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Book Chapter

The role of volatiles in subvolcanic processes and eruption triggering

Published:
January 01, 2015

Abstract

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.

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Geological Society, London, Special Publications

The Role of Volatiles in the Genesis, Evolution and Eruption of Arc Magmas

G. F. Zellmer
G. F. Zellmer
Massey University, New Zealand
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M. Edmonds
M. Edmonds
University of Cambridge, UK
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S. M. Straub
S. M. Straub
Columbia University, USA
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Geological Society of London
Volume
410
ISBN electronic:
9781862396982
Publication date:
January 01, 2015

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