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

The edifice of Mount Rainier, an active stratovolcano, has episodically col lapsed leading to major debris flows. The largest debris flows are related to argillically altered rock which leave areas of the edifice prone to fail ure. The argillic alteration results from the neutralization of acidic magm atic gases that condense in a meteoric water hydrothermal system fed by the melting of a thick mantle of glacial ice. 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 degrees C) in the caves were sampled for stable isotopic, gas, and geochemical studie s. Stable isotope data on fumarole condensates show significant excess deuteri um with calculated delta D and delta(18)O Values (-234 and -33.2 parts per thousand, respectively) for the vapor that are consistent with an origin as secondary steam from a shallow water table which has been heated by underl ying magmatic-hydrothermal steam. Between 1982 and 1997, delta D of the fum arole vapor may have decreased by 30 parts per thousand. The compositions of fumarole gases vary in time and space but typically con sist of air components slightly modified by their solubilities in water and additions of CO2 and CH4. The elevated CO2 contents (delta(13)C(CO2) = -11 .8 +/- 0.7 parts per thousand), with spikes of over 10,000 ppm, require the episodic addition of magmatic components into the underlying hydrothermal system. Although only traces of H2S were detected in the fumaroles, most no tably in a sample which had an air delta 13C(CO2) signature (-8.8 parts per thousand), incrustations around a dormant vent containing small amounts of acid sulfate minerals (natroalunite, minamiite, and wood-houseite) indicat e higher H2S (or possibly SO2) concentrations in past fumarolic gases. Condensate samples from fumaroles are very dilute, slightly acidic, and enr iched in elements observed in the much higher temperature fumaroles at Moun t St. Helens (K and Na up to the ppm revel; metals such as Al, Pb, Zn Fe an d Mn up to the ppb level and volatiles such as CI, S, and F up to the ppb l evel). The data indicate that the hydrothermal system in the edifice at Mount Rain ier consists of meteoric water reservoirs, which receive gas and steam from an underlying magmatic system. At present the magmatic system is largely f looded by the meteoric water system. However, magmatic components have epis odically vented at the surface as witnessed by the mineralogy of incrustati ons around inactive vents and gas compositions in the active fumaroles. The composition of fumarole gases during magmatic degassing is distinct and, i f sustained, could be lethal. The extent to which hydrothermal alteration i s currently occurring at depth, and its possible influence on future edific e collapse, may be determined with the aid of on site analyses of fumarole gases and seismic monitoring in the ice caves. (C) 2000 Elsevier Science B. V. All rights reserved.