EVOLUTION OF HYDROTHERMAL WATERS AT MOUNT ST-HELENS, WASHINGTON, USA

Hydrothermal water samples at Mount St. Helens collected between 1985 and 1989 and in 1994 are used to identify water types and describe the ir evolution through time. Two types of low temperature hydrothermal s ystems are associated with the 1980 eruptions and were initiated soon after emplacement of shallow magma and pyroclastic flows. The Loowit h ot spring system is located in the breach zone and is associated with the magma conduit and nearby avalanche deposits, whereas the Pumice Pl ain (PP) system is associated with pyroclastic flows and avalanche dep osits approximate to 3 to 5 km north of the volcano. The PP waters fir st discharged at the surface in 1981, whereas the Loowit waters began to issue at the surface in 1983. delta D, delta(18)O and H-3 indicated all thermal waters are dominantly derived from post-1980 recharge. Fl uids flow through, and are restricted to, the shallow 1980 avalanche a nd pyroclastic deposits. All water cooled with time (up to 43 degrees C on the PP and up to 20 degrees C in Loowit in 5 years), and chemical compositions have changed rapidly. All waters have highly variable, a nd unreliable geothermometer temperatures with maximum indicated tempe ratures < 185 degrees C. None of the fluids are at equilibrium with ho st rocks; dissolution of host rocks as a function of fluid temperature occurs at all sites. Although fluid chemistry varied dramatically in the early years of this study, all waters had similar Li/Cl ratios by 1989 indicating partial stabilization and demonstrating the similarity in host rock compositions at the thermal areas. Loowit waters exhibit delta D and delta(18)O enrichments from the meteoric water line and B /Cl ratios (0.02 to 0.05) similar to those in dome fumarole condensate s, indicating a less than or equal to 10% contribution of magmatic vol atiles to these waters. The PP waters generally do not exhibit isotopi c enrichments, and the B/Cl ratios are approximate to 1 order of magni tude less than those in Loowit. No magmatic volatiles enter PP waters, and this system is driven by meteoric water circulation within the co oling pyroclastic flows and underlying avalanche deposits. The PP syst em will Likely cool rapidly to form a nonthermal system. Similar, tran sient, low-temperature hydrothermal phenomena probably have been assoc iated with other ash-flow sheets, yet have thus far been largely undoc umented.