SUBSURFACE CHARACTERIZATION OF THERMAL SPRINGS IN CASCADE RANGE AND OLYMPIC MOUNTAINS, WASHINGTON USING MULTIPLE MINERAL EQUILIBRIA GEOTHERMOMETRY

In geothermal resource characterization, conventional cation and silica geothermometers are commonly used to estimate reservoir temperatures of geothermal resources because of their reliability. An established alternative to these standard geothermometers is Multiple Mineral Equilibria (MME) geothermometry. Additionally, the MME technique reveals any physical changes (dilution/mixing and boiling) the geothermal fluid may have undergone during ascent from its reservoir. Alongside local (Cascades vs Olympics) lithologic (igneous rocks vs marine turbidites) and tectonic (complex fault clusters vs spaced subvertical thrusts) comparisons, estimated subsurface temperatures and indicators of mineral-fluid conditions are used to diagnose the geothermal flow paths of five Cascade Range (Carson, Bonneville, Ohanapecosh, Baker, and Carson) and two Olympic Mountains (Olympic Complex and Sol Duc) thermal spring sites. This work continues analysis of thermal spring chemistry data collected in the summer of 2016. All calculations were done through SOLVEQ.

Most estimated temperatures for Cascade waters (53-153 ^oC) are hotter than Olympic waters (53-103 ^oC). These MME estimates generally fall within the ranges projected by some select conventional geothermometers (Cristobalite, Chalcedony, Quartz, Na/K, and Na-K-Ca). The average difference between reservoir and discharge temperatures is greater in the Cascade springs (x̅_difference: 53 ^oC vs 27 ^oC). Furthermore, the Cascade waters show more evidence of chemical evolution since ascent, as Carson (groundwater dilution=24%), Bonneville (CO₂ degassing=23%), and Ohanapecosh (CO₂ degassing=2%) were corrected for disequilibria. Conversely, all five Olympic springs are apparently still in equilibrium with their last reservoir.

The geothermal fluids feeding the Cascade springs may be undertaking an indirect or variable path to the surface, as reflected by a greater temperature difference between surface and reservoir and evidence of physical changes and a structurally complex volcanic arc setting. On the other hand, the fully equilibrated Olympic waters appear to have been cooled to a lesser extent during upflow and cycled through thrust-imbricated turbidites, so there may be a more direct connection between the reservoir and the springs.