CO2 AND H2S IN THE AIR OVER THE GROUND-SWELLING AREA WEST OF SOUTH SISTER VOLCANO, CENTRAL OREGON CASCADE RANGE

Intruding magma at ~6.5 km depth inferred to cause the InSAR-based inflation rate of 0.006 km³/y in the area of ground swelling west of South Sister volcano (C. Wicks, personal communication) would support only low levels of CO₂ emission. This assessment holds even if the inflation is due to intrusion of mafic magma, which is among the most CO₂-rich magmas and the most frequently erupted magma of the past 15,000 years in the Three Sisters region (Scott et al. 1999). Mafic magma with a liberal range of CO₂ contents (0.25-1.25 wt%) and accumulating at the inflation rate 6.5 km beneath the surface would degas only 100-500 metric tons/day (t/d) of CO₂. These degassing rates are on the low end of CO₂ emission rates observed at passively degassing volcanoes but are potentially detectable by airborne techniques—examples being the 200- to 300-t/d emission rates measured at Mt. Baker volcano, WA (McGee et al. 2001) and Mammoth Mountain, CA (Gerlach et al. 1999).

Helicopter surveys of CO₂ and H₂S, the least susceptible to scrubbing of the more abundant magmatic gases, were carried out ~60 m above the area of ground swelling on 25 April and 21 September 2001. Readings of latitude/longitude by GPS receiver and measurements of CO₂ and H₂S concentrations, as well as air temperature and pressure, were recorded synchronously at 1-s intervals. A total of some 2500-3000 data points were collected for each variable. Both surveys showed H₂S below detection limits (~5 ppb). The April survey showed possible trace levels of CO₂ at ~1 ppm above atmospheric background (371-372.5 ppm) just downwind of the swelling center. The September survey showed only background atmospheric CO₂ levels.

The surveys suggest very low rates of magma degassing and provide baselines for future airborne monitoring, should the inflation rate or other signs of unrest accelerate. If magma is accumulating at the inferred rate, the impact of degassed CO₂ on atmospheric CO₂ levels is apparently not significant. Perhaps degassed CO₂ has not yet reached the surface; or if it has, it may be dispersed over a large area and masked by variations in background atmospheric CO₂ levels; or most of it may be dissolved in subsurface water and carried off, or converted to HCO₃^- by water/rock reactions. Alternatively, magma may not be accumulating, and the ground swelling may reflect some other process.