TRAVERTINE-DEPOSITING SYSTEMS OF THE WESTERN U.S.: REGIONAL LINKS BETWEEN CENOZOIC TECTONISM AND WATER QUALITY

Regional geochemical investigation of CO₂-rich travertine-depositing cool springs of the Colorado Plateau and Rio Grande rift reveals genetic links to deeply circulating fluids associated with extensional tectonics, magmatism and mantle degassing.  Active springs are commonly located along basement-penetrating faults and associated with large accumulations of Quaternary travertine, implying that these hydrologic systems have persisted through time. These deposits are spatially associated with major tectonic features (e.g. rift-bounding structures, Jemez lineament, and Arizona Transition Zone) and overlie low mantle velocity zones as revealed by mantle tomography.  Extensive travertine accumulations are a surface expression of high fluxes of CO₂.  Our model focuses on the role of magmatism as a source of CO₂, and faults as conduits.  Gas analyses record high CO₂/N₂ and He/N₂, and ³He/⁴He ratios up to 1.15 Ra (ATZ), suggesting contributions of mantle-derived helium and its carrier gas CO₂ from nearby volcanic fields. Mixing models using Cl vs. Cl/Br, Sr vs. ⁸⁷Sr/⁸⁶Sr, and ³He/⁴He vs. d¹³C (of CO₂) indicate mixing of dilute, nonradiogenic ground waters with 1-14% of saline, radiogenic, deeply-circulated fluids to produce the observed spring chemistries. The radiogenic strontium isotope signal of CO₂-rich springs, contributed by waters that are deeply circulated through Precambrian basement, may account for the previously observed downstream increases in Colorado River ⁸⁷Sr/⁸⁶Sr. Similarly, natural inputs from deeply sourced saline spring waters affect the Colorado River salinity as it crosses the western Colorado Plateau and the Basin and Range province; and the Rio Grande as it travels southward.  Improved understanding of the western U.S. surface and ground water system requires consideration of chemical inputs from mantle-derived gases and deeply-circulated waters.