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Title: 桜島火山大正噴火の準備過程 −磁鉄鉱化学組成からの制約−
Other Titles: Incubation Processes for the Taisho Eruption of Sakurajima Volcano -Constraints from the Mineral Chemistry of Magnetite-
Authors: 中村, 美千彦  KAKEN_name
田村, 翔  KAKEN_name
伊藤, 嘉紀  KAKEN_name
奥村, 聡  KAKEN_name
井口, 正人  KAKEN_name
味喜, 大介  kyouindb  KAKEN_id
Author's alias: NAKAMURA, Michihiko
ITO, Yoshinori
OKUMURA, Satoshi
IGUCHI, Masato
MIKI, Daisuke
Keywords: 桜島火山
Sakurajima Volcano
Taisho eruption
magma mixing
zoning profile
Issue Date: 1-Jun-2010
Publisher: 京都大学防災研究所
Journal title: 京都大学防災研究所年報. B = Disaster Prevention Research Institute Annuals. B
Volume: 53
Issue: B
Start page: 261
End page: 268
Abstract: 桜島火山大正噴火に至るマグマ溜まりの噴火準備過程を調べるため,軽石噴出物に含まれる磁鉄鉱斑晶および斜長石内の磁鉄鉱包有物の鉱物化学組成分析を行った。包有物は斑晶よりも,低温側により幅広い組成分布を示す。これは,マグマ溜まりの温度が苦鉄質マグマの段階的な注入によって徐々に上昇していたことを示唆する。また個々の磁鉄鉱斑晶は組成が均質で顕著な累帯構造を示さない一方で,斑晶間の組成のばらつきは累帯構造の組成幅よりも大きい。このように斑晶組成が均質だが不揃いであることは,噴火前のマグマ溜まりには温度や化学組成の不均質構造が存在して,それが噴火直前の数年間の時期には,先駆的な噴火やマグマの注入などで大規模に擾乱されることが無かったことを意味する。大正噴火軽石の磁鉄鉱斑晶は,結晶の稜部で僅かな溶解組織を示すことから,高温マグマの注入が引き金となり,ごく短時間で噴火に至った可能性を検討する必要がある。
In order to examine incubation processes for the Taisho eruption of Sakurajima Volcano, chemical compositions of phenocrystic magnetite and magnetite inclusions in plagioclase phenocrysts were investigated. Compared to the phenocrysts, the inclusions have a wider compositional variation corresponding to the lower temperature. This suggests that magma chamber temperature had increased gradually by repeated injection of hotter magmas. Each magnetite phenocryst was homogeneous in composition, whereas the phenocrysts have significant compositional variation, which is larger than the zoning within each phenocryst. This shows that the pre-eruptive magma chamber was heterogeneous, and this heterogeneity was stable for a few years prior to the eruption, without perturbations by large-scale precursor eruptions or magma injections. The edges and corners of the magnetite phenocrysts show slightly dissolving texture, suggesting that injection of hotter magma immediately led to the eruption.
Appears in Collections:No.53 B

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