Fluid Geochemistry of the Makushin Geothermal Area, Unalaska Island, Alaska

Abstract

Several fumarole fields and numerous thermal springs occur on the eastern flanks of Makushin volcano. The thermal springs at Makushin are typically near neutral in pH, have very low chloride levels (10ppm), and are comparatively rich in Mg, Ca, SO4, HCO3 and SiO2. Similarities in the cation and isotopic compositions of Makushin thermal springs and local surface streams indicated the thermal waters are locally derived meteoric waters. These surface waters infiltrate to relatively shallow depths where they are heated by steam and gases rising from a vapor dominated zone. Predominate fumarolic gases are Co2, N2 and sulfur gases. All samples showed a significant level of H2 and a high H2/CH4 ratio. The ration of He3He3WRT atmospheric levels range from 4.5 to 6.0. Fumaroles and hot springs in the upper parts of Glacier and Makushin Valleys form a roughly linear trend, suggesting their distribution may be structurally controlled.