Heterogeneous pumice populations in the 2.08-Ma Cerro Galan Ignimbrite: Implications for magma recharge and ascent preceding a large-volume silicic eruption

Heather Wright, Christopher Folkes, Raymond Cas, Katharine Cashman

    Research output: Contribution to journalArticleResearchpeer-review

    Abstract

    Triggering mechanisms of large silicic eruptions remain a critical unsolved problem. We address this question for the 2. 08-Ma caldera-forming eruption of Cerro Gal? n volcano, Argentina, which produced distinct pumice populations of two colors: grey (5 ) and white (95 ) that we believe may hold clues to the onset of eruptive activity. We demonstrate that the color variations correspond to both textural and compositional variations between the clast types. Both pumice types have bulk compositions of high-K, high-silica dacite to low-silica rhyolite, but there are sufficient compositional differences (e. g., 150 ppm lower Ba at equivalent SiO2 content and 0.03 wt. higher TiO2 in white pumice than grey) to suggest that the two pumice populations are not related by simple fractionation. Trace element concentrations in crystals mimic bulk variations between clast types, with grey pumice containing elevated Ba, Cu, Pb, and Zn concentrations in both bulk samples (average Cu, Pb, and Zn concentrations are 27, 35, and 82 in grey pumice vs. 11, 19, and 60 in white pumice) and biotite phenocrysts and white pumice showing elevated Li concentrations in biotite and plagioclase phenocrysts. White and grey clasts are also texturally distinct: White pumice clasts contain abundant phenocrysts (44-57 ), lack microlites, and have highly evolved groundmass glass compositions (76.4-79.6 wt. SiO2), whereas grey pumice clasts contain a lower percentage of phenocrysts/microphenocrysts (35-49 ), have abundant microlites, and have less evolved groundmass glass compositions (69.4-73.8 wt. SiO2). There is also evidence for crystal transfer between magma producing white and grey pumice. Thin highly evolved melt rims surround some fragmental crystals in grey pumice clasts and appear to have come from magma that produced white pumice. Furthermore, based on crystal compositions, white bands within banded pumice contain crystals originating in grey magma. Finally, only grey pumice clasts form breadcrusted surface textures. We interpret these compositional and textural variations to indicate distinct magma batches, where grey pumice originated from an originally deeper, more volatile-rich dacite recharge magma that ascended through and mingled with the volumetrically dominant, more highly crystalline chamber that produced white pumice. Shortly before eruption, the grey pumice magma stalled within shallow fractures, forming a vanguard magma phase whose ascent may have provided a trigger for eruption of the highly crystalline rhyodacite magma. We suggest that in the case of the Cerro Gal? n eruption, grey pumice provides evidence not only for cryptic silicic recharge in a large caldera system but also a probable trigger for the eruption.
    Original languageEnglish
    Pages (from-to)1513 - 1533
    Number of pages21
    JournalBulletin of Volcanology
    Volume73
    Issue number10
    DOIs
    Publication statusPublished - 2011

    Cite this

    @article{9d643734dd764bac86bd9e15ca64602a,
    title = "Heterogeneous pumice populations in the 2.08-Ma Cerro Galan Ignimbrite: Implications for magma recharge and ascent preceding a large-volume silicic eruption",
    abstract = "Triggering mechanisms of large silicic eruptions remain a critical unsolved problem. We address this question for the 2. 08-Ma caldera-forming eruption of Cerro Gal? n volcano, Argentina, which produced distinct pumice populations of two colors: grey (5 ) and white (95 ) that we believe may hold clues to the onset of eruptive activity. We demonstrate that the color variations correspond to both textural and compositional variations between the clast types. Both pumice types have bulk compositions of high-K, high-silica dacite to low-silica rhyolite, but there are sufficient compositional differences (e. g., 150 ppm lower Ba at equivalent SiO2 content and 0.03 wt. higher TiO2 in white pumice than grey) to suggest that the two pumice populations are not related by simple fractionation. Trace element concentrations in crystals mimic bulk variations between clast types, with grey pumice containing elevated Ba, Cu, Pb, and Zn concentrations in both bulk samples (average Cu, Pb, and Zn concentrations are 27, 35, and 82 in grey pumice vs. 11, 19, and 60 in white pumice) and biotite phenocrysts and white pumice showing elevated Li concentrations in biotite and plagioclase phenocrysts. White and grey clasts are also texturally distinct: White pumice clasts contain abundant phenocrysts (44-57 ), lack microlites, and have highly evolved groundmass glass compositions (76.4-79.6 wt. SiO2), whereas grey pumice clasts contain a lower percentage of phenocrysts/microphenocrysts (35-49 ), have abundant microlites, and have less evolved groundmass glass compositions (69.4-73.8 wt. SiO2). There is also evidence for crystal transfer between magma producing white and grey pumice. Thin highly evolved melt rims surround some fragmental crystals in grey pumice clasts and appear to have come from magma that produced white pumice. Furthermore, based on crystal compositions, white bands within banded pumice contain crystals originating in grey magma. Finally, only grey pumice clasts form breadcrusted surface textures. We interpret these compositional and textural variations to indicate distinct magma batches, where grey pumice originated from an originally deeper, more volatile-rich dacite recharge magma that ascended through and mingled with the volumetrically dominant, more highly crystalline chamber that produced white pumice. Shortly before eruption, the grey pumice magma stalled within shallow fractures, forming a vanguard magma phase whose ascent may have provided a trigger for eruption of the highly crystalline rhyodacite magma. We suggest that in the case of the Cerro Gal? n eruption, grey pumice provides evidence not only for cryptic silicic recharge in a large caldera system but also a probable trigger for the eruption.",
    author = "Heather Wright and Christopher Folkes and Raymond Cas and Katharine Cashman",
    year = "2011",
    doi = "10.1007/s00445-011-0525-5",
    language = "English",
    volume = "73",
    pages = "1513 -- 1533",
    journal = "Bulletin of Volcanology",
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    publisher = "Springer-Verlag London Ltd.",
    number = "10",

    }

    Heterogeneous pumice populations in the 2.08-Ma Cerro Galan Ignimbrite: Implications for magma recharge and ascent preceding a large-volume silicic eruption. / Wright, Heather; Folkes, Christopher; Cas, Raymond; Cashman, Katharine.

    In: Bulletin of Volcanology, Vol. 73, No. 10, 2011, p. 1513 - 1533.

    Research output: Contribution to journalArticleResearchpeer-review

    TY - JOUR

    T1 - Heterogeneous pumice populations in the 2.08-Ma Cerro Galan Ignimbrite: Implications for magma recharge and ascent preceding a large-volume silicic eruption

    AU - Wright, Heather

    AU - Folkes, Christopher

    AU - Cas, Raymond

    AU - Cashman, Katharine

    PY - 2011

    Y1 - 2011

    N2 - Triggering mechanisms of large silicic eruptions remain a critical unsolved problem. We address this question for the 2. 08-Ma caldera-forming eruption of Cerro Gal? n volcano, Argentina, which produced distinct pumice populations of two colors: grey (5 ) and white (95 ) that we believe may hold clues to the onset of eruptive activity. We demonstrate that the color variations correspond to both textural and compositional variations between the clast types. Both pumice types have bulk compositions of high-K, high-silica dacite to low-silica rhyolite, but there are sufficient compositional differences (e. g., 150 ppm lower Ba at equivalent SiO2 content and 0.03 wt. higher TiO2 in white pumice than grey) to suggest that the two pumice populations are not related by simple fractionation. Trace element concentrations in crystals mimic bulk variations between clast types, with grey pumice containing elevated Ba, Cu, Pb, and Zn concentrations in both bulk samples (average Cu, Pb, and Zn concentrations are 27, 35, and 82 in grey pumice vs. 11, 19, and 60 in white pumice) and biotite phenocrysts and white pumice showing elevated Li concentrations in biotite and plagioclase phenocrysts. White and grey clasts are also texturally distinct: White pumice clasts contain abundant phenocrysts (44-57 ), lack microlites, and have highly evolved groundmass glass compositions (76.4-79.6 wt. SiO2), whereas grey pumice clasts contain a lower percentage of phenocrysts/microphenocrysts (35-49 ), have abundant microlites, and have less evolved groundmass glass compositions (69.4-73.8 wt. SiO2). There is also evidence for crystal transfer between magma producing white and grey pumice. Thin highly evolved melt rims surround some fragmental crystals in grey pumice clasts and appear to have come from magma that produced white pumice. Furthermore, based on crystal compositions, white bands within banded pumice contain crystals originating in grey magma. Finally, only grey pumice clasts form breadcrusted surface textures. We interpret these compositional and textural variations to indicate distinct magma batches, where grey pumice originated from an originally deeper, more volatile-rich dacite recharge magma that ascended through and mingled with the volumetrically dominant, more highly crystalline chamber that produced white pumice. Shortly before eruption, the grey pumice magma stalled within shallow fractures, forming a vanguard magma phase whose ascent may have provided a trigger for eruption of the highly crystalline rhyodacite magma. We suggest that in the case of the Cerro Gal? n eruption, grey pumice provides evidence not only for cryptic silicic recharge in a large caldera system but also a probable trigger for the eruption.

    AB - Triggering mechanisms of large silicic eruptions remain a critical unsolved problem. We address this question for the 2. 08-Ma caldera-forming eruption of Cerro Gal? n volcano, Argentina, which produced distinct pumice populations of two colors: grey (5 ) and white (95 ) that we believe may hold clues to the onset of eruptive activity. We demonstrate that the color variations correspond to both textural and compositional variations between the clast types. Both pumice types have bulk compositions of high-K, high-silica dacite to low-silica rhyolite, but there are sufficient compositional differences (e. g., 150 ppm lower Ba at equivalent SiO2 content and 0.03 wt. higher TiO2 in white pumice than grey) to suggest that the two pumice populations are not related by simple fractionation. Trace element concentrations in crystals mimic bulk variations between clast types, with grey pumice containing elevated Ba, Cu, Pb, and Zn concentrations in both bulk samples (average Cu, Pb, and Zn concentrations are 27, 35, and 82 in grey pumice vs. 11, 19, and 60 in white pumice) and biotite phenocrysts and white pumice showing elevated Li concentrations in biotite and plagioclase phenocrysts. White and grey clasts are also texturally distinct: White pumice clasts contain abundant phenocrysts (44-57 ), lack microlites, and have highly evolved groundmass glass compositions (76.4-79.6 wt. SiO2), whereas grey pumice clasts contain a lower percentage of phenocrysts/microphenocrysts (35-49 ), have abundant microlites, and have less evolved groundmass glass compositions (69.4-73.8 wt. SiO2). There is also evidence for crystal transfer between magma producing white and grey pumice. Thin highly evolved melt rims surround some fragmental crystals in grey pumice clasts and appear to have come from magma that produced white pumice. Furthermore, based on crystal compositions, white bands within banded pumice contain crystals originating in grey magma. Finally, only grey pumice clasts form breadcrusted surface textures. We interpret these compositional and textural variations to indicate distinct magma batches, where grey pumice originated from an originally deeper, more volatile-rich dacite recharge magma that ascended through and mingled with the volumetrically dominant, more highly crystalline chamber that produced white pumice. Shortly before eruption, the grey pumice magma stalled within shallow fractures, forming a vanguard magma phase whose ascent may have provided a trigger for eruption of the highly crystalline rhyodacite magma. We suggest that in the case of the Cerro Gal? n eruption, grey pumice provides evidence not only for cryptic silicic recharge in a large caldera system but also a probable trigger for the eruption.

    UR - http://www.springerlink.com/content/q6w17028214w9669/fulltext.pdf

    U2 - 10.1007/s00445-011-0525-5

    DO - 10.1007/s00445-011-0525-5

    M3 - Article

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    EP - 1533

    JO - Bulletin of Volcanology

    JF - Bulletin of Volcanology

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