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Mafic enclaves record syn-eruptive basalt intrusion and mixing

Plail, Melissa; Edmonds, Marie; Woods, Andrew; Barclay, Jenni; Humphreys, Madeleine C.S.; Herd, Richard A.; Christopher, Thomas

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Melissa Plail

Marie Edmonds

Andrew Woods

Jenni Barclay

Richard A. Herd

Thomas Christopher


Mafic enclaves hosted by andesite erupted at the Soufrière Hills Volcano between 1995 and 2010 yield insights into syn-eruptive mafic underplating of an andesite magma reservoir, magma mixing and its role in sustaining eruptions that may be widely applicable in volcanic arc settings. The mafic enclaves range in composition from basalt to andesite and are generated from a hybrid thermal boundary layer at the interface between the two magmas, where the basalt quenches against the cooler andesite, and the two magmas mix. We show, using an analytical model, that the enclaves are generated when the hybrid layer, just a few tens of centimetres thick, becomes buoyant and forms plumes which rise up into the andesite. Mafic enclave geochemistry suggests that vapour-saturated basalt was underplated quasi-continuously throughout the first three eruptive phases of the eruption (the end member basalt became more Mg and V-rich over time). The andesite erupted during the final phases of the eruption contained more abundant and larger enclaves, and the enclaves were more extensively hybridised with the andesite, suggesting that at some time during the final few years of the eruption, the intrusion of mafic magma at depth ceased, allowing the hybrid layer to reach a greater thickness, generating larger mafic enclaves. The temporal trends in mafic enclave composition and abundance suggests that basalt recharge and underplating sustained the eruption by the transfer of heat and volatiles across the interface and when the recharge ceased, the eruption waned. Our study has important implications for the petrological monitoring of long-lived arc eruptions.


Plail, M., Edmonds, M., Woods, A., Barclay, J., Humphreys, M. C., Herd, R. A., & Christopher, T. (2018). Mafic enclaves record syn-eruptive basalt intrusion and mixing. Earth and Planetary Science Letters, 484, 30-40.

Journal Article Type Article
Acceptance Date Nov 15, 2017
Online Publication Date Nov 19, 2017
Publication Date Feb 1, 2018
Deposit Date Nov 16, 2017
Publicly Available Date Dec 19, 2018
Journal Earth and Planetary Science Letters
Print ISSN 0012-821X
Publisher Elsevier
Peer Reviewed Peer Reviewed
Volume 484
Pages 30-40


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