Temporal geochemical variations in volatile emissions from Mount St. Helens, USA, 1980-1994

Fumarole discharges (95-560 degrees C) collected from the dacite dome insid e Mount St. Helens crater show temporal changes in their isotopic and chemi cal compositions. A delta D vs. delta(18)O plot shows that condensed waters from the gases are mixtures of meteoric and magmatic components, but that the apparent magmatic end-member in 1994 was depleted by about 7 parts per thousand in delta D relative to the apparent end-member in 1980. Based on d elta D modeling, approximately 63% of shallow, post-1980 magma has yet to d egas. Surprisingly, Cl and F contents in the 1994 samples were only 0.47 an d 3.8%, respectively, of the concentrations determined for end-member magma tic fluid in 1980. The data indicate that Cl (and F and B) is degassed from magma relatively quickly compared to water and/or that most of the Cl dega ssed in later years is dissolved into the shallow Mount St. Helens hydrothe rmal system. Because metals are often transported in magmatic and hydrother mal fluids as Cl complexes, rapid changes in surface volatile compositions may have implications for the timing and location of metals transport and d eposition in some volcanoes. (C) 2000 Elsevier Science B.V. All rights rese rved.