Time Series 210Pb-210Pb Data for Lavas Erupted From Mount St. Helens Volcano: Implications for Time-scales of Degassing and Crystallization

210Po has been shown to be highly volatile at magmatic temperatures and generally degasses almost completely during eruption. This has been true for nearly all of the lavas erupted from Mount St. Helens since November 2004, as the last day of complete degassing of 210Po from the Mount St. Helens dacite generally corresponds with their day of eruption within error of analysis and knowledge of eruption day. In contrast, samples SH 304 and SH 305, which erupted in October and November 2004, apparently last degassed Po about the day of the first phreatic explosions. Our sample of a lithified, interior portion of the gouge coating on a dacite spine erupted in April, 2005 also degassed Po until it erupted. The exterior less-lithified gouge erupted about July 1, 2005, however, was strongly enriched in 210Po over 210Pb, indicating that it was a primary conduit for escaping magmatic volatiles. Comparing our results with unpublished 226Ra activities (Donnelly and Cooper, this session) show SH 304 and SH 305 whole rocks to have near equilibrium (210Pb/226Ra) values. If the significant enrichment in Li observed for plagioclase in these samples by Adam Kent (personal communication, 2005) was generated by a flux of a Li and Rn-bearing volatile phase, then the duration of this fluxing must have been over a period that was long enough for Li to equilibrate in plagioclase, but too short to cause a significant 210Pb excess from 222Rn decay (i.e. less than 1-2 years). A preliminary 210Pb/226Ra ratio measured for plagioclase extracted from SH 304 yielded a model age of several decades, showing that a significant portion of the plagioclase grew in the decades leading to eruption. However, the lack of 210Po/210Pb disequilibrium in this same separate suggests that little of this growth occurred during the last year before eruption.