THE GEOCHEMISTRY OF THE GEOTHERMAL AND HYDROLOGIC SYSTEM OF THE SEPARATION CREEK DRAINAGE, THREE SISTERS AREA, CENTRAL OREGON CASCADES
The data from this study provide new insights into geothermal discharge through the complex hydrologic system of the Separation Creek drainage. Among other things our data show that a) the geothermal system is long-lived and not directly related to the current period of uplift; b) the magma recharge rate of the system is frequent enough to keep helium isotope ratios at upper mantle values; c) geothermal fluids discharge laterally away from the crest through the hydrologic system and interact along their flow paths with aquifer fluids/rocks, indicated by a correlation of decreasing helium and carbon isotopic ratios with distance from the area of uplift; d) A correlation between helium-3 and chloride suggests that the Separation Creek fluids are not unlike those discharging from the closest hot springs (located at least 25 km away from the volcano), but highly diluted with air-saturated groundwater. This indicates that at least part, possibly a major fraction, of the chloride in the geothermal water is magma-derived. Strong evidence for a geothermal system was not found in springs east of the Three Sisters crest. It is not clear, at this time, if the Separation Creek drainage presents a unique situation or if the strong magma-driven signal observed in the dilute cold springs is a common feature of Cascade volcanoes in regions of high annual precipitation.