Melt inclusion analysis of the Unzen 1991-1995 dacite: Implications for crystallization processes of dacite magma

Koshi Nishimura, Tatsuhiko Kawamoto, Tetsuo Kobayashi, Takeshi Sugimoto, Shigeru Yamashita

Research output: Contribution to journalArticle

18 Citations (Scopus)

Abstract

Dacitic magma, a mixture of high-temperature (T) aphyric magma and low-T crystal-rich magma, was erupted during the 1991-1995 Mount Unzen eruptive cycle. Here, the crystallization processes of the low-T magma were examined on the basis of melt inclusion analysis and phase relationships. Variation in water content of the melt inclusions (5.1-7.2 wt% H2O) reflected the degassing history of the low-T magma ascending from deeper levels (∼250 MPa) to a shallow magma chamber (∼140 MPa). The ascent rate of the low-T magma decreased markedly towards the emplacement level as crystal content increased. Cooling of magma as well as degassing-induced undercooling drove crystallization. With the decreasing ascent rate, degassing-induced undercooling decreased in importance, and cooling became more instrumental in crystallization, causing local and rapid crystallization along the margin of the magma body. Some crystals contain scores of melt inclusions, whereas there are some crystals without any inclusions. This heterogeneous distribution suggests the variation in the crystallization rate within the magma body; it also suggests that cooling was dominant cause for melt entrapment. Numerical calculations of the cooling magma body suggest that cooling caused rapid crystal growth and enhanced melt entrapment once the magma became a crystal-rich mush with evolved interstitial melt. The rhyolitic composition of melt inclusions is consistent with this model.

Original languageEnglish
Pages (from-to)648-662
Number of pages15
JournalBulletin of Volcanology
Volume67
Issue number7
DOIs
Publication statusPublished - Aug 1 2005

Keywords

  • Crystallization pressure
  • Crystallization process
  • Dacite
  • Magma chamber
  • Magmatic water content
  • Melt inclusions
  • Unzen

ASJC Scopus subject areas

  • Geochemistry and Petrology

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