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Textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens, USA

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Abstract

This study presents major- and trace-element chemistry of plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens volcano. Despite the considerable variation in textures and composition of plagioclase phenocrysts, distinct segments have been cross-correlated between crystals. The variation of Sr and Ba concentration in the melt, as calculated from the concentration in the phenocrysts using partition coefficients, suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector. In both of these melts Sr and Ba are correlated despite the abundance of plagioclase in the 1980 dacites. We propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower Sr and higher Ba, as documented in the rims of the phenocrysts. Although the cores of the phenocrysts crystallised at relatively shallow depths, they preserve the Sr and Ba of the deep-seated melts as they ascended from a deeper region. Further magma ascent resulted in microlite nucleation, which is responsible for a similar shift to even lower Sr concentration as observed in the groundmass of post-18 May 1980 samples.

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Acknowledgments

We thank Stuart Kearns for help with EMP and SEM analyses at the University of Bristol, Simone Kasemann, Richard Hinton and John Craven for assistance on the SIMS, and Norm Pearson for help with laser ablation analysis at Macquarie University which is supported by the Australian Research Council and Department of Education Science and Training. We also thank Kathy Cashman, Jon Davidson, Catherine Ginibre, Steve Sparks, David Pyle, Gerhard Wörner, Vincent van Hinsberg and an anonymous reviewer for helpful comments. Access to the SIMS facility in Edinburgh was provided by the Natural Environment Research Council. During the course of this research KB was supported by a University of Bristol postgraduate scholarship and by the DEST through an Endeavour award; JB was supported through a NERC Senior Research Fellowship and ST by an Australian Federation Fellowship.

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Authors and Affiliations

  1. Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s road, BS8 1RJ, Bristol, UK

    Kim Berlo, Jon Blundy & Chris Hawkesworth

  2. Department of Earth and Planetary Sciences, McGill University, 3450 University Street, H3A 2A7, Montreal, QC , Canada

    Kim Berlo

  3. GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, 2109, Australia

    Simon Turner

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  1. Kim Berlo
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Correspondence to Kim Berlo.

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Communicated by T.L. Grove.

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Berlo, K., Blundy, J., Turner, S. et al. Textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens, USA. Contrib Mineral Petrol 154, 291–308 (2007). https://doi.org/10.1007/s00410-007-0194-8

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