Degassing of mantle-derived CO₂ and He from springs in the southern Colorado Plateau region - Neotectonic connections and implications for groundwater systems

Groundwaters of the southern Colorado Plateau - Arizona Transition Zone region are a heterogeneous mixture of chemically diverse waters including meteoric (epigenic) fluids, karst-aquifer waters, and deeply sourced (endogenic) fluids. We investigate the composition of travertine-depositing CO₂-rich springs to determine the origin, transport, and mixing of these various components. The San Francisco Mountain recharge area has little surface flow. Instead, waters discharge through major springs hundreds of kilometers away. About 70% (9340 L/s) of the total recharge (13,500 L/s) discharges 100 km to the north in the incised aquifer system at Grand Canyon. Most of this water (85%; 8070 L/s) emerges through two travertine-depositing karst spring systems: Blue Springs (6230 L/s) and Havasu Springs (1840 L/s). About 30% of recharge (4150 L/s) flows to the south and discharges along NW-striking faults in the Arizona Transition Zone, forming the base flow for the Verde River. Geochemical data define regional mixing trends between meteoric recharge and different endogenic end members that range from bicarbonate waters to sulfate waters. Water quality in the region is dictated by the percentage and character of the endogenic inputs that cause a measurable degradation of groundwater quality for water supply. Sources for the high CO₂ include dissolution of limestone and dolostone (C_carb) and "external carbon" (C_external). C_external is computed as the bicarbonate alkalinity (dissolved inorganic carbon [DIC]) minus the C_carb (C_external = DIC - C_carb).C_external is deconvolved using carbon isotopes into biogenically derived sedimentary carbon (C_organic) and deep CO₂ inputs (C_endogenic). Measured δ¹³C values are -17‰ to +3‰ versus Pee Dee Belemnite (PDB). Assuming δ¹³ C_carb = +2‰, δ ¹³C_organic = -28‰, and δ ¹³ C_endogenic = -5‰, water chemistry mixing models indicate that an average of 42% of the total DIC comes from dissolution of carbonate rocks, 25% from organic carbon, including soil-respired CO₂, and 33% from deep (endogenic) sources. Helium isotope values (³He/⁴He) in gases dissolved in spring waters in the southern Colorado Plateau region range from 0.10 to 1.16 R_A (relative to air) indicating that a significant component of the deeply derived fluid is from the mantle (mean of 5% asthenospheric or 10% subcontinental lithospheric mantle source). Measured CO₂/³He ratios of 2 × 10⁹ to 1.4 × 10¹³ are adjusted by removing the proportion of CO₂ from C_carb and C_organic to give values <5 × 10¹⁰ for all but four samples. Various mixing models using CO₂/ ³He suggest that the mantle-derived components of the CO₂ load are highly variable from spring to spring and may make up an average of ∼10% of the total CO₂ load of the regional springs. Fluid-rock interactions involving endogenic fluids are suggested by ⁸⁷Sr/⁸⁶Sr, δ¹⁸O, and other tracers. The endogenic CO₂ component, multiplied by discharge for each spring, yields an integrated annual flux of deeply derived CO₂ to the groundwater system of ∼1.4 × 10⁹ mol/yr. This CO₂ emission from the Colorado Plateau region reflects a complex tectonic evolution involving Laramide hydration of the lithosphere above the Farallon slab, addition of fluids from mid-Tertiary mantle tectonism during slab removal, and ongoing fluid movement induced by neotectonic small-scale asthenospheric convection.