An ongoing episode of crustal uplift centered in the Separation Creek drainage of the Three Sisters area, central Oregon Cascades, may result from a magmatic intrusion that began in 1998. An investigation of springs in this drainage in summer 2001 revealed slightly elevated water temperatures and chloride (Cl-) concentrations of up to about 5?C and 20 milligrams per liter (mg/L), respectively, above background. The total discharge of anomalous Cl- in Separation Creek was 9.2 grams per second, which in combination with the temperature-Cl- relation in the springs results in a total advective heat discharge of 16 MW (megawatts). Comparison with similar findings obtained a decade earlier suggests that total Cl- and heat discharges in the groundwater drainage are unaffected by the current uplift. However, the isotopic composition of the dissolved inorganic carbon (DIC) in the spring waters (delta carbon-13 (13C) = -9.03 to -11.6?; carbon-14 (14C) <25 pmC) combined with helium-3/helium-4 (3He/4He) ratios near 8 RA and C/3He ratios <1010 in two of the springs are indicative of a magmatic source. The high 3He/4He ratios indicate that the magmatic gas is derived from a relatively recent, if not ongoing, intrusion. The concentration of magmatic carbon is low, a few millimoles per liter (mmol/L) at most, with an average value of 1.53 mmol/L for all the springs sampled in the drainage. Combining this average with the late-summer water flow in Separation Creek suggests a discharge of 21 tonnes/day of magmatic carbon dioxide (CO2). The presence of magmatic carbon in the shallow groundwater system, and the fact that DIC is uncorrelated with Cl-, suggests that some magmatic gas could escape diffusely through the soils.
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USGS Numbered Series
Report of hydrologic investigations in the Three Sisters area of central Oregon, Summer 2001