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Mineral disequilibrium in lavas explained by convective self-mixing in open magma chambers

Nature volume 411, pages 1037–1039 (2001)Cite this article

Abstract

Characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features, including reversely zoned crystals, resorption surfaces, wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix. These features are often interpreted as evidence of the mixing of magmas of contrasting composition, temperature and origin1,2. Here, however, we propose that such features can also be caused by convection within a magma body with a single composition, that is heated from below and cooled from above. We describe petrological observations of andesite lava erupted at the Soufrière Hills volcano, Montserrat, which indicate a heating event and the intermingling of crystals that have very different thermal histories. We present experimental data on a representative groundmass composition of this lava, which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature. The concept of convective self-mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions.

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Figure 1: Pressure–temperature diagrams to show conditions of Soufrière Hills magma.
Figure 2: Diagram of self-mixing concept and temperature profile.

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Acknowledgements

S.C. acknowledges a NERC studentship, R.S.I.S. a NERC professorship and M.R.C. support from the Gruppo Nazionale de Vulcanologia.

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Authors and Affiliations

  1. Department of Earth Sciences, Bristol University, Bristol, BS8 1RJ, UK

    S. Couch & R. S. J. Sparks

  2. INFM and Dipartimento di Scienze della Terra, Università di Camerino, Camerino, 62032, Italy

    M. R. Carroll

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  1. S. Couch
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  3. M. R. Carroll
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Correspondence to S. Couch.

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Couch, S., Sparks, R. & Carroll, M. Mineral disequilibrium in lavas explained by convective self-mixing in open magma chambers. Nature 411, 1037–1039 (2001). https://doi.org/10.1038/35082540

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