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Hornblende dehydration reactions during magma ascent at Soufrière Hills Volcano, Montserrat

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Abstract

Hornblende phenocrysts in recent andesites of the Soufrière Hills Volcano display reaction rims of microcrystalline plagioclase, pyroxene, Fe-oxides and interstitial glass, formed by decompression during magma ascent. Mass balance calculations give the following reactions with mineral proportions in agreement with modal abundances:

$$\begin{aligned} 100.0\,{\text{hbl}} & \to 49.8\,{\text{cpx}} + 27.6\,{\text{opx}} + 18.2\,{\text{plag}} + 4.5\,{\text{Ti - mag}} \\ 100.0\,{\text{hbl}} & \to 53.9\,{\text{cpx}} + 24.4\,{\text{opx}} + 18.1\,{\text{plag}} + 1.9\,{\text{ilm}} + 1.7\,{\text{mag}} \\ \end{aligned} $$

These reactions require an open chemical system with exchange of selected components with surrounding melt. Volatiles, TiO2 and alkalis are expelled and SiO2 and FeOT are consumed. Matrix glasses fall into two compositional groups. Glasses in pumice are relatively rich in CaO and poor in K2O and Na2O compared to glasses in dome samples. The former glasses formed by moderate amounts of groundmass crystallisation of plagioclase, associated with rapid magma ascent in explosive eruptions. The later glasses evolved in response to hornblende breakdown, groundmass crystallisation and mixing of melts from different levels during slow magma ascent and extended residence time in the dome. Interstitial glass compositions in reaction rims reflect the compositions of the surrounding matrix glasses, but show variable compositional differences mostly consistent with the proposed open-system reactions.

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Acknowledgements

This work benefited from helpful discussions with Jon Blundy, Alison Pawley and Tim Elliott. Stuart Kearns, Chloe Harford, Nicola Steen and Bruce Paterson are thanked for providing helpful analytical advice. We are also grateful to Mick Murphy for contributing photomicrographs from his collection (Fig. 1a–d), and to the Journal of Petrology for permitting us to reproduce Fig. 1d. The assistance of MVO staff and associates in obtaining the samples used in this study is gratefully acknowledged. V.J.E.B. acknowledges a Natural Environmental Research Council studentship and R.S.J.S support from a Royal Society Wolfson Merit Award. The manuscript was improved by the helpful reviews of Malcolm Rutherford and Mike Carroll.

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  1. Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, UK

    V. J. E. Buckley, R. S. J. Sparks & B. J. Wood

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  1. V. J. E. Buckley
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  2. R. S. J. Sparks
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  3. B. J. Wood
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Correspondence to V. J. E. Buckley.

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Communicated by J. Hoefs

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Buckley, V.J.E., Sparks, R.S.J. & Wood, B.J. Hornblende dehydration reactions during magma ascent at Soufrière Hills Volcano, Montserrat. Contrib Mineral Petrol 151, 121–140 (2006). https://doi.org/10.1007/s00410-005-0060-5

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