Comparison of the potential metamorphic and igneous carbon dioxide fluxes from the Salton Sea Geothermal System, California

The Salton Sea Geothermal System is located in the Salton Trough rift which is the boundary between the North American and Pacific Plates in south-central California. The Salton Trough rift is an extension of the Gulf of California rift system, and is filled with the sediments of the Colorado River delta. The Salton Sea Geothermal Field is located above the northernmost spreading center of the Gulf of California before the plate boundary becomes the San Andreas transform fault system. 327 MW of electricity is currently generated from the hot (< 390°C), hyper saline brines (-25% TDS) at shallow depths (>400 m). The geothermal brines of the Salton Sea Geothermal System contain abundant CO2 (-2,000 mg/ kg brine). The thermo-tectonic environment of the system suggests that both a magmatic and a metamorphic source of CO2 are plausible. In order to evaluate these possibilities this study estimates an annual CO2 flux for both metamorphic and igneous sources based on evaluation of devolatilization reactions occurring in the geothermal system, and an estimated volume of igneous material injected annually, respectively. Calculations of metamorphic CO2 flux are based on evaluation of the reaction progress of devolatilization reactions of four wells in the Salton Sea Geothermal Field. Measurement of reaction progress is based on petrography and XRD studies of well cuttings. When the average reaction progress from the observed well-bores is extrapolated to the full depth of the sediments in the Salton Trough [4 to 6 km] inferred form seismic data metamorphic CO2 fluxes of 78.7 to 637.8 g /m yr are calculated. The igneous CO2 flux is based on the potential size of an intrusive body emplaced beneath the geothermal field annually, which is constrained by various geophysical data, and is in the range of 0 to 88 g/m2 yr. The metamorphic CO2 flux estimate in this study is up to 7.25 times the maximum igneous CO2 flux.