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Magnetite-bubble aggregates at mixing interfaces in andesite magma bodies

Edmonds, M.; Brett, A.; Herd, R.A.; Humphreys, M.C.S.; Woods, A.

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M. Edmonds

A. Brett

R.A. Herd

A. Woods


Magnetite is a particularly favourable site for heterogeneous bubble nucleation in magma and yet only very rarely is evidence for this preserved due to the myriad of processes that act to overprint such an association. The possibility of bubble-magnetite aggregates in magmas carries with it interesting implications for the fluid mechanics of magma bodies and for the magma mixing process responsible for the formation of andesites. We use image analysis and statistical methods to illustrate a spatial association between magnetite and bubbles in mafic enclaves. There is a large range in magnetite contents in the enclaves (up to 7.5%) which is related to the porosity of the enclaves, indicating a mechanism of enrichment of the mafic magma in magnetite. In the andesite there is no spatial association between bubbles and magnetite and the magnetite content of the andesite is small. We suggest a mechanism for enclave formation whereby in vapour-saturated magma, bubbles nucleate on magnetite. Upon intrusion into the base of an andesite magma body, these bubble-magnetite aggregates rise and ‘sweep up’ other magnetites, resulting in the accumulation of aggregates at the magma interface. Instabilities lead to the flotation of enclaves, characterized by enrichment in magnetite and bubbles.


Edmonds, M., Brett, A., Herd, R., Humphreys, M., & Woods, A. (2015). Magnetite-bubble aggregates at mixing interfaces in andesite magma bodies. Geological Society Special Publications, 410(1), 95-121.

Journal Article Type Article
Acceptance Date May 19, 2014
Online Publication Date Aug 14, 2014
Publication Date Jan 1, 2015
Deposit Date Sep 4, 2014
Publicly Available Date Sep 11, 2014
Journal Geological Society Special Publications
Print ISSN 0305-8719
Electronic ISSN 2041-4927
Publisher The Geological Society
Peer Reviewed Peer Reviewed
Volume 410
Issue 1
Pages 95-121


Accepted Journal Article (25.9 Mb)

Copyright Statement
Accepted for publication in Geological Society of London special publications, volume 410. First published online August 14, 2014.

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