Fumaroles in ice caves on the summit of Mount Rainier-preliminary stable isotope, gas, and geochemical studies

Zimbelman, DR; Rye, RO; Landis, GP

US National Park Service, Mount Rainier National Park, Tahoma Star Woods Route, Ashford, WA 98304, USA

Journal of Volcanology and Geothermal Research Vol. 97, no. 1-4, pp. 457-473. Apr 2000. ISSN 0377-0273

The edifice of Mount Rainier, an active stratovolcano, has episodically collapsed leading to major debris flows. Two craters atop a 2000-year-old cone on the summit of the volcano contain the world's largest volcanic ice-cave system. In the spring of 1997 two active fumaroles (T=62 degree C) in the caves were sampled for stable isotopic, gas, and geochemical studies. Stable isotope data on fumarole condensates show significant excess deuterium with calculated delta D and delta super(18)O values (-234 and -33.2ppt, respectively) for the vapor that are consistent with an origin as secondary steam from a shallow water table which has been heated by underlying magmatic-hydrothermal steam. Between 1982 and 1997, delta D of the fumarole vapor may have decreased by 30ppt. The compositions of fumarole gases vary in time and space but typically consist of air components slightly modified by their solubilities in water and additions of CO sub(2) and CH sub(4). The elevated CO sub(2) contents ( delta super(13)C sub(CO2)=11.80 plus or minus 0.7ppt), with spikes of over 10,000 ppm, require the episodic addition of magmatic components into the underlying hydrothermal system. Although only traces of H sub(2)S were detected in the fumaroles, most notably in a sample which had an air delta super(13)C sub(CO2) signature (-8.8ppt), incrustations around a dormant vent containing small amounts of acid sulfate minerals (natroalunite, minamiite, and woodhouseite) indicate higher H sub(2)S (or possibly SO sub(2)) concentrations in past fumarolic gases. Condensate samples from fumaroles are very dilute, slightly acidic, and enriched in elements observed in the much higher temperature fumaroles at Mount St. Helens (K and Na up to the ppm level; metals such as Al, Pb, Zn Fe and Mn up to the ppb level and volatiles such as Cl, S, and F up to the ppb level). The data indicate that the hydrothermal system in the edifice at Mount Rainier consists of meteoric water reservoirs, which receive gas and steam from an underlying magmatic system. At present the magmatic system is largely flooded by the meteoric water system. However, magmatic components have episodically vented at the surface as witnessed by the mineralogy of incrustations around inactive vents and gas compositions in the active fumaroles. The composition of fumarole gases during magmatic degassing is distinct and, if sustained, could be lethal.