Degassing of mantle-derived C[O.sub.2] 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 arc 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 C[O.sub.2]-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 C[O.sub.2] include dissolution of limestone and dolostone ([C.sub.carb]) and 'external carbon' ([C.sub.external]). [C.sub.external] is computed as the bicarbonate alkalinity (dissolved inorganic carbon [DIC]) minus the [C.sub.carb] ([C.sub.external] = DIC - [C.sub.carb]). [C.sub.external] is deconvolved using carbon isotopes into biogenically derived sedimentary carbon ([C.sub.organic]) and deep C[O.sub.2] inputs ([C.sub.endogenic]). Measured [[delta].sup.13]C values are -17 [per thousand] to + 3 [per thousand] versus Pee Dee Belemnite (PDB). Assuming [[delta].sup.13] [C.sub.carb] = +2 [per thousand] [[delta].sup.13] [C.sub.organic] = -28 [per thousand], and [[delta].sup.13] [C.sub.endogenic] = -5 [per thousand], 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-rcspired C[O.sub.2], and 33% from deep (endogenic) sources. Helium isotope values ([sup.3]He/[sup.4]He) in gases dissolved in spring waters in the southern Colorado Plateau region range from 0.10 to 1.16 [R.sub.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 C[O.sub.2]/[sup.3]He ratios of 2 x [10.sup.9] to 1.4 x [10.sup.13] are adjusted by removing the proportion of C[O.sub.2] from [C.sub.carb] and [C.sub.organic] to give values