Geochemistry and geothermometry of Breitenbush Hot Springs, Oregon, USA

摘要

The aqueous chemistry, isotopic composition (delta O-18 and delta H-2), and geothermometry of the Breitenbush Hot Springs area in the central Oregon Cascades were examined. Water samples were collected from springs (5), wells (8), and the Breitenbush River (2). Geothermometry was interpreted using the Reservoir Temperature Estimator (RTEst) software package and constrained by the reported mineralogy. Reservoir temperature was calculated assuming equilibration with chalcedony, celadonite, laumontite, heulandite, and epidote. Because of its relatively rapid reaction rate, calcite was presumed to remain equilibrated during cooling of the ascending reservoir waters. CO2(g) degassing from the springs was accounted for by adjusting the carbon content and temperature to be consistent with a cooling curve passing through the chemical composition of the deep well waters. The average estimated reservoir temperature of 137.1 +/- 2.0 degrees C is lower than the 174 to 180 degrees C reported in other multicomponent geothermometric studies. However, our new estimate is consistent with mineralogical, aqueous geochemical, and fluid inclusion data, and with geothermal borehole temperature measurements near the site.Stable oxygen and hydrogen isotope data indicate that the thermal waters at Breitenbush and Austin Hot Springs are a mixture of 4 to 8% "andesitic water" (Giggenbach, 1992) and local meteoric waters recharged at elevations of 1750 to 2200 m, along the crest of the Cascade Mountains. Br/Cl ratios, delta O-18, and delta D, and their correlation with Cl- concentrations from our study combined with data and analyses from other sources, suggests that Cl-, other halogens, and CO2 in the Breitenbush hot springs are primarily derived from degassing fluids rising from the serpentinized forearc mantle.The calculated reservoir temperature and average measured thermal water Cl- concentration were used with previously reported chloride-flux-based measurements by the USGS to estimate a hydrothermal discharge of 13.7 +/- 1.6 L/s and an advective heat discharge of 7.1 +/- 0.8 MW. This calculated hydrothermal heat loss is slightly less than Ingebritsen et al.'s (1992) previous estimate of 9 MW. However, it still represents a substantial transfer of heat from a relatively small, constant groundwater discharge.