Helium isotope (3He/4He) data from geothermal springs in the Colorado Rocky Mountains (western United States) provide unequivocal evidence for a remarkable mantle-to-groundwater connection, with contributions of up to 27% mantle-derived helium. Hydrochemical modeling of springs shows the mantle helium is associated with high pCO2 with 76 ± 20% of the CO2 also derived from endogenic (deep geologic) sources. These springs occur preferentially along faults, have highest 3He/4He values above domains of low mantle velocity, and demonstrate unexpectedly widespread neotectonic mantle degassing. Total CO2 flux through these springs is 3 × 108 mol/yr, a small but persistent contribution to the CO2 budget and an important baseline for carbon sequestration/leakage studies.

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