INFLUENCE OF THE VALLES CALDERA IN SUPPLYING CO2 TO GEOTHERMAL SPRINGS IN NORTHERN NEW MEXICO

Carbonic warm and hot springs extend NE of the Valles Caldera, which had major eruptions 1.6 and 1.25 Ma, subsequent rhyolite eruptions lasting until 40-60 ka, and now hosts an active magmatically driven geothermal system. We examined a suite of springs NE of the caldera along the Jemez lineament in north-central New Mexico. Springs were categorized based on geochemistry of both water and dissolved gas for the purpose of understanding deep flow pathways of the regional geothermal system. Spring groups differ in the eastern (Taos) versus western (Ojo Caliente and La Madera) regions. Springs of the western region are associated with the Ojo Caliente fault; those of the eastern regions are associated with Taos Gorge and Picuris-Pecos faults. Helium isotope analysis reveals the presence of a mantle component in both areas; 0.14 to 0.32 R_A in west and 0.13 to 0.20 R_A in the east, hence 1.6 to 4.0% mantle derived helium assuming a MORB end member of 8 R_c/R_A (R_A = ³He/⁴He ratio of air; R_c is air-corrected value). d¹³C from springs at Ojo Caliente are -3.5‰ and springs in the east range from -9.33 to -4.11‰, interpreted to reflect a mixture of sources; however higher values at La Madera, -1.02 to 2.33‰, are similar to Valles Caldera and may reflect decarbonation of limestone. The ⁸⁷Sr/⁸⁶Sr value at La Madera is 0.718, the ⁸⁷Sr/⁸⁶Sr values are lower along the Taos Gorge fault at 0.708, suggesting that western springs have flowpaths through granitic basement. Modeled PCO₂ in western springs is 10^-0.336 to 10^-0.842, which is higher than eastern springs of 10^-2.270 to 10^-3.614; this corresponds with CO₂ data, highest western value is 30.721% and eastern is 4.273%. These tracers suggest that the Jemez volcanic system is supplying CO₂ leakage along faults of the Jemez lineament that are not evident farther east. We conclude that western springs underwent mixing with endogenic fluids derived in part from the Jemez Mountains whereas eastern springs are more meteoric, but still have ³He/⁴He ratios that suggest magmatic fluid input. The combined tracer study suggests geothermal waters traveling along fault pathways likely influenced western springs whereas eastern springs show a higher mixing proportion from meteoric water and a Taos Plateau endogenic endmember that, while still containing a magmatic component, has less ³He and CO₂.