火山 第 59 巻 ( 2014)
第 2 号 41-54 頁

論 説

富士火山，太郎坊に露出する新期スコリア層の全岩化学組成
─富士黒土層形成期付近を境とするマグマ供給系の変化─
金 子 隆 之＊・安 田敦＊・嶋 野 岳 人＊＊・吉 本 充 宏＊＊＊, †・藤 井 敏 嗣＊＊＊＊, ††（2013 年 10 月 18 日受付，2014 年 4 月 2 日受理）

Whole-rock Chemical Composition of Scoria Layers of the Younger Stage Exposing at Taro-bo, Mt Fuji
─ A Change in the Magma Plumbing System Occurred around Formation Periodof the Fuji-black Soil Layer (FB)
Takayuki KANEKO＊, Atsushi YASUDA＊, Taketo SHIMANO＊＊,
Mitsuhiro YOSHIMOTO＊＊＊, † and Toshitsugu FUJII＊＊＊＊, ††Mt Fuji had preferentially effused basaltic magma since its initiate of activity at about one hundred thousand yearsago. In the Younger stage (5,600 yrsBP〜), however, it became to spout occasionally andesitic or dacitic materials to beinvolved in air-fall deposits or pyroclastic flows, such as, Hoei scoria or Osawa pyroclastic flows. Andesitic magma issolely found as melt-inclusions in phenocrystic minerals in the Older stage (100,000〜10,000 yrsBP). This suggests thatsome kind of changes might have occurred in the magma plumbing system of Mt Fuji. To investigate such possibility,
we measured whole-rock chemical compositions of the representative scoria layers of the Younger stage erupted in andaround the summit and Hoei craters, and interpreted the results based on our recent model of magma plumbing system ofMt Fuji, consisting of the deep basaltic and the shallow andesitic chambers. The compositions of the scoria layers of theyounger stage showed high FeO*/MgO ratio with high and varied TiO2 and K2O contents, which are the same as thegeneral characteristics of Mt Fuji as pointed by previous studies. Detailed comparison with the scoria layers showed thatcompositions of the Younger stage had slightly but systematically higher SiO2 content - occasionally basaltic andesite than those of the Older stage. These chemical differences can be explained by existence of more differentiated magma
having higher SiO2 content in the shallow chambers in the Younger stage, compared to those in the Older stage, rather
than by increase of the mixing ratio of the similar differentiated magma of the Older stage. Mixing such a differentiatedmagma with a basaltic magma newly raised from the deep chamber can generate basalts having slightly high SiO2
content as erupted in the Younger stage. Although cause of generating SiO2-rich magma in the Younger stage is unclear,it might have occurred associated with the inactive period suggested by the Fuji-black soil layer lying between the twostages. The shallow dacitic magma chamber assumed in the model of the Hoei eruption might be an extreme case wherethe magma in the shallow chamber evolved to be highly SiO2-rich composition.
Key words : Fuji, magma plumbing system, andesite, magma chamber, melt-inclusion＊〒113-0032 東京都文京区弥生 1-1-1
東京大学地震研究所
Earthquake Research Institute, University of Tokyo, 1-1-1
Yayoi, Bunkyo-ku, Tokyo 113-0032＊＊〒417-0801 静岡県富士市大渕 325
常葉大学大学院環境防災研究科
Graduate School of Environment and Disaster Research,
Tokoha University, 325 Obuchi, Fuji-shi, Shizuoka 4170801＊＊＊〒060-0810 札幌市北区北 10 条西 8 丁目
北海道大学大学院理学院
Faculty of Science, Hokkaido University, N10 W8, Kitaku, Sapporo 060-0810＊＊＊＊〒160-0011 東京都新宿区若葉 1-22
環境防災総合政策研究機構
Crisis and Environment Management Policy Institute,
1-22 Wakaba, Shinjuku-ku, Tokyo 160-0011†
現在：〒403-0005 富士吉田市上吉田字剣丸尾 5597-1
山梨県富士山科学研究所
Mount Fuji Research Institute, Yamanashi PrefecturalGovernment 5597 Kenmarubi, Kamiyoshida, Fujiyoshida,
Yamanashi 403-0005††
兼任：同上
Corresponding author: Takayuki Kaneko
e-mail: kaneko@eri.u-tokyo.ac.jp