Kari M. Cooper John S. Pallister Carl R. Thornber Matthew Wortel David R. Sherrod William E. Scott Peter H. Stauffer Mark K. Reagan 2008 Disequilibrium between ²¹⁰Po, ²¹⁰Pb, and ²²⁶Ra was measured on rocks and plagioclase mineral separates erupted during the first year of the ongoing eruption of Mount St. Helens. The purpose of this study was to monitor the volatile fluxing and crystal growth that occurred in the weeks, years, and decades leading up to eruption. Whole-rock samples were leached in dilute HCl to remove ²¹⁰Po precipitated in open spaces. Before leaching, samples had variable initial (²¹⁰Po) values, whereas after leaching, the groundmasses of nearly all juvenile samples were found to have had (²¹⁰Po) ≈ 0 when they erupted. Thus, most samples degassed ²¹⁰Po both before and after the magmas switched from open- to closed-system degassing. All juvenile samples have (²¹⁰Pb)/(²²⁶Ra) ratios within 2 δ of equilibrium, suggesting that the magmas involved in the ongoing eruption did not have strong, persistent fluxes of ²²²Rn in or out of magmas during the decades and years leading to eruption. These equilibrium values also require a period of at least a century after magma generation and the last significant differentiation of the Mount St. Helens dacites. Despite this, the elevated (²¹⁰Pb)/(²²⁶Ra) value measured in a plagioclase mineral separate from lava erupted in 2004 suggests that a significant proportion of this plagioclase grew within a few decades of eruption. The combined dataset suggests that for most 2004-5 lavas, the last stage of open-system degassing of the dacite magmas at Mount St. Helens is confined to the period between 1-2 years and 1-2 weeks before eruption, whereas plagioclase large enough to be included in the mineral separate grew around the time of the 1980s eruption or earlier. application/pdf 10.3133/pp175037 en U.S. Geological Survey Timing of degassing and plagioclase growth in lavas erupted from Mount St. Helens, 2004-2005, from 210Po-210Pb-226Ra disequilibria reports