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

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• 作者：Kim Berlo ; Simon Turner ; Jon Blundy ; Stuart Black and Chris Hawkesworth
• 关键词：Pb-210 ; Ra-226 ; Mount St. Helens ; degassing ; time scales
• 刊名：Earth and Planetary Science Letters
• 出版年：2006
• 期刊代码：18_0012821x
• 类别：ph
• 出版时间：30 September 2006
• 卷：249
• 期：3-4
• 页码：337-349
• 文件大小：385 K

摘要

Disequilibria between ²¹⁰Pb and ²²⁶Ra can be used to trace magma degassing, because the intermediate nuclides, particularly ²²²Rn, are volatile. Products of the 1980–1986 eruptions of Mount St. Helens have been analysed for (²¹⁰Pb/²²⁶Ra). Both excesses and deficits of ²¹⁰Pb are encountered suggesting rapid gas transfer. The time scale of diffuse, non-eruptive gas escape prior to 1980 as documented by ²¹⁰Pb 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 ²¹⁰Pb excess is much shorter (months) as can be observed from steady increases of (²¹⁰Pb/²²⁶Ra) with time during 1980–1982. The formation of ²¹⁰Pb 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 (²¹⁰Pb/²²⁶Ra) in a single eruption caused by tapping magma from various depths. The two time scales of gas transport, to create both ²¹⁰Pb deficits and ²¹⁰Pb excesses, cannot be reconciled in a single event. Rather ²¹⁰Pb deficits are associated with pre-eruptive diffuse degassing, while ²¹⁰Pb excesses document the more vigorous degassing associated with eruption and recharge of the system.