Helium and Carbon Isotope Systematics of Springs in the Separation Creek Drainage System, Three Sisters area, Central Oregon Cascades.

In response to recent and on-going uplift in the Separation Creek drainage system, 5 km west of South Sister volcano in the central Oregon Cascades (e.g. Wicks et al., 2001), a hydrogeochemical monitoring project was initiated by the U.S. Geological Survey in the summer of 2001. When compared to existing literature data, we found no significant changes in the helium isotope composition of hot springs located in the vicinity of South Sister volcano, but outside the area of uplift. Nor were there significant changes in fluid chemistry or conductivity of cold springs within the area of uplift. For the latter group, there are no pre-uplift helium or carbon isotope data. Therefore, the implications of the strong magmatic helium and carbon isotope signals measured in two of these samples and their possible relationship to the recent uplift could not be evaluated (Van Soest et al., 2001; Evans et al., 2002). Within the scope of the hydrogeochemical monitoring project, a detailed survey of cold springs in the Separation Creek drainage area was planned for the spring, summer and fall of 2002. Preliminary results for spring 2002 samples suggest a relationship between helium isotope composition and distance from South Sister volcano, but not the center of uplift: 8.6R_A at 3 km (from a sample nearest the youngest erupted volcanics), 7.4R_A at 5 km (near the center of uplift), 7.0R_A at 10 km, 6.8R_A at 18 km, and 5.2R_A at 25 km from South Sister volcano. The last value is from the hot spring closest to the area of uplift for which there is pre-uplift data and it suggests a constant helium isotope ratio over time (1982-present). The new carbon isotope results confirm the existence of a mixing relationship between deep abiogenic (magmatic) carbon and shallow biogenic carbon that was apparent in the 2001 samples. The carbon isotope results appear to correlate with the Cl and conductivity anomalies in the springs. At this time, whether a similar correlation exists for the helium isotope data remains unclear. To date there is no clear evidence that links the geochemical data to the recent and on-going uplift, but work currently in progress may shed more light on this issue. Evans, W. C., Mariner, R. H., Ingebritsen, S. E., Kennedy, B. M., Van Soest, M. C., and Huebner, M. A. Report of hydrologic investigations in the Three Sisters area of central Oregon, Summer 2001. USGS Water-Resources Investigations Report 02-4061, 11p., 2002. Van Soest, M. C., Kennedy, B. M. Evans, W. C., and Mariner, R. H. Helium Isotope Compositions in Springs from the Three Sisters Region, Central Oregon, USA. Eos Trans. AGU 82(47), Fall Meet. Suppl., Abstract V51C-1028, 2001. Wicks, C. Jr., Dzurisin, D, Ingebritsen, S. E., Thatcher, W., Lu, Z., and Iversen, J. Magmatic activity beneath the quiescent Three Sisters volcanic center, central Oregon Cascade Range, USA, inferred from InSAR. Eos Trans. AGU 82(47), Fall Meet. Suppl., Abstract G31C-0155, 2001.