New Mexico Geological Society Annual Spring Meeting — Abstracts

Mantle helium has been identified within surface and shallow groundwater in the Colorado Rockies, Jemez Mountains, and northern New Mexico in several previous studies. We examine a suite of springs in the southern San Luis basin and surrounding areas and categorize them based on geochemistry of both water and gas. Consideration of multiple chemical tracers (including gases) will help to better understand the nature of the regional geothermal systems. The springs and thermal wells of northern New Mexico have been grouped by geologic setting (major fault structures and hydrostratigraphic units) and we use multiple tracers to identify end member chemistry. Our methods include major ion chemistry, stable isotopes, gas abundance, and helium isotope analysis. Each tracer aids in understanding mixing between shallow and geothermal sources.

Our preliminary results show mixing trends using major ion compositions, non-reactive gas and helium isotope diagrams, with deeply-circulated fluid end members defined by Ojo Caliente and Ponce de Leon geothermal springs. The major ion compositions show two possible deep end members, Ojo Caliente, Na-HCO₃ waters, and Ponce de Leon, Na-SO₄ waters. Gas abundances, in particular the non-reactive gases Ar-N₂-He, compare dissolved gases in spring samples to air. Deeply derived end members typically have higher helium relative abundances. Our springs show water mixing ranging from deeply derived sources to air-like compositions, with Ponce de Leon along the mixing line. Helium isotope analysis reveals the presence of a mantle component. Several of our samples show 3-4% mantle derived helium assuming a MORB and crustal end member of 8 and 0.02 RA (where RA is the ³He/⁴He ratio of air). Our initial conclusions are that Ojo Caliente is a carbonic spring with mantle derived volatiles, and springs along the Embudo fault have similar R_c/R_a values as Ojo Caliente but are not carbonic suggesting different geothermal end members. The use of multiple tracers will allow us to make further conclusions concerning proportions of mixing, groundwater quality degradation, and to apply the gas compositional and isotopic results to better understand geothermal influences on surface systems of the southwestern US.