TY - JOUR
T1 - Melt inclusion analysis of the Unzen 1991-1995 dacite
T2 - Implications for crystallization processes of dacite magma
AU - Nishimura, Koshi
AU - Kawamoto, Tatsuhiko
AU - Kobayashi, Tetsuo
AU - Sugimoto, Takeshi
AU - Yamashita, Shigeru
N1 - Funding Information:
Fig. A1 a Relationship between melt fraction and temperature (solid line) for water-saturated granodiorite composition at 200 MPa (modified after Piwinskii and Wyllie 1968). Starting material is Sample 766 with 69.4 wt% SiO2 in Piwinskii and Wyllie (1968). The stable phase assemblage changes at the intersections of solid line and dotted lines. b Assumption of a linear relationship between temperature and crystal fraction used in the thermal evolution modeling. T S and T L denote the solidus and liquidus temperatures, respectively Acknowledgements We thank S. Sparks for his constructive comments on an earlier version of the manuscript. Careful and thoughtful reviews by M. Rutherford, H. Sato and H. Shinohara improved the manuscript. K.N. thanks S. Nakada, C-H. Chen, Y. Yamaguchi, G. Saito, M. Nakamura, M. Yamamoto, T. Yanagi, T. Nishiyama, K. Takemura, S. Ohsawa, T. Shibata, M. Yoshikawa and J. Yamamoto for valuable discussions and encouragement during this work. This work was financially supported by the Nissan Science Foundation, Kyoto University, and 21st century COE program
PY - 2005/8
Y1 - 2005/8
N2 - 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.
AB - 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.
KW - Crystallization pressure
KW - Crystallization process
KW - Dacite
KW - Magma chamber
KW - Magmatic water content
KW - Melt inclusions
KW - Unzen
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U2 - 10.1007/s00445-004-0400-8
DO - 10.1007/s00445-004-0400-8
M3 - Article
AN - SCOPUS:23944464725
SN - 0258-8900
VL - 67
SP - 648
EP - 662
JO - Bulletin of Volcanology
JF - Bulletin of Volcanology
IS - 7
ER -