A geochemical investigation of springs near Three Sisters volcanoes was conducted in response to the detection of crustal uplift west of the peaks. Dilute, low-temperature springs near the center of uplift show 3He/4He ratios ≥7RA (RA is the ratio in air), and transport in total ∼16 MW of heat and ∼180 g/s of magmatic carbon (as CO2). These anomalous conditions clearly reflect the influence of magma, but they seemingly predate the onset of the present uplift and derive from a previous event. Episodes of intrusion may thus be more common in this area than the age of eruptive vents would imply.

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