Timing of Degassing and Plagioclase Growth in Lavas Erupted from Mount St. Helens, 2004–2005, from ²¹⁰Po-²¹⁰Pb-²²⁶Ra Disequilibria

Disequilibrium between (super 210) Po, (super 210) Pb, and (super 226) Ra 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 (super 210) Po precipitated in open spaces. Before leaching, samples had variable initial ( (super 210) Po) values, whereas after leaching, the groundmasses of nearly all juvenile samples were found to have had ( (super 210) Po) nearly equal 0 when they erupted. Thus, most samples degassed (super 210) Po both before and after the magmas switched from open- to closed-system degassing. All juvenile samples have ( (super 210) Pb)/( (super 226) Ra) ratios within 2sigma of equilibrium, suggesting that the magmas involved in the ongoing eruption did not have strong, persistent fluxes of (super 222) Rn 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 ( (super 210) Pb)/( (super 226) Ra) 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 degassed (super 210) Po both before and after the magmas switched from open- to