New Mexico Geological Society Annual Spring Meeting — Abstracts

Extension rates in the Rio Grande rift (RGR) have slowed since the Miocene within the Albuquerque Basin, and the Valles Caldera geothermal system is presumably cooling, yet large amounts of CO₂ are degassing along active Quaternary faults in these areas. This study measures present day CO₂ flux (g_CO2/m²d) along faults in order to provide evidence that faults are conduits for fluids from deep crustal and magmatic sources. We used a CO₂ flux device (EGM-4 CO₂ gas analyzer; PP Systems, with an accumulation chamber) to measure transects across faults at three sites within the Valles Caldera (Alamo Canyon-AC, Sulphur Springs-SS, and Soda Dam-SD) and near the western and northern borders of the Albuquerque Basin at five sites (Penasco Springs-PS, Zia Pueblo-ZP, San Ysidro-SY, Sand Hill Fault-SHF, and Carrizo Arroyo-CA). These sites (except for ZP) characterize the western RGR faults from the Valles Caldera southward along the rift related Jemez, Nacimiento, Sand Hill, and Comanche fault networks. Mantle He and CO₂ are well documented at 5 of the 8 sites, and decreases in the mantle derived He component is shown by decreasing ³He/⁴He ratios away from the Caldera. Targets along these fault systems included carbonic and geothermal springs and cemented faults. The cemented (silica and carbonate) faults demonstrate past fluid migration and CO₂ degassing. Results are as follows: carbonate-cemented faults at Zia Pueblo show little to no CO₂ flux (0-1.2 g_CO2/m²d), indicating an impermeable barrier to cross and up-flow fluid migration. The silica cemented fault associated with the Santa Ana fault network showed a moderate CO₂ flux (9.6 g_CO2/m²d), indicating it is still a conduit for upward and potentially cross-flow fluid migration.

Cumulative probability plots were constructed to distinguish between CO₂ fluxes produced by background (biological and soil), diffuse, and/or endogenic (magmatic or tectonic) sources, also referred to as populations. Active carbonic and geothermal spring sites (AC, SS, SD, PS, and SY) have at least 3 populations, whereas sites with no active springs (ZP, SHF, and CA) have only 2 populations. Our analyses show that background values vary between sites, especially within the Valles Caldera where more vegetation and precipitation occurs. Generally, CO₂ fluxes that range from 0.2 to 8.6 and from 0.2 to 2.5 g_CO2/m²d in the Valles Caldera and Albuquerque Basin, respectively, are considered background. CO₂ fluxes ranging from 2.0-148 g_CO2/m²d are considered diffuse in the Valles Caldera. Sites in the Albuquerque Basin with active carbonic springs (PS and SY) and those without active springs (ZP, SHF, and CA) have diffuse CO₂ flux rates that range from 0.8-35.5 and 1.7-9.6 g_CO2/m²d, respectively. Endogenic fluxes from the Valles Caldera sites range from 100-2,400 (maximum flux detection) g_CO2/m²d. Endogenic fluxes from the Albuquerque Basin sites with active carbonic springs range from 13.2-2,400 g_CO2/m²d, and no endogenic CO₂ was measured at sites that had no active springs. These high flux rates are comparable to Yellowstone and other geothermal systems around the world, and provide evidence that large amounts of CO₂ are contributed to the atmosphere from dormant volcanoes and intercontinental rift systems.