噴出物中の熱水変質鉱物の特徴::十勝岳火山噴出物の例  [in Japanese] Characteristics of hydrothermally altered minerals in volcanic products at Tokachidake volcano, central Hokkaido, Japan  [in Japanese]

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Author(s)

    • 井村 匠 Imura Takumi
    • 秋田大学大学院国際資源学研究科資源学専攻 Department of Geosciences, Geotechnology, and Materials Engineering For Resources, Graduate School of Earth Resource Sciences, Akita University
    • 大場 司 Ohba Tsukasa
    • 秋田大学大学院国際資源学研究科資源地球科学専攻 Department of Earth Resource Science, Graduate School of International Resource Sciences, Akita University
    • 中川 光弘 Nakagawa Mitsuhiro
    • 北海道大学大学院理学研究院地球惑星科学部門 Department of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University

Abstract

<p>十勝岳火山噴出物の変質火山灰粒子について岩石学的観察を行った.4.7ka噴火,3.3ka噴火,1926年噴火噴出物を対象とした.これらの火山灰粒子は様々な程度に変質し,鉱物組み合わせからシリカタイプ(シリカ鉱物のみ),ミョウバン石タイプ(シリカ鉱物+ミョウバン石±カオリン鉱物),カオリンタイプ(シリカ鉱物+カオリン鉱物)に分類される.いずれの鉱物組み合わせも酸性熱水変質を示している.全試料において,酸性変質部と未変質部からなる弱変質した火山灰粒子が卓越する.このような岩石組織は,熱水流通系における酸性熱水-岩石反応により形成したものである.これには多量の熱水が岩石と反応しながら排出されるような反応過程が考えられ,反応時間はごく短期間であった.これらの特徴は,マグマ貫入時に一時的に形成した高温酸性熱水系による非定常プロセスに由来している.以上の点はマグマ貫入頻度が高い十勝岳の火山熱水系の特徴であると考えられる.</p>

<p>Volcanic products from Tokachidake volcano were observed mineralogically and petrographically, focusing on hydrothermal alteration of ash particles to interpret the pre-eruptive interaction between magma and the subvolcanic hydrothermal fluid. The 4.7 ka Ground Crater pyroclastic flow deposit, the 3.3 ka Ground Crater pyroclastic flow deposits, and the 1926 AD lahar deposits were examined. Each sample consists of ash grains undertaken alteration in various degrees from unaltered to intensely altered. Based on mineral assemblage, the alteration of ash is classified into three types: silica type (silica mineral alone), alunite type (silica mineral-alunite±kaolin), and kaolin type (silica mineral-kaolin). Most ash grains in the 4.7 ka and 1926 AD products belong to either of silica and alunite types. The 3.3 ka products are rich in kaolin-type ash and exhibit a stratigraphic variation in the proportion of unaltered ash: unaltered ash increases with the stratigraphy. All the alteration observed in the ash imply acid hydrothermal alteration, regardless of degrees of alteration. The weakly altered ash grains consist of unaltered volcanic rock and acid alteration parts, indicating that an acid hydrothermal fluid-rock interaction at open-fluid flow system formed their altered rock texture. It shows the reaction process proceeding during discharging the reactant fluid from the inside of the rock. These features reflect an unstable process of a short-lived subvolcanic hydrothermal system accompanying with a magma intrusion to the shallow potion and also with the reaction between the rock and acidic fluid that passed through the rocks.</p>

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 125(3), 203-218, 2019

    The Geological Society of Japan

Codes

  • NII Article ID (NAID)
    130007660928
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • ISSN
    0016-7630
  • NDL Article ID
    029690330
  • NDL Call No.
    Z15-174
  • Data Source
    NDL  J-STAGE 
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