2:45 PM - 3:00 PM
[SCG53-05] Talc formation by hydrothermal alteration at active volcanoes
Keywords:Talc, Hydrothermal alteration, Active volcanoes, Silica activity
1. Introduction
Various alteration minerals are formed by interactions between volcanic rocks and thermal waters derived from volcanic gases. In the case of clay minerals, pyrophyllite and kaolin-group minerals form in acidic environments, while smectite, chlorite, and illite form in near-neutral environments. In contrast, talc, which is generally produced by hydrothermal alteration of ultramafic rocks, such as serpentinite, is uncommon in hydrothermal alteration at active volcanoes, and its formation process has not been well studied. We investigated hydrothermal alteration environments at active volcanoes in Hokkaido, and identified talc in altered rocks from several volcanoes. In this study, we summarize the occurrence and characteristics of talc and discuss its formation process at active volcanoes.
2. Occurrence and characteristics of talc
We identified talc in altered lavas and welded tuffs from Komagatake volcano, Tarumae volcano, and the Nukkakushi crater area of Tokachidake volcano. Based on the altered mineral assemblages, it is suggested that talc formed in near-neutral alteration environments.
At the Nukkakushi crater area of Tokachidake volcano, talc has been observed replacing olivine phenocrysts in altered dense andesitic lava, whereas other phenocrystic minerals remain unaltered. Magnetite often coexists with talc, and cristobalite, pyrite, and gypsum are occasionally recognized. The groundmass has partially undergone hydrothermal alterations to form saponite.
At Komagatake and Tarumae volcanoes, orthopyroxene phenocrysts in altered welded tuff and lava are completely replaced by talc, smectite, and chlorite, while clinopyroxene phenocrysts often show partial replacement. Plagioclase, the most dominant mineral phase, is generally unaltered. The groundmass has undergone hydrothermal alterations to form clay minerals, such as talc, smectite and chlorite.
Chemical analysis using EDS indicated that talc is Fe-rich, containing approximately 10 wt.% FeO.
3. Formation processes of talc
Pyroxene and olivine are replaced by serpentine, brucite, and talc by serpentinization, and talc becomes stable at a high silica activity environment (e.g., Frost & Beard, 2007). Based on the characteristics of the altered dense lava at the Nukkakushi crater area, it is unlikely that a large amount of acidic thermal waters permeated the lava. Instead, it is more probable that a small amount of H2S gas and heat supply caused near-neutral hydrothermal alterations. In the case of Komagatake and Tarumae volcanoes, the hydrothermal alteration environments associated with talc formation were near-neutral (Takahashi, 2022). In these volcanoes, the matrix glass was replaced by clay minerals, such as smectite (saponite) and chlorite, leading to excess Si, which produced the high silica activity environment necessary for talc formation. Talc contains approximately 10 wt.% FeO, implying that it was formed at relatively low temperatures (<100 °C) (e.g., Mayhew et al., 2013). Magnetite and pyrite often coexist with talc, suggesting that residual Fe from talc formation precipitated as these minerals.
4. Conclusions
Our investigations revealed that, even in active volcanoes, talc forms through hydrothermal alterations of olivine and pyroxene phenocrysts in near-neutral environments. In such environments, a high silica activity environment required for talc formation can be established through alteration of the matrix glass. Moreover, talc is presumed to have formed in a relatively low-temperature (<100 °C) due to its Fe-rich characteristics.
Various alteration minerals are formed by interactions between volcanic rocks and thermal waters derived from volcanic gases. In the case of clay minerals, pyrophyllite and kaolin-group minerals form in acidic environments, while smectite, chlorite, and illite form in near-neutral environments. In contrast, talc, which is generally produced by hydrothermal alteration of ultramafic rocks, such as serpentinite, is uncommon in hydrothermal alteration at active volcanoes, and its formation process has not been well studied. We investigated hydrothermal alteration environments at active volcanoes in Hokkaido, and identified talc in altered rocks from several volcanoes. In this study, we summarize the occurrence and characteristics of talc and discuss its formation process at active volcanoes.
2. Occurrence and characteristics of talc
We identified talc in altered lavas and welded tuffs from Komagatake volcano, Tarumae volcano, and the Nukkakushi crater area of Tokachidake volcano. Based on the altered mineral assemblages, it is suggested that talc formed in near-neutral alteration environments.
At the Nukkakushi crater area of Tokachidake volcano, talc has been observed replacing olivine phenocrysts in altered dense andesitic lava, whereas other phenocrystic minerals remain unaltered. Magnetite often coexists with talc, and cristobalite, pyrite, and gypsum are occasionally recognized. The groundmass has partially undergone hydrothermal alterations to form saponite.
At Komagatake and Tarumae volcanoes, orthopyroxene phenocrysts in altered welded tuff and lava are completely replaced by talc, smectite, and chlorite, while clinopyroxene phenocrysts often show partial replacement. Plagioclase, the most dominant mineral phase, is generally unaltered. The groundmass has undergone hydrothermal alterations to form clay minerals, such as talc, smectite and chlorite.
Chemical analysis using EDS indicated that talc is Fe-rich, containing approximately 10 wt.% FeO.
3. Formation processes of talc
Pyroxene and olivine are replaced by serpentine, brucite, and talc by serpentinization, and talc becomes stable at a high silica activity environment (e.g., Frost & Beard, 2007). Based on the characteristics of the altered dense lava at the Nukkakushi crater area, it is unlikely that a large amount of acidic thermal waters permeated the lava. Instead, it is more probable that a small amount of H2S gas and heat supply caused near-neutral hydrothermal alterations. In the case of Komagatake and Tarumae volcanoes, the hydrothermal alteration environments associated with talc formation were near-neutral (Takahashi, 2022). In these volcanoes, the matrix glass was replaced by clay minerals, such as smectite (saponite) and chlorite, leading to excess Si, which produced the high silica activity environment necessary for talc formation. Talc contains approximately 10 wt.% FeO, implying that it was formed at relatively low temperatures (<100 °C) (e.g., Mayhew et al., 2013). Magnetite and pyrite often coexist with talc, suggesting that residual Fe from talc formation precipitated as these minerals.
4. Conclusions
Our investigations revealed that, even in active volcanoes, talc forms through hydrothermal alterations of olivine and pyroxene phenocrysts in near-neutral environments. In such environments, a high silica activity environment required for talc formation can be established through alteration of the matrix glass. Moreover, talc is presumed to have formed in a relatively low-temperature (<100 °C) due to its Fe-rich characteristics.