Mantle Helium and Carbon Isotopes in Separation Creek Geothermal Springs, Three Sisters Area, Central Oregon: Evidence for Renewed Volcanic Activity or a Long Term Steady State System?

Here we present the helium and carbon isotope results from the initial study of a fluid chemistry-monitoring program started in the summer of 2001 near the South Sister volcano in central Oregon. The Separation Creek area which is several miles due west of the volcano is the locus of strong crustal uplift currently occurring at a rate of 4-5 cm/yr (Wicks, et. al., 2001).Helium [R_C/R_A = 7.44 and 8.61 R_A (R_C/R _A = (³He/⁴He)_sample-. _{air corrected}/(³He/⁴He)_air))] and carbon (δ¹³C = -11.59 to -9.03‰ vs PDB) isotope data and CO₂/³He (5 and 9 × 10⁹) show that bubbling cold springs in the Separation Creek area near South Sister volcano carry a strong mantle signal, indicating the presence of fresh basaltic magma in the volcanic plumbing system. There is no evidence though, to directly relate this signal to the crustal uplift that is currently taking place in the area, which started in 1998. The geothermal system in the area is apparently much longer lived and shows no significant changes in chemistry compared to data from the early 1990s. Hot springs in the area, which are relatively far removed from the volcanic edifice, do not carry a strong mantle signal in helium isotope ratios (2.79 to 5.08 R_A), unlike the cold springs, and also do not show any significant changes in helium isotope ratios compared to literature data for the same springs of over two decades ago. The cold springs of the Separation Creek area form a very diffuse but significant low temperature geothermal system, that should, due to its close vicinity to the center of up uplift, be more sensitive to changes in the deeper volcanic plumbing system than the far removed hot springs and therefore require much more study and consideration when dealing with volcano monitoring in the Cascade range or possibly with geothermal exploration in general.