ORIGIN OF THERMAL FLUIDS AT LASSEN VOLCANIC NATIONAL PARK: EVIDENCE FROM NOBLE AND REACTIVE GAS ABUNDANCES.

Thermal fluid discharges at Lassen are dominated by high-altitude fumaroles and acid-sulfate hot springs in the Park, and lower altitude, neutral, high-chloride hot springs in Mill Valley 7-10 km to the south. The interrelations of these fluids have been studied by noble and reactive gas analyses. Atmospheric noble gas (ANG) contents of superheated fumaroles are similar to those of air-saturated recharge water (ASW) at 5 degree C and 2500-m elevation. Low-elevation, high-chloride, hot-spring waters are highly depleted in ANG, relative to the ASW. The surface temperatures and gas chemistry of the fumaroles and hot springs suggest that steam originating from partial to near-complete vaporization of liquid from a boiling, high-chloride, hot water aquifer is decompressed adiabatically, and more or less mixed with shallow groundwater to form superheated and drowned fumaroles within the Park. Refs.