New Mexico Geological Society Annual Spring Meeting — Abstracts

Carbonic warm and hot springs extend NE of the Valles Caldera toward Taos, NM. The Valles Caldera 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. From studying spring chemistries at a range of distances from the Valles Caldera, we found that the Valles geothermal system influences water and gas chemistry at Ojo Caliente via a geothermal outflow plume (possible lateral transport) along the Embudo- Ojo Caliente fault systems of the Jemez lineament. This finding is supported by external carbon (C_ext – carbon from carbonates removed) concentrations of 0.04 mol/L and δ¹³C of -3.5‰ (corresponding with Valles Caldera values), ⁸⁷Sr/⁸⁶Sr ratios of 0.747, Sr abundances of 1.35 ppm and higher ⁴He concentrations than the Valles Caldera (suggesting long lateral transport through granitic bedrock), Na concentrations at 983 ppm and lithium at 4.3 ppm.

Our second finding is that regional fault systems provide conduits for both lateral and vertical migration of deep gases and solutes to rise and mix with meteoric fluids; this is verified by helium and carbon isotopes. Throughout the study site helium isotope analysis reveals the presence of a mantle component of 0.13 to 0.32 Rc/Ra, hence 1.6 to 4.0% MORB, assuming a MORB end member of 8 Rc/RA (Ra = ³He/⁴He ratio of air; Rc is air-corrected value). Carbon isotopes throughout the site have δ¹³C values ranging from -9.05 to -3.5‰ showing mixing among an endogenic source, at -3.5 (Goff et al., 2000), and an organic source, typically -28‰.

Our third finding is that the eastern Rio Grande Rift springs have different chemistry than Ojo Caliente and La Madera springs (springs on the western side of the Rift) due to less lower world influence, meaning the eastern springs are mainly influenced by small geothermal input coming vertically upwards along faults rather than laterally from the Valles Caldera. We argue this because the eastern springs have lower C_ext concentrations of 0.001 to 0.004 mol/L, and they plot closer to the epigenic end member on mixing lines, in particular trace element and non-reactive gas plots.

In addition to these regional trends we discovered extensive degassing on a smaller geographic scale from Ojo Caliente to La Madera and Statue springs. The δ¹³C values at La Madera and Statue springs range from 3.6 to 8.6‰ and have similar C_ext concentrations to Ojo Caliente at 0.03 to 0.04 mol/L. La Madera values can be modeled by using degassing Rayleigh fractionation equations, with an alpha for HCO₃ to CO₂, starting the degassing trend from Ojo Caliente values. The driver for CO₂ degassing is the fault network at Ojo Caliente where there is an intersection of NW and NE faults.

We conclude that Ojo Caliente springs underwent mixing with endogenic fluids derived in part from the Valles Caldera whereas eastern Rio Grande Rift springs are more meteoric, but still have ³He/⁴He ratios suggesting mantle or deep crustal degassing mostly from vertical transport along deeply penetrating faults.