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?

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? M. C. van Soest 1 , B. M. Kennedy 1 , W. C. Evans 2 , & R. H. Mariner 2 CIG-ESD, LBNL, 1 Cyclotron Rd, Berkeley CA 94720, USA. Ph: 510 486 5659; Fx: 510 486 5496; Em: mcvansoest@lbl.gov, bmkennedy@lbl.gov. USGS, 345 Middlefield Rd, Menlo Park, CA 94025, USA. Em: wcevans@usgs.gov, rmariner@usgs.gov. Abstract 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 = ( 3 He/ 4 He) sample-air corrected /( He/ He) air ))] and carbon (δ C = -11.59 to -9.03‰ vs PDB) isotope data and CO 2 / He (5 and 9 x 10 ) 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 2 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. Introduction InSAR imagery data indicates that the Three Sisters volcanic area in Central Oregon has been the locus of strong crustal uplift since about 1998 (Wicks et al. 2001, 2002). Uplift is occurring in a concentric area with its center several km west of South Sister volcano on the Cascades crest. Maximum uplift rates for the center of ‘the bulge’ are 4-5 cm/yr and modeling suggests this is consistent with intrusion of 0.006 km 3 /yr magma at a depth of 6-7 km in the crust (Wicks et al. 2001, 2002). The area of the uplift has no young (<1 ka) volcanic vents and so far there have been no clearly identifiable occurrences of ground deformation or increases in seismicity. A decade prior to the onset of the uplift a source of geothermal fluid was identified by means of a Cl - anomaly in the Separate Creek drainage, which is the main drainage