²¹⁰PB-²²⁶RA DISEQUILIBRIA IN DACITE FROM MOUNT ST. HELENS, 1980-1986

The US Geological Survey reference suite of four Mount St. Helens samples erupted between 1980 and 1986, were analyzed for activities of ²¹⁰Po (half-life = 138 d), which are assumed to be equal to activities of ²¹⁰Pb (half-life – 22.6 y) because of the ages and freshness of the samples. These data will be compared with ²²⁶Ra activities obtained by analyzing the same sample powders for its descendent ²¹⁴Pb. The goal will be to investigate whether large scale fluxing of ²²²Rn-bearing volcanic gasses affected these samples, as has been postulated for other samples erupted from Mount St. Helens in the 1980s (Berlo et al., 2006, Earth and Planetary Science Letters), but not for samples erupted in 2004-2005 (Reagan et al., in press). Disequilibria in (²¹⁰Pb/²²⁶Ra) can be produced through the fractioning of ²¹⁰Pb from ²²⁶Ra through extensive crystal fractionation or partial melting of the crust. Another mechanism for producing ²¹⁰Pb -²²⁶Ra disequilibria are ²²²Rn excesses or deficits caused by persistent degassing over a number of years. The reference suite rocks show a slight a variation in ²¹⁰Pb activities between the samples (1.02-1.09 dpm/g), which is above the 1-sigma analytical error of 0.02, but not 2-sigma error, suggesting a relatively steady-state system during the six year period.