Long-lived radon decay products in Mount St. Helens emissions: An estimation of the magma reservoir volume

Abstract

The size of the Mount St. Helens magma reservoir is estimated from long-lived radon decay products in fumarole emissions. Samples were collected in September 1981 on filters and by a new sampling technique developed for fumarole condensate. The Po-210/Pb-210 activity ratios were significantly higher in the condensate than on the filters. The difference is attributed to Po-210 losses from the filters by a variety of processes and to trapping of Po-210 in sulfur precipitate that prevents detection of alpha particles. Model calculations based on Po-210, Pb-210, and Bi-210 activities give volume estimates for the degassing magma of about either 1 × 10 8 m 3 or 4 × 10 5 m 3/d according to the model utilized.