NEW INTERPRETATIONS OF THE GEOTHERMOMETRY AND GEOCHEMISTRY OF BREITENBUSH HOT SPRINGS, OREGON CASCADES
Geothermometric estimates were then made using the reconstructed discharge waters and the RTEst software package. Al+3 concentrations were evaluated along with reservoir temperature via parameter estimation and compared to measured concentrations. Fe(III) was calculated from goethite equilibrium. An estimated reservoir temperature of 137.1 ±2.0°C was obtained, assuming equilibrium with chalcedony, celadonite, laumontite, heulandite, and epidote. While this estimate is lower than the 174 to 180°C range reported in other multicomponent geothermometric studies, it is consistent with available mineralogical, fluid inclusion, water chemistry and borehole temperature measurements near the site.
A subset of samples has Na+ and Cl- concentrations similar to the thermal samples but are depleted in K+ and SiO2(aq). We simulated these waters by cooling the calculated reservoir composition while maintaining equilibrium with chalcedony, K-feldspar, calcite and heulandite and allowing gibbsite to precipitate when it became supersaturated. The calculated Ca2+, SiO2(aq), and K+ concentrations and pH are congruent with measured values.
Stable oxygen and hydrogen isotopic data indicate that thermal waters at both Breitenbush and Austin Hot Springs (located ~40 km north) are a mixture of 4 to 8% “andesitic waters” and meteoric water recharged at elevations of 1750 to 2200 m, along the crest of the Cascade Mountains. The correlation of stable isotopes with Cl- concentrations, measured Br/Cl ratios, and analyses from other sources suggests that Cl-, other halogens, and CO2 in the Breitenbush thermal waters are primarily derived from degassing fluids rising from the serpentinized forearc mantle.