Assessing the plausibility of direct constraints on ancient atmospheric pCO2 from fluid inclusions in halite: A theoretical and experimental approach

Michael Naylor Hudgins, Justin G Park, Alex M Ryan, Jessika A Rogers, Morgan F Schaller

Fluid inclusions in halite formed in surficial environments have recently gained attention for their ability to capture and preserve samples of ancient air, and by mechanical decrepitation, these inclusion gases can be quantified via mass spectrometry. However, it has yet to be demonstrated that the CO2 content measured on halite accurately represents the overlying air at various CO2 concentrations and not the exsolution of some or all of the dissolved inorganic carbon. Based on the kinetics of the carbonate system equilibrium and aqueous solubility of CO2, we hypothesize that the CO2 measured by bulk analysis of fluid inclusion gas is derived solely from a mixture between CO2 (g) and [CO2]aq (CO2 from air and air-saturated brine). To address this, we first conducted depressurization experiments on atmospherically equilibrated carbonate solutions, and by mass balance we show that only CO2(aq) degasses during solution depressurization, while the remaining HCO3– and CO32– combine with Na+ cations to precipitate NaHCO3 salt. In addition, we performed mechanical decrepitation experiments on lab-grown halite (LGH) that was precipitated in a sealed pressure vessel under various pCO2 conditions (450 ppm, 3000 ppm, and 5000 ppm). Volatile contents of the carbonate solutions and the LGH were analyzed with a quadrupole mass spectrometer. Measured CO2 content falls between the expected values of air and air-saturated brine for the different pCO2 conditions, and by partitioning the gas contributions from each phase, we observe atmospheric pCO2 that closely matches the starting conditions. Our findings demonstrate that inclusions in halite faithfully entrap and preserve CO2 from the overlying atmosphere and that the exsolution of DIC does not meaningfully contribute to the measured CO2, giving confidence in applying our methods to new and existing analyses of both modern and ancient natural halite. Bulk gas analysis of fluid inclusions is one of the only methods to directly constrain ancient atmospheric composition through deep time and extend the record of atmospheric pCO2 well beyond the ice cores.