Application of Noble Gas Isotopic Signatures at McElmo Dome-DOE Canyon to Investigate CO2 Source and System Characterization
The McElmo Dome-DOE Canyon field in the Four Corners region is one of the largest sources of CO2 in the Rocky Mountain region. In prior studies, hypotheses in favor of CO2 generation by thermal in situ decomposition of carbonate-sulfate assemblages in the Leadville Limestone or magmatic-gas release were proposed. The fundamental source of the gases, however, remained poorly understood. In this investigation, noble gas isotope signatures were used in an attempt to fingerprint the source of the CO2 gas and test competing hypotheses on its origin, migration, and evolution. Analyses of noble gas isotopes, stable isotopes, and major gas compositions across the McElmo-DOE field reveal variable and mixed mantle-crust signatures which are dominated by the addition of radiogenic crustal signatures (4He, 21Ne, 40Ar). A comparison of CO2/3He against CO2 concentrations are consistent with a magmatic 3He source that mixed with crustal contributions. The crustal contributions are indicated by helium isotope ratios 3He/4He (where the ratio of RAIR=1) from 0.057 to 0.215 (R/Ra), nucleogenic (following U and Th decay) 20Ne/22Ne (<8.5), 21Ne/22Ne (>0.10), and highly elevated radiogenic Ar with 40Ar/36Ar* >15,000. Our preliminary data suggests that CO2 gas was likely sourced from Cenozoic magmatic activity in the region that filled Leadville Formation traps at the time of magmatism. Magmatic events spanned the period from 75–5 Ma and involved melting of Proterozoic lithospheric mantle which was a key source of carbonated mantle melts in the Oligocene. Mafic rocks generated from these melts have elevated K, U, Th and F, and these magmas could have been a major source of the exceptionally high nucleogenic (21Ne, 22Ne) and radiogenic (4He,40Ar) signatures of noble gases in the McElmo Dome and Doe Canyon CO2 fields.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015