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<p>In active volcanoes, various hydrothermal alteration minerals form through the interaction between edifice rocks and thermal waters derived from magmatic gases. Regarding clay minerals, the presence of pyrophyllite and kaolin minerals indicates acidic environments, while smectite, chlorite, and illite reflect near-neutral hydrothermal conditions. Although talc has generally been regarded as uncommon in hydrothermally altered rocks at active volcanoes, our investigations reveal its presence at several sites, suggesting that talc is no as absent from subvolcanic hydrothermal environments as was previously assumed. This study summarizes the occurrence of talc and discusses its formation environments and processes at active volcanoes. The alteration mineral assemblages at the studied volcanoes indicate that talc replaces pyroxene and olivine under near‑neutral hydrothermal environments. In addition, talc is Fe‑rich (approximately 10 wt.% FeO), implying formation at relatively low temperatures (up to 100&#xa0;°C). A high silica activity environment required for talc formation is established by the release of excess silica during matrix glass argillation. In open systems where element diffusion occurs, saponite predominantly replaces pyroxene and olivine, accompanied by minor talc, owing to the supply of Al from surrounding areas during hydrothermal alteration. In contrast, in quasi‑closed systems with limited element diffusion, talc alone replaces olivine and pyroxene in situ because of the minimal Al supply. These insights improve our understanding of hydrothermal alteration environments and alteration processes at active volcanoes.</p>

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Occurrence, formation processes, and significance of talc in hydrothermal alteration environments at active volcanoes

  • 期刊论文

  • 发表日期：

    2026年4月25日

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  • Ryo Takahashi,
  • Masahiro Yahata

摘要

In active volcanoes, various hydrothermal alteration minerals form through the interaction between edifice rocks and thermal waters derived from magmatic gases. Regarding clay minerals, the presence of pyrophyllite and kaolin minerals indicates acidic environments, while smectite, chlorite, and illite reflect near-neutral hydrothermal conditions. Although talc has generally been regarded as uncommon in hydrothermally altered rocks at active volcanoes, our investigations reveal its presence at several sites, suggesting that talc is no as absent from subvolcanic hydrothermal environments as was previously assumed. This study summarizes the occurrence of talc and discusses its formation environments and processes at active volcanoes. The alteration mineral assemblages at the studied volcanoes indicate that talc replaces pyroxene and olivine under near‑neutral hydrothermal environments. In addition, talc is Fe‑rich (approximately 10 wt.% FeO), implying formation at relatively low temperatures (up to 100 °C). A high silica activity environment required for talc formation is established by the release of excess silica during matrix glass argillation. In open systems where element diffusion occurs, saponite predominantly replaces pyroxene and olivine, accompanied by minor talc, owing to the supply of Al from surrounding areas during hydrothermal alteration. In contrast, in quasi‑closed systems with limited element diffusion, talc alone replaces olivine and pyroxene in situ because of the minimal Al supply. These insights improve our understanding of hydrothermal alteration environments and alteration processes at active volcanoes.