MANTLE SOURCE FOR CO2-RICH SPRINGS IN THE SOUTHWESTERN U.S.: LINKS BETWEEN MANTLE TOMOGRAPHY, NEOTECTONICS AND WATER QUALITY

The southwestern U.S. has CO2-rich thermal springs, cool springs, and gas fields. Based on studies in Grand Canyon and Rio Grande rift, these are hypothesized to be linked to deeply circulating fluids associated with extensional tectonics, magmatism and mantle degassing. CO2-rich springs are close to tectonic features such as the Walker Lane, Cenozoic normal faults, St George volcanic trend, Jemez lineament, and Rio Grande rift. Springs are commonly located along basement penetrating faults, are found with accumulations of Quaternary travertine and are often associated with Quaternary basalts and low mantle velocity zones.

Aqueous and gas-phase geochemical tracers can be used to identify possible origins of spring waters. Grand Canyon and Rio Grande rift waters are characterized by T < 25ēC, pH from 6 to 9, conductivity from 500 - 50,000 ĩS, and alkalinity from 300 - 3000 ppm. Waters range from Ca-Mg-HCO3 to Na-Cl-SO4 type, deposit travertine, and are characterized by salts with Cl/Br (wt/wt) from 4 - 4000. d18O values cluster between -14 to -9 ‰ SMOW, and range as heavy as -5 ‰. The 87Sr/86Sr ratio of representative springs range between 0.7106 and 0.7342. The dissolved gases moving with the spring waters are CO2 – rich, ranging up to 99% of the dry gas (water-free); some have He far in excess of atmospheric levels. N2/CO2 and N2/He range from 0.003 – 10 and 7 – 10,000, respectively, suggesting that the gases are influenced by magma and mantle degassing. . 3He/4He ratios indicate components of mantle 3He ranging from R/Ra of 6 (Walker Lane) to 0.2 (Grand Canyon).

These data provide an unexpected link between neotectonics and water quality. Colorado River and Rio Grande salinity increases downstream. Prior explanations used agricultural impacts, and water exchange with marine deposits. However, the range of Cl/Br values is greater than what can be explained by meteoric water evaporation and rock-water interactions with marine deposits, thus requiring the input of basin or crustal brines. d18O indicates mixing between meteoric and formation waters/brines. 87Sr/86Sr values require water exchange with basement rocks. Improved understanding of the western U.S. surface and ground water system requires characterization of the chemistry, distribution, and volume of mantle-derived gases traveling with deeply circulated waters.