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 their evolution through time. Two types of low temperature hydrothermal systems are associated with the 1980 eruptions and were initiated soon after emplacement of shallow magma and pyroclastic flows. The Loowit hot spring system is located in the breach zone and is associated with the magma conduit and nearby avalanche deposits, whereas the Pumice Plain (PP) system is associated with pyroclastic flows and avalanche deposits ≈ 3 to 5 km north of the volcano. The PP waters first discharged at the surface in 1981, whereas the Loowit waters began to issue at the surface in 1983. 8O and 3H indicated all thermal waters are dominantly derived from post-1980 recharge. Fluids flow through, and are restricted to, the shallow 1980 avalanche and pyroclastic deposits. All water cooled with time (up to 43 °C on the PP and up to 20 °C in Loowit in 5 years), and chemical compositions have changed rapidly. All waters have highly variable, and unreliable geothermometer temperatures with maximum indicated temperatures < 185 °C. None of the fluids are at equilibrium with host 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 δD and δ18O enrichments from the meteoric water line and B/Cl ratios (0.02 to 0.05) similar to those in dome fumarole condensates, indicating a ≤ 10% contribution of magmatic volatiles to these waters. The PP waters generally do not exhibit isotopic enrichments, and the B/C1 ratios are ≈ 1 order of magnitude less than those in Loowit. No magmatic volatiles enter PP waters, and this system is driven by meteoric water circulation within the cooling pyroclastic flows and underlying avalanche deposits. The PP system will likely cool rapidly to form a nonthermal system. Similar, transient, low-temperature hydrothermal phenomena probably have been associated with other ash-flow sheets, yet have thus far been largely undocumented.