Tracing pre-eruptive magma degassing using (210Pb/226Ra) disequilibria in the volcanic deposits of the 1980–1986 eruption of Mount St. Helens

Kim Berlo, Simon Turner, Jon Blundy, Stuart Black, Christopher John Hawkesworth

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    40 Citations (Scopus)

    Abstract

    Disequilibria between Pb-210 and Ra-226 can be used to trace magma degassing, because the intermediate nuclides, particularly Rn-222, are volatile. Products of the 1980-1986 eruptions of Mount St. Helens have been analysed for (Pb-210/Ra-226). Both excesses and deficits of Pb-210 are encountered suggesting rapid gas transfer. The time scale of diffuse, non-eruptive gas escape prior to 1980 as documented by Pb-210 deficits is on the order of a decade using the model developed by Gauthier and Condomines (Earth Planet. Sci. Lett. 172 (1999) 111-126) for a non-renewed magma chamber and efficient Rn removal. The time required to build-up Pb-210 excess is much shorter (months) as can be observed from steady increases of (Pb-210/Ra-226) with time during 1980-1982. The formation of Pb-210 excess requires both rapid gas transport through the magma and periodic blocking of gas escape routes. Superposed on this time trend is the natural variability of (Pb-210/Ra-226) in a single eruption caused by tapping magma from various depths. The two time scales of gas transport, to create both Pb-210 deficits and Pb-210 excesses, cannot be reconciled in a single event. Rather Pb-210 deficits are associated with pre-eruptive diffuse degassing, while Pb-210 excesses document the more vigorous degassing associated with eruption and recharge of the system. (c) 2006 Elsevier B.V. All rights reserved.

    Original languageEnglish
    Pages (from-to)337-349
    Number of pages13
    JournalEarth and Planetary Science Letters
    Volume249
    Issue number3-4
    DOIs
    Publication statusPublished - 30 Sept 2006

    Keywords

    • Pb-210
    • Ra-226
    • Mount St. Helens
    • degassing
    • time scales
    • SO2 EMISSIONS
    • PERMEABILITY
    • CRYSTALLIZATION
    • CONSTRAINTS
    • LAVAS
    • PB-210-RA-226
    • DECOMPRESSION
    • SYSTEMATICS
    • WASHINGTON
    • DYNAMICS

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