Timing of degassing and plagioclase growth in lavas erupted from Mount St. Helens, 2004-2005, from 210Po-210Pb-226Ra disequilibria

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.