APPLICATION OF AR-HE-NE ISOTOPES AT MCELMO DOME-DOE CANYON TO INVESTIGATE CO2 SOURCE AND SYSTEM CHARACTERIZATION

The McElmo Dome-Doe Canyon field (MDO) is one of the largest subsurface CO₂ reservoirs in the Four Corners region. In prior studies, hypotheses favored CO₂ generation by thermal in situ decomposition of carbonate-sulfate assemblages in the Leadville Limestone or from magmatic degassing of Laramide plutons. The source of the CO₂ gas, however, was not fully evaluated. In this investigation, noble gas isotope signatures (Ne, Ar, He) were used to fingerprint the CO₂ gas produced in the field and test competing hypotheses on its origin.

Analyses of noble gas isotopes and major gas composition data across the MDO reveals signatures that are consistent with a mixture of crustal, magmatic, and atmospheric contributions, dominated by elevated radiogenic and nucleogenic isotopes (⁴He, ²¹Ne, ⁴⁰Ar). A comparison of CO₂/³He to CO₂ volume shows our helium concentrations are consistent with magmatic ³He sources, but other isotopic in gas signatures reveal a dominant crustal or lithospheric mantle signature.

Our preliminary data shows mixed signatures of crustal and mantle input, possibly reflecting different sources of CO₂ and noble gases. We argue that the CO₂ gas was sourced from 30-4 Ma magmas that degassed and filled stratigraphic traps in the Leadville Formation. Oligocene mantle melts are the favored candidate for a mantle magmatic source. Magmatic events in the area span from 75-5 Ma and involved melting of Proterozoic lithospheric mantle, which was a key source of carbonate-rich mantle melts. Oligocene mafic rocks generated from these melts have elevated K, U, Th and F, and provide a possible explanation of exceptionally high nucleogenic (²¹Ne, ²²Ne) and radiogenic (⁴He, ⁴⁰Ar) noble gases concentrations observed in the MDO.