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

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.