Continental-scale links between the mantle and groundwater systems of the western United states: Evidence from travertine springs and regional He isotope data

摘要

To understand regional mantle degassing, we compiled new and existing helium isotope data measured in hot springs, gas fields, and travertine-depositing cool springs and compared these geochemical data with mantle velocity structure determined from tomographic studies. These data suggest heterogeneous mantle degassing, with regions of highest ~3He/~4He in groundwaters (hence, highest mantle helium contribution) corresponding to regions of lowest mantle velocity, a reflection of tectonically active and partially molten mantle. New He isotope and water chemistry data from travertine-depositing cool springs of the western United States show marked variability' consistent with mixing between surface water recharge and inputs from deep crustal and mantle sources. The deeply sourced end-member fluids of these mixing trends have high 3He/'He, high dissolved CO_2, and high salinity compared to shallow recharge waters, and commonly have elevated trace element concentrations. Consequently, these fluids cause degradation of water quality in western U.S. aquifers. Our conclusions highlight a connection between neotectonics (e.g.. mantle degassing) and water quality in the western United States.