none 10.1144/SP410.7 Geological Society of London Geological Society of London 1881 70411549 65091 2024072310103500855 10.1144 2024-07-23T21:13:44Z 2014-08-15T04:40:58Z 34 Geological Society, London, Special Publications SP 0305-8719 2041-4927 02 16 2015 01 2015 410 1 M. Edmonds Earth Sciences Department, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK A. Brett Earth Sciences Department, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK R. A. Herd School of Environmental Sciences, University of East Anglia, Norwich, UK M. C. S. Humphreys Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE, UK A. Woods Earth Sciences Department, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK 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. 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