NEW INTERPRETATIONS OF THE GEOTHERMOMETRY AND GEOCHEMISTRY OF BREITENBUSH HOT SPRINGS, OREGON CASCADES

The geochemistry and geothermometry of Breitenbush hot springs are reassessed using chemical and isotopic data from wells and springs at the site. Our presumption that ascending thermal water remains equilibrated with calcite is supported by waters from thermal wells with calcite SI » 0 at their discharge temperatures. However, samples from most springs suggest near-surface CO₂ loss. The amount of CO₂ loss and temperatures at which the samples last equilibrated with calcite were reconstructed using an iterative technique presuming a common source with the thermal wells.

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.