COMBINED CLUMPED CARBONATE THERMOMETRY - FLUID INCLUSION STUDY: INVESTIGATING PRESSURE-TEMPERATURE CONDITIONS DURING CALCITE PRECIPITATION, FARNHAM DOME, EAST-CENTRAL UTAH

This study combines carbonate clumped isotope thermometry (CIT) with burial history data as a means to develop an independent geobarometer to make pressure corrections to homogenization temperatures (Th) obtained from fluid inclusion microthermometry. Data come from carbonate veins sampled from faults and fractures of the Farnham Dome anticline, Utah. Calcite δ¹⁸O and δ¹³data are consistent with an initial period of Sevier-related deformation involving heated meteoric and formation fluids migrating along faults that terminate into a detachment within the Jurassic Carmel Formation at temperatures ranging from 80 to 165 ºC. A broad four way anticlinal trap formed as a result of Laramide deformation and subsequently filled with CO_2. Fill-to-spill reservoir models indicate fluids could have migrated within the damage zone of antithetic thrust faults and/or around fault-tip terminations. Calcite veins associated with the western (youngest) antithetic thrust faults are more enriched in the heavy isotopes of carbon and formed at temperatures from 65-114 ºC. These values are similar to the isotopic values of veins obtained from drill cores that penetrate the CO₂ reservoir.

Measured Th differ from temperatures obtained by CIT by up to 30° C. However, depths calculated from pressure corrected Th values plotted on their respective isochores fall within 100 to 200 meters of those predicted from the combined CIT-burial curve history data. Petrographic evidence indicates multiple vein generations within a given sample. Thus, CIT results likely represent bulk values of multiple fluid flow events. One sample with the highest 1σ standard deviation (Δ_{47 =} 0.0434 ‰, ±12° C) results in a trapping pressure error of ±15 MPa, equivalent to 500 meters of uncertainty in depth. Despite this large uncertainty, the results demonstrate that combined CIT-burial history data can provide improved constraints on P-T conditions at the time of carbonate precipitation. The current data set is consistent with vein formation at pressures below the fracture gradient during syntectonic fluid flow events.