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Intermittent generation of mafic enclaves in the 1991–1995 dacite of Unzen Volcano recorded in mineral chemistry

  • Hiroaki SatoEmail author
  • Francois Holtz
  • Roman E. Botcharnikov
  • Setsuya Nakada
Original Paper

Abstract

Mafic enclaves in the 1991–1995 dacite of Unzen volcano show chemical and textural variability, such as bulk SiO2 contents ranging from 52 to 62 wt% and fine- to coarse-grained microlite textures. In this paper, we investigated the mineral chemistry of plagioclase and hornblende microlites and distinguished three enclave types. Type-I mafic enclaves contain high-Mg plagioclase and low-Cl hornblende as microlites, whereas type-III enclaves include low-Mg plagioclase and high-Cl hornblende. Type-II enclaves have an intermediate mineral chemistry. Type-I mafic enclaves tend to show a finer-grained matrix, have slightly higher bulk rock SiO2 contents (56–60 wt%) when compared with the type-III mafic enclaves (SiO2 = 53–59 wt%), but the overall bulk enclave compositions are within the trend of the basalt–dacite eruptive products of Quaternary monogenetic volcanoes around Unzen volcano. The origin of the variation of mineral chemistry in mafic enclaves is interpreted to reflect different degree of diffusion-controlled re-equilibration of minerals in a low-temperature mushy dacitic magma reservoir. Mafic enclaves with a long residence time in the dacitic magma reservoir, whose constituent minerals were annealed at low-temperature to be in equililbrium with the rhyolitic melt, represent type-III enclaves. In contrast, type-I mafic enclaves result from recent mafic injections with a mineral assemblage that still retains the high-temperature mineral chemistry. Taking temperature, Ca/(Ca + Na) ratio of plagioclase, and water activity of the hydrous Unzen magma into account, the Mg contents of plagioclase indicate that plagioclase microlites in type-III enclaves initially crystallized at high temperature and were subsequently re-equilibrated at low-temperature conditions. Compositional profiles of Mg in plagioclase suggest that older mafic enclaves (Type-III) had a residence time of ~100 years at 800 °C in a stagnant magma reservoir before their incorporation into the mixed dacite of the 1991–1995 Unzen eruption. Presence of different types of mafic enclaves suggests that the 1991–1995 dacite of Unzen volcano tapped mushy magma reservoir intermittently replenished by high-temperature mafic magmas.

Keywords

Mafic enclaves Mg in plagioclase Cl in hornblende Magma mixing Diffusion re-equilibration Unzen volcano 

Notes

Acknowledgements

H.S. is grateful to Profs. Keiko Suzuki-Kamata, Yusuke Seto, and Eiichi Sato for their discussions and technical assistance during the study. The manuscript was highly improved by the journal reviewers and the editor. H.S. and S. N. were financially supported by Unzen Scientific Drilling Project of MEXT and funds by Japan Society for Promotion of Science. F.H. and R.B. thank the DFG (German Research Foundation) for the research support on Unzen volcano (German ICDP priority program).

Supplementary material

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Supplementary material 5 (PDF 18 KB)

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Copyright information

© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  • Hiroaki Sato
    • 1
    Email author
  • Francois Holtz
    • 2
  • Roman E. Botcharnikov
    • 2
  • Setsuya Nakada
    • 3
  1. 1.Department of Earth and Planetary SciencesKobe UniversityKobeJapan
  2. 2.Leibniz Universität HannoverInstitute of MineralogyHanoverGermany
  3. 3.Earthquake Research InstituteThe University of TokyoTokyoJapan

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