecasting a similar style of eruption at Ascension Island in the future.
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In memory of Jon Davidson, who inspired so many. The authors wish to thank the Ascension Island Government, the Ascension Island Heritage Society, Ascension Island Conservation Department and Ascension Island residents (in particular Drew Avery) for logistical support during field campaigns. Thoughtful reviews by Brian Jicha and Charlotte Gordon, discussion with Jane Scarrow and Bridie Davies, and expedient editorial handling by Sami Mikhail have all enriched this publication. The authors are grateful to Chris Hayward, Ian Schipper, Richard Hinton, John Craven, Cees-Jan de Hoog, Bertrand Lezé, Chris Ott-ley and George Cooper for their laboratory and technical assistance during the course of analyses for this project. This project was funded by a Leverhulme Trust Research Project Grant (RPG-2013-042), with a 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).
© The Author(s) 2020.