Addition of magmatic volatiles into the hot spring waters of loowit canyon, Mount St. Helens, Washington, USA

Abstract

Geochemical studies on cold meteoric waters, post-1980 hot spring waters, fumarole emissions from the dacite dome, and volcanic rocks at Mount St. Helens (MSH) from 1985 to 1989 show that magmatic volatiles are involved in the formation of a new hydrothermal system. Hot spring waters are enriched in δ18O by as much as 2‰ and display enrichments in δD relative to cold waters. A well-defined isotopic trend is displayed by the isotopic composition of a>400°C fumarole condensate collected from the central crater in 1980 (-33‰ δD, +6‰ δ18O), of condensate samples collected on the dome, and of cold meteoric and hot spring waters. The trend indicates that mixing occurs between local meteoric water and magmatic water degassing from the dacite dome. Between 30 and 70% magmatic water is present in the dome fumarole discharges and ≈10% magnatic water has been added to the waters of the hydrothermal system. Relations between Cl, SO4 and HCO3 indicate that the hot spring waters are immature volcanic waters formed by reaction of rocks with waters generated by absorption of acidic volcanic fluids. In addition, the B/Cl ratios of the spring waters are similar to the B/Cl ratios of the fumarole condensates (≈0.02), values of δ13C in the HCO3 of the hot springs (-9.5 to-13.5‰) are similar to the magmatic value at MSH (-10.5‰), and the 3He/4He ratio, relative to air, in a hot spring water is 5.7, suggesting a magmatic origin for this component.